3.1. Macrophytes in the Oder/Szczecin Lagoon – the historic state
In the 1890’s, Brandt (1896) carried out a field survey and mapping of macrophytes in the eastern part of the lagoon, the Wielki Zalew. He reported bulrush (Juncus l.), Potamogeton species and other macrophytes down to a colonization depth of at least 2 m and mentioned a rich and diverse fauna in emerse macrophytes stands. Based on comments by Neuhaus (1933), data of Neubaur (1927) and Holtz (1892) and conclusions by Gosselck und Schabelon (2007) it can be assumed that charophytes were present in the 1890’s in different part of the lagoon, as well. Studies of Schubert et al. (2003) indicate that the following species were present in the lagoon a century ago: Chara contraria, Chara hispida, Chara tomentosa, Chara globularis, Nitellopsis obtuse, Potamogeton lucens and Ranunculus reptans.
Figure 2 extrapolates the field data to the entire lagoon assuming a maximum colonization depth of 2.5 m and that no gradients between different parts of the lagoon exist. This colonization depths shows the best agreement with the map of Brandt (1896). We consider the resulting 36% macrophyte coverage as the likely maximum historical coverage with macrophytes and as reference for the WFD (40% of the Wielki Zalew and 32% of the Kleines Haff). In comparison, assuming a maximum colonization depth of 2 m would result in a total macrophyte covered area of 27% of the total lagoon surface area. It is likely that the existing gradients in water transparency between both parts of the lagoon (Friedland et al. 2019) existed a century ago, as well. This means that the past spatial macrophyte coverage in the Kleines Haff (Fig. 2) is possibly overestimated, but data that would allow an estimation of the maximum colonization depths 130 years ago is lacking. Transferring the present relative transparency gradient to the past would result in past maximum macrophyte coverage in the Kleines Haff of only about 20%. These facts suggest that the lagoon was never a macrophyte dominated, clear water system. However, it does not mean that macrophytes do not play an important role in the lagoon’s ecology. Further, ongoing sea-level rise increases the colonization area for macrophytes, especially for reed and bulrush, and may increase their importance.
3.2. Present state of macrophyte coverage and composition
The results combine own data on spatial macrophyte coverages and colonization depths, with a literature analysis and transect data obtained from WFD monitoring. Focus is on the Kleines Haff. Reed (Phragmites australis) and bulrush (Schoenoplectus lacustris), littoral helophytes, are the dominant species and are abundant at the entire lagoon coastline. During the sampling campaign in 2016, reed was observed down to a water depth of 1.5 m and bulrush down to 2.6 m. These emerse macrophytes compete with submerged vegetation for space. The reed belts in the lagoon are dense. Three metres inside the reed belt (from the sea front) near the town Bellin, an average number of up to 312 reed stems/ m2 with an average diameter of 7 mm was counted.
Only in sheltered areas of the Kleines Haff, submerse macrophytes are abundant and diverse. In front of emerse macrophyte belts and in shallow exposed areas the coverage is patchy with low densities (Gosselck and Schabelon 2007; Dumke 2001). Species and their share are compiled in Table 1. In the Kleines Haff, Potamogeton species are most abundant and cover plots of 5–50 m² (Gosselck and Schabelon 2007) followed by Ceratophyllum demersum. The recent monitoring shows a significant coverage with Myriophyllum spicatum, as well.
Table 1 Compilation of data on submerse macrophyte species and their maximum observed colonization depth in Kleines Haff based monitoring data and complementing literature (Selig et al., 2006, Gosselck and Schabelon 2007; Porsche et al. 2008; Schadach 2013). The shares are calculated based on the number of individuals (total = 1920) found on all transects. Potamogeton pectinatus = Stuckenia pectinata.
A historical data compilation covering the last two centuries (Schubert et al. 2014) documents the presence of seaweed (Zostera marina and Zostera noltei) in most of the south-western Baltic coastal waters. The data does not indicate the presence of seagrass in any part of the Oder Lagoon, because its low salinity. Neubauer (1927) reports a dominance of charophytes in parts of the northern Wielki Zalew. Still in the 1960s, Garbacik-Wesolowska, 1969, 1973 in Wolnomiejski and Witek 2013) mentions an area of 65 ha covered by charophytes in the Wielki Zalew and a 15.5% total macrophyte coverage of the Wielki Zalew. Until 2013, data does not prove the presence of charophytes in the Kleines Haff.
The most recent monitoring of 2015, 2018 and 2021, in the Kleines Haff reports 25 species for the Kleines Haff. The species spectrum includes the Charophytes Chara aspera and Chara baltica, which are found only sporadically, and the spermatophytes Ceratophyllum demersum, Elodea nuttallii, Myriophyllum spicatum, Phragmites australis, Potamogeton crispus, Potamogeton friesii, Potamogeton pectinatus, Potamogeton perfoliatus and, locally even, Zostera marina.
