Composition of sedimentary organic matter in Thrissur Kole wetland, southwest India

Spatio-temporal distribution dynamics of various biochemical components in Thrissur Kole wetland were analyzed to assess the quality of organic matter (OM) and trophic status. Thrissur Kole wetland forms part of the Vembanad wetland system, located on the southwest coast of India. Surface sediment samples were collected from sixteen sites during the pre-monsoon (February 2019) and monsoon (August 2019) seasons. Concentration of biochemical parameters displayed the trend as follows: total carbohydrates > total protein > tannin and lignin > total lipids. Intermediate values of C/N ratios [average: 17.15 ± 0.86 (pre-monsoon), 9.81 ± 0.42 (monsoon)], indicated a mixed origin of OM. The occurrence of high tannin and lignin content substantiated terrestrial vascular plant input to the wetland ecosystem. Elevated levels of PRT were due to anthropogenic inputs via aquaculture activities and guano of birds. Majority of the sites exhibited lower values of protein to carbohydrate ratios (< 1) revealed the occurrence of aged sedimentary OM. Lower values of total lipid to total carbohydrate ratio (< 1) indicated lower nutritional quality of the OM. Lower % contribution of labile OM to total OM, lower biopolymeric carbon to total organic carbon ratio and lower algal contribution to the biopolymeric carbon suggested that a bulk fraction of OM exists as refractory. Based on protein and carbohydrate content, biopolymeric carbon and algal contribution to biopolymeric carbon, trophic state of the wetland system was evaluated as hypertrophic which implies deterioration in ecosystem health.


Introduction
Organic matter in aquatic environment usually originates from autochthonous and allochthonous sources. Autochthonous sources of OM include decomposition of macrophytes, algae, bacteria and plankton in the aquatic environment and sedimentation. Meanwhile, allochthonous contribution occurs through particulate and dissolved OM via run-off, and emission of organic pollutants from agricultural sector (Yamamuro 2000;Liu et al. 2007;Wu et al. 2013). The deposited OM in sediments has its own unique character and their chemical composition has been used for assessing various processes such as rate of sedimentation, water mass dynamics, presence of land-derived materials, redox potential, and productivity (Jacob et al. 2008;Shaji et al. 2020).
Preservation of OM in sediments depends on dissolved oxygen level and sedimentation rate (Kumar et al. 2013).
OM in sediments is a key component that plays a significant role in biogeochemical cycles and growth of aquatic organisms (Du et al. 2022;Wen et al. 2023). The biochemical composition of OM in the aquatic system results from a dynamic equilibrium between heterotrophic utilization, external input and autochthonous production. Assessment of the quality of OM is mandatory to distinguish between the labile/refractory OM. Use of biochemical composition has been recognized as an effective methodology for characterization of OM in sediments. Composition of biochemical fractions has been recognized as a reliable and efficient tool to unravel the origin of OM and its diagenetic reorganization (Cotano et al. 2006). The summative measure of total OM content in sediments provide only limited knowledge on its bioavailability to consumers (Incera et al. 2003). Knowledge on composition of OM in sediments is crucial in assessing the nutritional quality. Biochemical constituents reflect the changes in the productivity, quality of OM as available food 1 3 174 Page 2 of 17 to benthic organisms and provide implications on the trophic status of the aquatic environment (Cividanes et al. 2002;Pusceddu et al. 2003;Joy et al. 2019;Gardade et al. 2020).
Wetlands occupy peculiar geographic regions and have been recognized as productive ecosystems. These biodiverse aquatic systems can accumulate bulk quantities of OM delivered from various sources and contribute to the global carbon budget. Kole wetlands in Kerala have been declared as Ramsar Site in 2002, based on ecological importance. It forms a part of Central Asian Flyway of migratory birds (Greeshma and Jayson 2020). The Kole lands support a wide variety of migratory birds that were included in the Red Data Book list (Sivaperuman and Jayson 2002;Greeshma and Jayson 2020). Kole is an ecologically fragile area that is beset by several natural and man-made activities. Modernized progressive mechanical and aquaculture activities prevailing in this sensitive environment adds to severe ecological risks (Nisari and Sujatha 2021).
Only a few scientific reports are available with regard to physico-chemical characteristics (Vineetha et al. 2016) and status of metal contamination (Nisari and Sujatha 2021) in Kole wetlands of Thrissur. Detailed investigation on the quantity and quality of OM using biochemical parameters in sediments of the study region is still lacking. Hypothesis of the study include: whether the physico-chemical parameters exert any control over the spatial distribution of biochemical components in the study area, check whether biochemical composition is suitable to distinguish labile or refractory OM, and predict the trophic status of the system. Objectives of the study were to explore: (i) the spatio-temporal distribution of various biochemical components in the surface sediments of Thrissur Kole wetlands (ii) the quality of OM, possible sources and trophic state.