Based on the PHYtoBenthic Index used within the WFD assessment, the present state of macrophytes in the Oder Lagoon is classified as non-satisfactory. Reasons are a relatively poor species composition and the lack of ecologically valuable species. The recent local observation of charophytes alone can hardly be interpreted as an improved ecological state of the lagoon. However, the data at least suggests a tendency towards an improvement.
Another important aspect that negatively affects the state assessment is the insufficient spatial coverage of macrophytes compared to the potential area at the present Secchi depth of 0.6 m (Fig. 3). The reference value for the lower distribution limit of submerged macrophytes in the Kleines Haff is 3.0 m, according to the WFD assessment. The colonization depth for an excellent state is ≥ 2.7 m and for the good state between 2.4 m and 2.7 m, based on calculations by Domin et al. (2004). On average over the years 2015 and 2021 and over all transects, the present lower colonization depth is only 1.2 m and far below the threshold for a good status. Only very locally, on one transect at the northern coast near Gummlin, a colonization depth between 1.9 m and 2.2 m was recorded.
Assuming that water depths down to 1.5 m potentially could be fully covered by macrophytes would result in an area of about 13% of the total areas of the Kleines Haff. Our survey data complemented with exiting WFD transect sampling data suggests a very patchy distribution and a real coverage close to 5% of the Kleines Haff surface area.
The loss of macrophytes in Baltic inner coastal waters is commonly regarded as indirect effect of eutrophication (Schiewer and Glocke 1996). During the last century nutrient loads to the lagoon increased. In the early 1970’s, this increase intensified and caused strong eutrophication with increased phytoplankton concentrations, increased resuspendable organic material and subsequently declining light conditions. However, the limited macrophyte coverage 130 years ago suggests that macrophytes were either lost due to earlier eutrophication or as a result of long-lasting human impact.
3.3. Macrophyte coverage in a potential good water quality state
The question is how large would macrophytes covered areas be, compared to the situation today and in the past, assuming that a good water transparency in the lagoon exists? The present water transparency threshold according the WFD is a Secchi depth of 1.7 m in the Kleines Haff (Sagert et al. 2008). Based on model simulations, Friedland et al. (2019) suggest 1.97 m for the Kleines Haff and 2.87 m Secchi depth for the Wielki Zalew. For a Secchi depth of 1.7 m, Middelboe and Markager (1997) provide a colonization depth for charophytes of 2.19 m and for angiosperms of 1.99 m for many Danish aquatic systems that are comparable to the Oder Lagoon. Comparing the thresholds for a good water transparency and the threshold for a good macrophyte colonization depth show an existing mismatch that requires a harmonization. It is likely that a good water transparency status of 1.7 m Secchi depth would not allow a colonization depth of above 2.4 m.
Reference state for the lagoon according to the WFD is a dominance of charophytes (Schubert et al. 2003; Selig et al. 2006). Therefore, charophytes and angiosperms represent the ecologically preferred target groups describing the good ecological status. As a consequence, we focus on the potential spatial coverage of these groups. The potential areas covered by angiosperms and charophytes are shown in Fig. 4. At least 27% of the Kleines Haff areas would be covered by macrophytes in a good ecological status. Taking into account gradients between the two parts of the lagoons, with a higher transparency in the Wielki Zalew, this could result in a total macrophyte coverage of about 35%. This coverage is very close to our historic maximum coverage. Therefore, a Secchi depth of 1.7 m for the Kleines Haff represents a situation before the 1890’s and seems to be a too ambitious threshold for a good ecological status.
The Secchi depths for a good ecological status suggested by Friedland et al. (2019) is even much larger and would results a macrophyte coverage in above 50% of the lagoon. The same is true for the exiting target (> 2.4 m) describing a good ecological state according to the WFD. This is far beyond what we consider as maximum possible historic coverage of 36% and does not seem realistic, not even as reference state according to the WFD.
However, all these macrophyte coverage calculations are theoretical. It is well known that the distribution of macrophytes is not only controlled by light availability. The sediment plays an important role. Macrophytes usually prefer consolidated, stable sediments and are not able to settle on fine, muddy sediments. The sediment map (Fig. 4) indicates that sandy sediments prevail near-shore and macrophyte growth in the lagoon is hardly restricted by unsuitable bottom conditions. Other important factors are exposition to wind, waves and currents (Scheffer 1998; Yousef 1999; Schneider 2004). Since the lagoon is west-east oriented, it is exposed to the dominating westerly winds and frequent storms. Resulting waves, strong currents and high critical shear stress at the bottom restrict the macrophyte distribution in reality, as well.