Description of the study area
Thrissur District is located in the central region of Kerala, South India. The total geographic area of Thrissur District is 3032 km 2 . Thrissur Kole lands is a part of India's second largest Ramsar site-Vembanad wetland (No: 1214). It runs parallel to the Arabian Sea and the low-lying flood plains located at 0.5-1 m below Mean Sea Level covering an area of 13,632 hectares are spread over the districts of Thrissur and Malappuram. The Malayalam word "Kole" means bumber yield or high returns under favorable conditions (Srinivasan 2012) and it refers to a peculiar type of cultivation practice adopted in these lands. The study area lies in the Thrissur district between 10° 20 ' and 10° 40' N and 75° 58' and 76° 11' E. The geographical distribution of Kole lands in Thrissur district is estimated to be 11,674 hectares and 1958 hectares in Ponnani Kole of Malappuram district (Johnkutty and Venugopal 1993). The Kole system is fed by ten rivers. All these rivers originate from Western Ghats, flow westwards and join the Arabian Sea. The area is also exposed to diurnal tidal cycles. Kole lands are split into three divisions by the rivers draining them: Thrissur South Kole lies to the south of Karuvannur River; Thrissur North Kole lies in between Kechery River and Karuvannur River; and Ponnani Kole lies to the north of Kechery River.
As per Thrissur District Plan, about 233.74 mm 3 water is contained in the Kole lands for irrigation of summer crop in the wetland system which is in fact more than water is stored in the Chimmony and Peechi dams for irrigation of summer crop in the Kole lands. The area lies parallel to the Sea and flood waters are mainly brought by two rivers Kechery and Karuvannur which ultimately drain to the Arabian Sea. A network of main and cross canals provides external drainage and connects the different regions of the Kole to the rivers.
Kole lands are influenced by monsoon, which contributes approximately 2763 mm of average rainfall with an average temperature of 28 °C (Sarath et al. 2017;Nisari and Sujatha 2021). Kole wetlands remains submerged under flood water for about six months in monsoon period. During the Southwest monsoon, the slope from the upland areas facilitates the accumulation of nutrients in the water bodies through land run-off and riverine discharge (Nisari and Sujatha 2021). However, the Kole lands have been suffering from various kinds of human-induced alterations since last few decades. Indiscriminate conversion of vast areas to non-agricultural land, urban development, unsustainable cultivation methods being adopted, increased mining activities causing fine sand deposition in the subsurface of the banks and waste disposal are the major threats to this sensitive wetland ecosystem.

Sampling and analytical methodology
Sampling stations were selected based on geographical features and environmental significance (Table 1). Environmental conditions of the study area were influenced by agricultural activities, riverine influx and tidal ingression from Arabian Sea. Estuarine sites are represented by S 1, S 2 , S 3 , and S 13 , whereas riverine zone is characterized by S 5 , S 6 , S 12 , S 14 , S 15 , and S 16 . Meanwhile, zones with irrigation canal and agricultural activities are denoted by S 4 , S 7 , S 8 , S 9 , S 10 , and S 11 . Surface sediment samples were collected from 16 sites of Thrissur Kole lands ( Fig. 1) during February 2019 (pre-monsoon) and August 2019 (monsoon). Collection of sediment was carried out on board with Van Veen grab (0.042 m 2 ) and subsampled to polythene bags using a clean plastic spatula to prevent any cross-contamination. Triplicate samples were collected for each sampling site and mixed together to obtain true representative composite sample. The samples were placed in an ice box, transported to the laboratory and kept at -20 °C in a deep freezer until the analysis. Determination of pH and E h of sediment samples was done in situ using hand-held probe (Eutech Instruments-Cyberscan PCD 650). Grain size of the sediments was determined by pipette analysis (Krumbein and Pettijohn 1938;Folk 1974). Sediments for analysis of remaining parameters were dried using freeze dryer (BK-FD12S Freeze Dryer, Biobase, China -80 °C) and finely powdered using agate mortar and pestle. Instrumental method was used to estimate total sulfur (TS) in sediments (Vario EL III CHNS Analyser, Germany). Total organic carbon (TOC) was determined by TOC analyser (VARIO TOC SELECT-Elementar, Germany). The total organic matter (TOM) was calculated by multiplying the TOC content with 1.724 (Nelson and Sommers 1996). Total organic nitrogen (TON) was determined using the Kjeldahl method. Estimation of total carbohydrate (CHO) in sediments was done by spectrophotometric method (Dubois et al. 1956) using glucose as calibration standard. Total protein (PRT) was analyzed (Lowry et al. 1951;Rice 1982) utilizing bovine albumin as calibration standard. The total lipid (LPD) content of sediments was extracted with chloroform and methanol mixture (1:1 v/v) according to Bligh and Dyer (1959) and were determined according to Barnes and Blackstock (1973) employing cholesterol as calibration standard. The sum of concentrations of CHO, PRT, and LPD was defined as Labile Organic Matter (LOM) (Danovaro et al. 1993;Cividanes et al. 2002). Analysis of tannin and lignin (TL) was carried out spectrophotometrically by the sodium tungstate phosphomolybdic acid method (Kloster 1974). Tannic acid was used as a calibration standard. Chlorophyll and phaeopigments measurements were established according to standard procedures (Lorenzen and Jeffrey 1980;APHA 1995). Pigments were extracted using 90% acetone (24 h in the dark at 40 °C). After centrifugation, the supernatant was used to determine chlorophyll pigments (Chl a , Chl b and Chl c ) by spectrophotometry-multiwavelength mode. Chlorophyll pigment extract was acidified with 0.1 N HCl, measured absorbance spectrophotometrically (multi wavelength mode) to estimate the concentration of phaeopigments. Total carbohydrate, total protein and total lipid were converted into the carbon equivalents using the conversion factors 0.45, 0.50 and 0.75 g of C/g, respectively, and their sum was reported as biopolymeric carbon (BPC) (Fabiano and Danovaro 1994;Pusceddu et al. 2009). Algal contribution to BPC was calculated as the percentage of Chl a to biopolymeric carbon concentrations, after converting Chl a concentration into carbon equivalents, by applying a mean value of 40 (Pusceddu et al. 2009). Estimation of all the variables was carried out in triplicate and the average value was reported.
Spatial and seasonal variations in estimated variables were determined using two-way analysis of variance (ANOVA) (Software: Microsoft Excel). Pearson correlation matrix was generated (Software: SPSS version-26) to evaluate the interrelationship among variables. Principal Component Analysis (PCA) was employed to identify the source of OM as well as the process governing the distribution of various geochemical variables in sediments . Hierarchical Cluster Analysis (HCA) was carried out in the normalized data set using Ward's method with squared Euclidean distances as a measure of similarity. Ward's method is an ANOVA-based approach to evaluate the distances among the clusters in an attempt to minimize the sum of squares of any two clusters that can be formed at each step. The software SPSS Version-26 was used to run the whole process of HCA.