3.4. Effects of macrophytes on water quality
Guiding for this sub-chapter is one question: How relevant are macrophytes for the water quality in the lagoon? As mentioned before, the effects of macrophytes on aquatic ecosystems and especially water quality are well known and well documented (e.g. Scheffer 1998; Horppila and Nurminen 2003; Hilt et al. 2006; Blindow et al. 2014). Can macrophytes affect water quality in the entire lagoon, can changes in macrophyte coverage explain changes in water quality and have macrophytes to be taken into account when defining water quality thresholds according to the WFD?
The model suggests that a macrophyte colonization depth of 2 m water depth would reduce the concentration of organic matter in the water column in a narrow near coast strip by more than 50% (Fig. 5a). Especially sheltered shallow systems such as Lake Neuwarp and Lake Usedom are strongly affected. Macrophytes would affect even central parts of the lagoon by reducing organic matter concentration by 10%-20%. Changes in zooplankton grazing pressure (Fig. 5b) are restricted to near shore areas and hardly affect central parts of the lagoon. Shading by macrophytes is limited to the coastal macrophyte covered areas (Fig. 5c). The cumulative effect of all changes resulting from increased macrophyte coverage on the phytoplankton concentration in the lagoon, expressed in terms of chl.a, is shown in Fig. 5d. Sheltered and semi-closed areas would face a chl.a reduction of about 10% and offshore areas of about 3%. Central parts of the lagoon are even less affected. This is especially true for the Kleines Haff. Altogether, macrophytes have effects on nearshore water quality, while open parts of the lagoon are not much affected. We cannot expect that changes in macrophyte coverage during the last 140 years affected water quality parameters in the central parts of the lagoon significantly.
Since water quality thresholds are determined based on data from central lagoon stations, an effect cannot be expected. The existing thresholds can be regarded as reliable. Another question is whether data from the central lagoon is really representative for the state of lagoon. The introduction of additional near shore stations would certainly provide a more complete picture of the state of the lagoon and is therefore recommendable.
Figure 6 summarizes the effects of macrophytes on chl.a concentrations integrated over the areas of the two parts of the lagoon. Reduced resuspension increases the light availability in the water body and favours phytoplankton while the other macrophyte effects, e.g. shading or increased zooplankton concentrations, hamper phytoplankton growth (Fig. 6a, b). The combination of all effects result in a chl.a reduction of 5% in the Wielki Zalew and below 2% in the Kleines Haff. Assuming the much lower historic loads of 1880 in the model simulations result in a chl.a reduction of below 4% in the Wielki Zalew and below 1% in the Kleines Haff. The lower the loads, the lower are the effects of macrophytes on water quality. It becomes obvious, that the Wielki Zalew is and always was much more affected by macrophytes and changes in coverages than the Kleines Haff. Model results suggest that the effects of macrophytes on water quality in the entire Oder Lagoon is and always was very limited. For model based assessments within the WFD, such as the lagoon’s behaviour on nutrient load increases and reductions, macrophytes can be neglected. The benefit of introducing state variables describing macrophytes in the model does not justify the effort and is not recommendable for the Oder Lagoon. In other smaller or shallower coastal waters this will certainly be different. A consequence is that the analysis of long-term changes and management perspectives for the lagoon can neglect macrophytes and focus on fundamental relationships between external loads and lagoon water quality. This is in agreement with Blindow and Meyer (2015) who mention a macrophyte containing volume of 15–20% as prerequisite for strong controlling effects in shallow lakes. Assuming the maximum colonization depth of 2.5 m in the Oder Lagoon, the macrophyte containing volume would be close to 10% and assuming a colonization depth of 2 m the volume would be reduced to only 6–7%.
3.5. Relationships between eutrophication controlling factors
Guiding question is whether eutrophication in the lagoon already took place centuries ago or if the lagoon is even a naturally eutrophied system. The latter would explain the relatively low coverage with macrophytes centuries ago. The old comprehensive OECD study of world-wide lakes by Vollenweider (1976) and later up-dates by Jones and Lee (1986) can give an insight into major relationships between nutrient loads and basic water quality parameters. Lee and Jones (1981) confirm the transferability of the relationships to estuaries and Reynolds (1992) introduce light as limiting factor. This allows answering the additional question, whether the lagoon can be regarded as a system with a behaviour that is typical for lagoons and lakes.
Figure 7a shows that both parts of the lagoon have and had for the last 30 years a molar N/P close to 7/1 (expressed by weight). This indicates that P is not the limiting element for primary production in the lagoon, but that N may play an important role in controlling primary productivity. However, in comparison to the OECD lakes, the lagoon shows high concentrations for both nutrients in the water. The relationship between P and chl.a can be regarded as typical, as well (Fig. 7b). Here too, the concentrations for both parameters are very high when compared to the OECD lakes. This is true for the situation today and 30 years ago. Water transparency in both parts of the lagoon is and was very low compared to the investigated OECD lakes. In the Wielki Zalew, the relationship between water transparency and average chl.a-concentrations is comparable to the lakes (Fig. 3c). In contrast, the Kleines Haff shows a relatively low transparency at the given chl.a-concentration. Due to its shallowness, and longer retention time, sediment resuspension is more prominent in the Kleines Haff. The important role of sediment resuspension on water transparency is confirmed by frequent Secchi depths below 1 m even during winter seasons.