Results
Spatial and seasonal distribution of fundamental sediment quality parameters are depicted in Table 2. Values of sediment pH varied from 6.53 to 7.58 with an average value of 7.13 ± 0.30 and displayed maximum value at S 16 during pre-monsoon (Fig. 2a). Meanwhile, the variation in pH during monsoon was from 6.62 to 7.47 with higher values recorded at S 6 and S 11 and exhibited average value of 7.15 ± 0.23. Redox potential of sediment samples exhibited a variation from -16 to 45 mV (pre-monsoon) and − 29 to 22 mV (monsoon). Texture analysis revealed silt and clay as the dominant fractions in majority of the sites during pre-monsoon ( Fig. 2b) with an average of 58.57 ± 1.08% and 26.81 ± 0.87%, respectively. However, during monsoon ( Fig. 2c), sand and silt fractions exhibited dominance with an average of 37.22 ± 2.58% and 34.03 ± 1.48%, respectively. TOC content during pre-monsoon varied from 0.59 to 7.35% with its maximum recorded at S 14 (Fig. 2d). Meanwhile, TOC content during monsoon exhibited its maximum at S 10 , with a variation from 0.43 to 4.71%. Concentration of TON in sediments ( Fig. 2e) varied from 0.04 to 0.42% (premonsoon) and 0.07 to 0.52% (monsoon). Observed variation in total sulfur during pre-monsoon was from ND to 1.28% with its maximum content noticed at S 14 . However, during monsoon, it varied from 0.23 to 1.19% with its maximum recorded at S 13 .
CHO content in sediments ranged from 17.42 to 656.81 mg/g and displayed maximum at S 11 ( Fig. 3a) during pre-monsoon. However, during monsoon, CHO content varied from 2.41 to 393.35 mg/g with its maximum exhibited at S 8 . Estimated PRT content displayed wide fluctuations in the study region (Fig. 3b) and it varied from 0.99 to 74.46 mg/g (pre-monsoon) and 1.28 to 36.60 mg/g (monsoon). Concentration of total lipids in the sediments (Fig. 3c) varied from 0.24 to 2.69 mg/g and displayed maximum at S 7 during premonsoon and 0.31 to 3.26 mg/g and exhibited maximum at S 12 during monsoon. Observed tannin and lignin content in the study region ( Fig. 3d) varied from 0.84 to 9.83 mg/g (pre-monsoon) and 0.51 to 3.83 mg/g (monsoon). Chlorophyll a (Chl a ) recorded variation in concentration from 4.70 to 59.48 mg/kg with an average of 20.53 ± 1.44 mg/kg (pre-monsoon) and 3.90 to 35.45 mg/kg with an average of 15.67 ± 1.02 mg/kg (monsoon). The maximum content for Chl a was observed at S 14 (pre-monsoon) and at S 11 (monsoon). Concentration of Chlorophyll b (Chl b ) ranged from 1.20 to 36.24 mg/kg and exhibited maximum at S 14 (premonsoon) and 1.33 to 22.32 mg/kg and displayed maximum at S 2 (monsoon). Also, Chlorophyll c (Chl c ) exhibited variation from ND to 14.23 mg/kg during (pre-monsoon) and ND to 12.59 mg/kg (monsoon). Average content of phaeophytin was higher during pre-monsoon (34.20 ± 1.20 mg/kg) than monsoon season (32.35 ± 1.30 mg/kg). The concentration of phaeophytin pigments varied from 7.64 to 97.80 mg/kg (pre-monsoon) and with its maximum content at S 14 . Meanwhile, it varied from 5.69 to 94.82 mg/kg during monsoon and displayed its maximum at S 1 .

Seasonal and spatial variation in fundamental sediment quality parameters
Higher pH values were observed at estuarine zone of the wetland viz. S 1 , S 2 , S 3 and S 13 (Fig. 2a) compared to other stations due to seawater ingression. Seasonal variation in pH (P < 0.01) was observed as its distribution was controlled by freshwater inputs from rivers and seawater ingression from the Arabian Sea. Highly depleted values of E h observed in the study implied the enhanced respiration process in the wetland. Redox potential (E h ) is the measure of the availability of electrons and their activity in the medium. Majority of stations recorded depleted E h values during monsoon indicating anaerobic conditions (Kumar et al. 2017) created by respiration of OM derived from autochthonous and allochthonous sources. Remarkable seasonal and spatial variation was observed for both sand (p < 0.01) and silt (spatial: p < 0.05; seasonal: p < 0.01). The dominance of high sand fraction during monsoon ( Fig. 2c) was due to the accelerated hydrodynamic conditions (S 1 , S 2 , S 3 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11 , S 13, and S 16 ). However, the occurrence of high silt and clay fractions ( Fig. 2b) at sites (S 2 , S 4 , S 5 , S 6 , S 7 , S 8 , S 9 , S 11 , S 12 , S 14 , and S 15 ) during pre-monsoon was due to lower hydrodynamic energy conditions that promote the deposition of fine sediment fractions. The complex current system prevailing in the wetland along with terrestrial run-off contributes to wide variations in the sediment grain size distribution.
Spatial and seasonal distribution of TOC is displayed in Fig. 2d. TOC contents were influenced by many factors viz. textural characteristics, deposition rate, addition of terrestrial materials, and in situ primary production. The content of TOC was comparatively higher during premonsoon compared to monsoon. Higher TOC concentrations were recorded at S 2 , S 7 , and S 14 due to the combined effect of river inputs and discharge of organic wastes from agricultural and aquaculture units. Lower TOC concentrations were recorded at S 1 , and S 2 in both seasons due to the inability of sandy sediments to trap fine particles. The increasing trend of TOC observed at the central zone (S 6 , S 7 , S 8 , S 9 , and S 10 ) of the study region (Fig. 2d) during monsoon was due to OM input from agriculture activities. The obtained TOC concentrations in sediments were in good agreement with previous investigations (Gireeshkumar et al. 2013;Kumar et al. 2017;Mathew et al. 2019). Concentration of TOC exhibited highly significant correlation with fine-grained sediment fraction viz., silt (r = 0.57) signaled its size-dependent scavenging (Nair et al. 2002). Fine-grained sediment was found to be the main factor influencing the accumulation of OM due to higher rate of adsorption (Cotano and Villate 2006;Ramaswamy et al. 2008;Gireeshkumar et al. 2013).
TON content was relatively lower in sediments and displayed elevated levels during monsoon (p < 0.01). Higher levels of TON were recorded at S 2 , S 4 , S 7 , S 8 , S 9 , S 10 , S 11 , S 12 , and S 14 during monsoon (Fig. 2e) was due to the finegrained sediment texture and the direct input of nitrogenous compounds from agriculture and land run-off (Subramanian 2004;Kumar et al. 2017). Accumulation of TON in fine sediment fraction was also supported by correlation analysis (TON vs. silt; r = 0.56). Mineralization of organic nitrogen and biological N fixation can be an important source of sedimentary nitrogen in the aquatic environment (Gruber and Galloway 2008;Kumar et al. 2017). Strong positive correlation existed between TON and TOC (r = 0.90), since both are components of TOM.

Characterization of depositional environment
Stoichiometric ratios were used to assess the origin and transformation of sediment-bound OM (Yamamuro and Hiroshi 2014). TOC/TS ratios give relevant implications on the redox conditions prevailing in the depositional environment (Raisewell et al. 1988). Usually, the dissolved SO 4 2− is reduced to H 2 S gas, which reacts with iron minerals to form Fe sulfides which cause a qualitative redox status of the environment under deposition (Akhil et al. 2013;Kumar et al. 2017). TOC/TS ratios (Fig. 4a) varied from ND to 23.22 during pre-monsoon and 1.61 to 12.33 during monsoon. Higher values of TOC/TS ratio were observed during premonsoon than monsoon due to increased terrestrial inputs of OM. In general, TOC/TS > 5 suggests oxic sediment with oxygenated bottom water, TOC/TS = 1.5-5 reflect sediment deposition under periodic anoxia and TOC/TS < 1.5 implied anoxic sediment with anoxic water (Benny 2009;Kumar et al. 2017). Majority of the sites S 1 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11 , S 14 , and S 15 in pre-monsoon; S 4, S 5 , S 7 , S 8 , S 9 , S 10 , S 11 , S 12 , S 15 , and S 16 in monsoon recorded TOC/TS > 5 category and the sites S 2 , S 3 , S 12, S 13 , and S 16 in pre-monsoon; S 1 , S 2 , S 3 , S 6 , S 13 , S 14 in monsoon exhibited TOC/TS = 1.5-5 Fig. 4 Variations of TOC/TS ratio (a), C/N ratio (b), PRT/CHO ratio (c), LPD/CHO ratio (d), LOM/TOM ratio (e) and BPC to TOC ratio (f) in surface sediments of the study region category which pointed out that the sediments undergo sulfate reduction below an oxygenated water system (Hedges and Keil 1995;Renjith et al. 2011;Gireeshkumar et al. 2013).

Organic matter source assessment
Elemental ratio C/N ratio has been recognized as a useful tool to distinguish the sources of OM, environmental depositional condition, sediment quality and productivity indicators and pollution indices (Avramidis et al. 2015). The generalization is that OM derived according to preferential remineralization of their protein composition (Mayer 1993) has atomic C/N ratios ranging from 4 to 10, while the average ratio for terrestrial vascular plants is > 15 (Wakeham 2002). It has been well established that selective degradation of different minerals/nutrients in sediments can influence the C/N ratio (Muller 1997). Estimated C/N ratios exhibited prominent seasonal variation (p < 0.01) in the study region which ranged from 10.27 to 33.54 (avg: 17.15 ± 0.86; pre-monsoon) and 5.75 to 13.92 (avg: 9.81 ± 0.42; monsoon). The observed values of C/N ratio were in good agreement with previous studies (Table 3) from Vembanad wetland system Salas et al. 2015;Mathew et al. 2019). Biogeochemical process viz., leaching, microbial mediated remineralization and autolysis modify the C/N ratios in sediments (Kumar et al. 2017). Enhanced inputs of terrestrial vascular plant OM were observed at sites (S 1 , S 2 , S 3 , S 5 , S 6 , S 7 , S 8 , S 11 , S 13, S 14 , and S 15 ) during pre-monsoon (Fig. 4b), while lower C/N ratios during monsoon implied enhanced algal derived OM. Intermediate values of C/N ratio pointed out mixed contribution from both terrestrial and autochthonous OM to sediments (Muri et al. 2004).

Phenolic compounds as OM source indicator
Polycyclic phenolic group of compounds with high molecular weight viz., tannin and lignin are synthesized by vascular plants (Hernes and Hedges 2000) and are delivered to sedimentary systems due to the terrestrial run-off (Akhil et al. 2013;Salas et al. 2015). This class of organic compounds forms a significant fraction of refractory OM and their estimation gives useful information on the amount of terrestrially derived organic detritus in aquatic systems (Lin et al. 2006;Salas et al. 2015). The higher content of tannin and lignin was recorded in pre-monsoon than monsoon. Elevated levels of tannin and lignin exhibited at S 7 , and S 14 implied origin of OM from agriculture sources, river inputs, and from terrestrial run-off. Prevalence of these refractory OM compounds has already been recorded in the Vembanad wetland system Akhil et al. 2013;Salas et al. 2015).

Organic matter quality assessment
Estimation of the biochemical composition of sedimentary OM is to account for the origin and quality of deposited material in the system (Fabiano et al. 1995). Spatial and temporal distribution in the estimated biochemical composition followed the trend: CHO > PRT > LPD during both seasons without any significant spatial and seasonal variations.

Total carbohydrates
CHO represents the dominant fraction of sedimentary OM. Carbohydrates include polyhydroxylated compounds ranging in size from 5 to 6 carbon sugars to large biopolymers and major constituents of vascular plants. Pre-monsoon season exhibited higher carbohydrate content than the monsoon season due to enhanced sinking settling of OM. Higher levels of CHO were due to inputs from autochthonous and allochthonous sources. The peak concentration of CHO (Fig. 3a) was noticed at S 11 (pre-monsoon) and S 8 (monsoon) due to the detrital material input from agricultural land and the contribution from vascular plant debris. Additionally, the abundance of CHO over PRT and LPD during both seasons implied the deposition of aged organic detritus and active exploitation of PRT than CHO by microbial process Venturini et al. 2012;Salas et al. 2015).

Total proteins
The second dominant fraction of LOM has been identified as proteins which are the most significant nitrogen bearing compounds in living organisms. PRT content was higher during pre-monsoon than monsoon. PRT comprises a part of LOM in the sediments contributed by allochthonous as well as autochthonous inputs. Sediment-bound PRT can provide information on productivity (Dell' Anno et al. 2002) and has been used as a powerful tool to evaluate the benthic trophic status of aquatic environment (Pusceddu et al. 2004;Shilla 2021). Riverine and irrigation canal zones recorded higher levels of PRT in both seasons (Fig. 3b) indicating the contribution of anthropogenic inputs via aquaculture activities (Salas et al. 2015). During pre-monsoon, elevated levels of PRT observed at S 7 was due to the detrital material inputs from the decay of aquatic vegetation coupled with terrestrial inputs and at S 14 due to the municipal sewage input. In addition, the guano of birds also serves as a prominent source of protein to the wetland sediments (Joy et al. 2019). PRT contents are subjected to intense microbial activity resulting in a lowering of concentration compared to other organic fractions in aquatic systems .

Total lipids
Sediment-bound LPDs are more dominant in eutrophic systems than in oligotrophic systems. Lipids are produced by living organisms (Burdige et al. 2000;Salas et al. 2015) and are derived not only from aquatic biota but also from higher plant wax (Shilla 2021). Like PRT, LPD also serves as the best descriptor of the productivity of the system. Observed LPD content in the study indicated the effective biological activity related to enhanced productivity of the aquatic system (Akhil et al. 2013;Salas et al. 2015) and enhanced levels of LPD concentrations during monsoon than pre-monsoon. The general distribution pattern of the LPD exhibited a higher concentration at the central zone (S 7 , S 9 , S 10 , S 11 , S 12 , and S 13 ) of the study region pointing out the origin of organic compounds in sediments by terrestrial run-off (supported by enhanced C/N ratio) coupled with anthropogenic inputs. In addition, the higher levels of LPD in the sediments pointed out the greater availability of easily assimilated OM (Gremare et al. 2002;Kumar et al. 2017). Lower content of LPDs recorded at S 3 (Fig. 3c) in both seasons may be because of coarse sediments or the utilization of lipids as an energy source by heterotrophic organisms (Ittekkot et al. 1984;Joy et al. 2019). The proliferation of minerals may also influence the degradation of lipids into other compounds (Cheriyan 2017). Better preservation of lipid constituents is also facilitated by frequent sedimentation, consequent sealing of OM in the deposited sediments and anoxic conditions (Shibini et al. 2019).
Observed trends of biochemical composition in the study area were compared with previous studies (Table 3). Biochemical composition in Vembanad wetland has been extensively investigated and the following trends have been reported: LPD > PRT > CHO (Joseph et al. 2008); LPD > PRT > CHO ; CHO > LPD > PRT ; CHO > PRT > LPD (Salas et al. 2015); PRT > CHO > LPD . Estimated levels of CHO and PRT in the study were higher than that of Vembanad wetland system, while LPD content was low (Table 3). The concentration of biochemical constituents exhibited a similar trend with Salas et al. (2015).
The ratios of biochemical constituents provide fundamental knowledge on sources and cycling of OM in the aquatic system. Generally, the protein to carbohydrate ratio (PRT/ CHO) has been utilized as an index to assess the origin of OM in the sediments, the status of the biochemical degradation processes and to distinguish the nature (fresh/aged) of sedimentary OM (Danovaro et al. 1993;Joseph et al. 2012). PRT/CHO ratio in the study ranged from 0.02 to 1.62 (avg: 0.36 ± 0.01) during pre-monsoon and 0.01-7.65 (avg: 1.09 ± 0.02) during monsoon. The observed values were comparable with previous reports Salas et al. 2015). PRT/CHO > 1 are connected usually with the recently produced OM; typically observed immediately after the deposition of freshly produced phytoplankton (Pusceddu et al. 2000;Joseph et al. 2012). The sites S 6 and S 7 during pre-monsoon and S 5 , S 7 , S 15 , and S 16 during monsoon (Fig. 4c) recorded PRT/CHO > 1 indicating the presence of freshly deposited OM. Lower PRT/CHO ratios (< 1) in the study region were indicative of aged or less degradable OM like terrestrial vascular plant debris. Sediments from majority of the stations were under the stress of strong hydrodynamic conditions and, hence, exhibited a lower PRT/CHO ratio. Also, the low PRT/CHO ratio pointed out the presence of aged sedimentary OM and the reduced availability of OM for the consumption to benthic organisms (Pusceddu et al. 2009).
The nutritional value of sedimentary OM is assessed by its biochemical composition (Fabiano and Danovaro 1994;Dell' Anno et al. 2002;Bianchelli et al. 2020;Shilla 2021). LPD and LPD/CHO ratio has been used as an effective tool to describe the nutritional quality of sedimentary OM (Gremare et al. 2002;Joseph et al. 2008Joseph et al. , 2012. Moreover, the LPD content in the sediments is a good indicator of the benthic trophic status (Dell' Anno et al. 2002). LPD/ CHO ratio in the study region (Fig. 4d) varied from 0.003 to 0.058 with an average of 0.016 ± 0.001 (pre-monsoon) and 0.002 to 0.897 with an average of 0.093 ± 0.001 (monsoon), which were comparable with earlier observations (Salas et al. 2015). Lower values (LPD/CHO < 1) recorded during the study denoted the presence of lower nutritional value of the OM content in the sediments, which might be connected with terrigenous run-off (Jacob et al. 2009). The LPD/CHO ratio recorded its maximum at the irrigation canal zone and agricultural dominated area (S 7 ) during both seasons (Fig. 4d), which indicating the freshness of OM as well as high nutritional value of sedimentary OM in the wetland. Comparatively, higher LPD/CHO ratios were observed during monsoon than pre-monsoon implied enhanced allochthonous input of OM (vascular plant debris enriched with LPD). A few stations with TOC/TS ratio in the category (sediment deposition under periodic anoxia) implied a better preservation of aged OM (as indicated by PRT/CHO < 1) and lower nutritional quality of OM as per LPD/CHO < 1.
Labile organic matter characterizes the easily assimilable fraction of OM available to aquatic benthic organisms. LOM widely fluctuated in the study region (Fig. 3e), ranging from 38.87 to 686.12 mg/g (avg: 142.99 ± 1.49 mg/g) in pre-monsoon and 15.67 to 427.40 mg/g (avg: 100.84 ± 1.04 mg/g) in monsoon. S 11 during pre-monsoon and S 8 during monsoon showed peak levels of LOM can be attributed to agricultural sources. Enhanced levels of LOM reflected the increased in situ productivity coupled with the external inputs of terrigenous materials (Kumar et al. 2017). LOM to TOM ratio in the present investigation (Fig. 4e) ranged from 0.64 to 9.48 (avg: 3.17 ± 0.13) in pre-monsoon and 0.47 to 24.23 Page 11 of 17 174 (avg: 3.13 ± 0.21) in monsoon, reflected the fact that a large portion of TOM represented refractory material (Joseph et al. 2008).
Observed chlorophyll pigments (Fig. 5) recorded the following trend in both seasons: Chl a > Chl b > Chl c . Remarkable levels of chlorophyll pigments in the sediments were controlled by light availability and DO content in the water column (Moreno and Niell 2004;Salas et al. 2015). Enormous quantities of suspended particulate matter leached via terrestrial run-off to the system lead to increase turbidity, eventually reducing the availability of sunlight in the water column (Salas et al. 2015). Autochthonous inputs in the water column contribute enhanced chlorophyll content in sediments. Depleted levels of pigments during monsoon might be due to the high flushing which leads to the accelerated removal of phytoplankton towards the coastal regions (Jyothibabu et al. 2006). Phaeophytin pigments are an early breakdown product of Chl a that degrade more slowly in the system. The estimated ratio of Chl a to phaeopigments in the study region was low (< 1) and varied from 0.53 to 0.65 (pre-monsoon) and 0.15 to 0.69 (monsoon). Lower values of the Chl a to phaeopigments ratio implied a higher sedimentation rate from the overlying water column or a relatively faster rate of detritus phytoplankton deposition in the system (Dell' Anno et al. 2002;Resmi 2015). The ratio of Chl a / (Chl a + Phaeopigments) varied from 0.34 to 0.39 (premonsoon) and 0.13 to 0.41 (monsoon).

Trophic status of the Kole wetland
Characterization of the trophic status of aquatic ecosystems is of utmost importance to understand the food web linkages, biogeochemical characteristics, and water quality of the study region. Benthic trophic status of the system was elucidated based on the threshold levels of PRT and CHO values according to Dell Anno et al. (2002) (Meso-oligotrophic: PRT < 1.5 mg/g; CHO < 5 mg/g; PRT/CHO < 1, Eutrophic: PRT = 1.5-4.0 mg/g; CHO = 5-7 mg/g; PRT/ CHO > 1, Hypertrophic: PRT = 4 mg/g; CHO = 7 mg/g; PRT/CHO > 1). Based on total PRT and CHO concentrations, the majority of the stations were classified as hypertrophic, while PRT/CHO ratios indicated meso-oligotrophic conditions in most of the stations during both seasons. A few stations (S 5 , S 9 , S 15 , and S 16 ) recorded transformation from meso-oligotrophic to hypertrophic/eutrophic conditions during monsoon due to terrestrial run-off and associated depositional changes.
Another benthic classification based on BPC and algal contributions to the BPC (Pusceddu et al. 2011). According to this concept, aquatic systems are classified as follows: Oligotrophic (BPC < 1 mgC/g, algal fraction > 25% of BPC),

Fig. 5
Spatial and seasonal variation pattern of pigments in the surface sediments of the study region mesotrophic (BPC = 1-3 mgC/g, algal fraction = 12-25% of BPC) and eutrophic (BPC > 3 mgC/g, algal fraction < 12% of BPC). Algal contribution to BPC was calculated as the % of Chl a to BPC concentrations, after converting Chl a concentration into carbon equivalents (Chl carbon) using a mean value of 40 (Pusceddu et al. 2009). In this study, biopolymeric carbon (Fig. 3f) varied from 18.73 to 310.68 (pre-monsoon) and 7.42 to 194.26 (monsoon).
The ratio of BPC to TOC has been regarded as an indicator of the quality of OM available to its consumers (Danovaro et al. 1997). Estimated BPC/TOC ratio (Fig. 4f) varied from 0.53 to 7.40 (pre-monsoon) and 0.42 to 18.84 (monsoon). Most of the sites exhibited low BPC/TOC ratio which confirmed that a large part of OM in those stations was refractory in nature. According to BPC and algal contribution to BPC, almost all the stations were classified as hypertrophic. Enriched protein levels, enhanced BPC content and low algal contribution to BPC indicated the probability of eutrophication. The abundance of degraded phytodetritus and the low algal contribution to the BPC suggested the persistence of refractory OM in the sediments. This may have relevant implications for heterotrophic nutrition in this system due to the dominance of low-quality food for benthic organisms (Venturini et al. 2012).

Statistical analysis
ANOVA revealed the absence of significant spatio-temporal variations among the biochemical components. Pearson correlation (Table 4) recorded remarkable positive correlation between fine sediment fractions (silt and clay) with various biochemical fractions (silt vs. PRT and TL, clay vs. LPD) and negative correlation between sand with various biochemical fractions (sand vs. PRT, LPD, TL) suggesting granulometric dependence. LPD exhibited highly significant negative correlation with pH (r = − 0.44) indicating mineralization process. Meanwhile, significant positive correlation of LPD with clay (r = 0.46), TOC (r = 0.66) and TON (r = 0.64) indicated granulometric influence. LOM displayed a highly prominent correlation with CHO (r = 0.99) and illustrates a major contribution of carbohydrates to OM in the wetland system.

Principal component analysis (PCA)
Quality and quantity of sediment OM in wetland systems are influenced by biogeochemical processes and characteristics of the depositional environment. Factor analysis was employed to assess the biogeochemical processes controlling the composition and distributional characteristics of OM in the system. Rotation method: Varimax with Kaiser normalization was applied to identify the variables which are significant to each other based on the significance of their correlations. Extracted components explained a total variance of 87.05% derived from PCA (Fig. 6). Factor 1 accounted for 44.11% of the total variance and exhibited strong positive loadings on silt, TOC, TOM, TON, PRT, LPD, TL as well as pigments (Chl a and Chl b ) pointing out the role of sediment grain size in the accumulation and preservation of biochemical components. Moreover, significant loadings of TL along with other biochemical components signaled OM contribution from terrestrial sources. Prominent positive correlations: TOC vs. silt, TON vs. silt, PRT vs. silt, TL vs. silt, LPD vs. clay and negative correlations: PRT vs. sand, LPD vs. sand and TL vs. sand (Table 4) also substantiate the role of sediment grain size in the distribution of various biochemical components. Positive loading of TON as well as TOC together with biochemical constituents and pigments pointed out accumulation of nitrogen rich OM via autochthonous production, terrestrial run-off and the agriculture activities. However, factor 2 explained 23.19% of the total variance and displayed positive loadings on TS, LPD, and pigments (Chl a , Chl b , Chl c and phaeophytin) suggested the influence of redox state of the depositional environment. The strong positive correlations among: TS vs. Chl b (r = 0.44), TS vs. Chl c (r = 0.57), indicated enhanced deposition of phyto pigments in the sediments under reducing conditions. Positive loadings on CHO, LOM and BPC were indicated by the third factor unraveled 19.75% of the total variance, suggesting a major contribution of CHO to LOM. The strong positive correlations among: CHO vs. LOM, CHO vs. BPC, reflected a major contribution of carbohydrates towards LOM.

Hierarchical cluster analysis (HCA)
HCA highlights a comprehensive understanding and relatable group interpretation of the 16 sampling points and geochemical variables in two seasons. Sampling sites in each cluster exhibited synonymous characteristic nature with respect to the general sedimentary variables and trophic status of the wetland. Based on their value of dissimilarity between observations, cluster analysis formulated a dendrogram using ward linkage (Fig. 7).
The dendrogram for pre-monsoon displayed two prominent clusters. First cluster comprises thirteen sampling stations viz. S 2 , S 4 , S 5 , S 6 , S 7 , S 8 , S 9 , S 10 , S 11 , S 12 , S 13 , S 14 , and S 15 with low sandy and high muddy substratum, majority of them located in the irrigation zone. These regions could be designated as mixed status of hypertrophic and mesooligotrophic areas. Second cluster includes three sampling stations (S 1 , S 3 , and S 16 ) with high sandy and low muddy substratum in the estuarine zone and could be designated as mixed status of hypertrophic, eutrophic and meso-oligotrophic areas.

Table 4
Pearson correlation matrix for the sedimentary parameters in the study region (n = 32) The bold values are useful for easy identification of significant correlations among variables Parameter The dendrogram for monsoon exhibited two remarkable clusters. First cluster constitutes six sampling sites viz. S 2 , S 4 , S 5 , S 7 S 8 , S 9 , S 10 , S 11 , S 12 , S 13 , S 14 , S 15 , and S 16 with intermediate sand and mud substratum, majority being in the irrigation zone. Meanwhile, the second cluster includes three sampling stations (S 1 , S 3 , and S 6 ) with high sandy and low muddy substratum located in the estuarine zone and could be designated a mixed status of hypertrophic, eutrophic and meso-oligotrophic areas.
Even though Kole wetlands are biodiverse, highly productive and provide numerous ecosystems services, the ecological health of these ecosystems has been deteriorating due to anthropogenic activities. A regular monitoring and assessment of the trophic state of the wetland systems through biogeochemical studies is essential for their conservation and sustainable management. This investigation can be considered as one of the pioneer research focusing on geochemical characterization of OM in Kole wetlands of Thrissur using biochemical composition. The study could distinguish the possible sources of OM, quality of OM, nutritional status and evaluated trophic state.

Conclusion
Texture analysis inferred the dominance of silt and clay fractions during pre-monsoon, while monsoon season exhibited the dominance of sand and silt due to run-off. Bulk sediment analysis revealed high OM content in sediments sourced to in situ production, terrestrial run-off and agriculture activities. C/N ratios indicated a mixed input of allochthonous and autochthonous OM during both seasons. Among the estimated biochemical parameters, carbohydrate was the most dominant component (both seasons), suggesting contributions from in situ production coupled with vascular plant debris through terrestrial run-off. Terrestrial vascular plant input to sediments was supported by elevated the tannin and lignin levels. Predominance of carbohydrates over proteins and lipids indicated the enhanced mineralization of proteinaceous OM.
Observed chlorophyll pigment concentration in sediments was as follows: Chl a > Chl b > Chl c (in both seasons). Lower values of Chl a /phaeopigments ratios (< 1) signaled the relative rate and recent phytoplankton deposition to the sediments. Strong correlations of silt, clay and TOC with biochemical composition indicated adsorption and biogeochemical process controlling the dynamics of OM. Interrelationships among the biochemical constituents indicate similarity in origin and distribution pattern. PCA unraveled the role of sediment grain size in accumulation and preservation of biochemical components in periodically changing reducing conditions. Based on total PRT and CHO concentrations, BPC and algal contribution to BPC categorized the study area as hypertrophic (during both seasons). Lower PRT/CHO (≤ 1) ratios observed at majority of sites during both seasons reflected the dominance of aged sedimentary OM and detrital heterotrophic conditions in the wetland. Meanwhile, depleted LPD/CHO ratio (≤ 1) suggested lower nutritional quality of sedimentary OM. Lower values of LOM /TOM and BPC/TOC ratio reflected the fact that a large portion of TOM represented refractory material and its reduced availability to benthic consumers. The Kole wetland system exhibited hypertrophic status which warrants a regular monitoring of the ecosystem health and the essentiality to implement remedial measures for conservation.