The very high nutrient concentrations, the tendency that N is the element with the shorter availability and the low water transparency indicates that light is the limiting factor for primary production in both parts of the lagoon, but that light limitation in the Kleines Haff is even stronger. The lagoon shows a situation beyond P-limitation as described by Reynolds (1991).
The OECD study of world-wide lakes by Vollenweider (1976) provides a relationship between external P-loads and the sensitiveness of a lake towards eutrophication. The shallower a system and the higher the water residence time, the higher the sensitiveness towards eutrophication and the lower the acceptable external P-load (Fig. 7d). The P-loads to the entire Oder Lagoon and to each part of the lagoon is today and was 40 years ago above the acceptable loads for a non-eutrophied system. Compared to the Wielki Zalew, the higher water residence time and the slightly lower average depths makes the Kleines Haff more sensitive towards eutrophication.
The main source of external P is the Oder river, draining into the Wielki Zalew. As a consequence, the Kleines Haff receives significantly less external P compared to the Wielki Zalew. In both parts of the lagoon, the P-loads are far above the acceptable level and keep the system in a eutrophic state. Even if we assume that the maximum allowable P-input (MAI), required for a good ecological status of the Baltic Sea according to Helcom, would be reached in future this would not cause a change in the lagoon. The MAI is still far above the critical load and would keep all parts of the lagoon in a eutrophic state. The historic P-loads reflect the situation around the 1880’s, about 140 years ago (Gadegast et al. 2012; Hirt et al. 2014; Gadegast and Venohr 2015). At that time, we can assume emissions into surface waters of the Oder catchment below 6 kg N/ha and around 0.1 kg P/ha. Very likely, the loads around the 1880’s were not significantly higher compared to earlier centuries. As a consequence, we can assume that the P-loads were above the critical level and kept the lagoon in a eutrophic state already for centuries. This allows to address the lagoon as a naturally eutrophied system, with limited submerse macrophyte coverage.
3.6. Long-term development of water quality
For the last decades, the question how water quality in the lagoon is and was affected by external nutrient load reductions, can be assessed in more detail. The external nutrient loads had a maximum in the 1980’s of 115,000 t TN (10,500 t TP) and strongly declined to 56,750 t TN (2,800 t TP) in 2010–2014 (Friedland et al. 2019). Figure 8 compiles all existing data for N, P and chl.a from the central stations in Kleines Haff and Wielki Zalew. The dissolved inorganic N concentrations in the Wielki Zalew partly correspond to the Oder/Odra river nitrogen loads. For example, the flood year 2010 discharged about 90,000 t N to the lagoon and increased the DIN concentration to about 140 µmol/l. However, the strong variability between the years cannot be explained by external annual N loads (Fig. 8c). The N concentrations in the Kleines Haff show a less strong inter-annual variability and do not follow the pattern in the Wielki Zalew (Fig. 8a).
The annual N and P concentrations in both parts of the lagoon do not show a systematic relationship to each other and the P concentrations in the lagoon are not clearly related to external P loads. Altogether the inter-annual variability of P-concentrations in Kleines Haff is stronger compared to Wielki Zalew. Schernewski et al. (2011) suggest that P-peaks in 1989 are resulting from temporal hypoxia and the release of iron-bound P from the sediments. This could be an explanation for the P-peak in the hot year 2003, as well. Bachor (2005) estimated an N content of 14,200 t and a P content of 2,400 t in the upper sediment layer (0–5 cm) for the Kleines Haff. In Wielki Zalew, P release from the sediment under temporary hypoxic conditions might explain the P peak in 2003, as well. This anoxic P-release is a process often observed in shallow aquatic systems (Boström and Pettersson 1982; Jensen and Andersen 1992). The Oder/Odra river influence, shipping induced turbulence and a higher water exchange with the Baltic Sea are reasonable explanations that internal eutrophication is less obvious in the Wielki Zalew data.
The chl.a-concentrations show a strong inter-annual variability in both parts of the lagoon. Especially in the Kleines Haff, the data suggests an opposite behaviour of N and chl.a-concentrations and in the last decade, the N concentrations in both parts of the lagoon are in some years close to zero.
The aggregated annual data is not suitable to analyse processes in detail. For the Wielki Zalew, a higher temporal resolution of the data could possibly prove a relationship between especially external N loads and concentrations in the lagoon water. However, what we can conclude is that the Kleines Haff and the Wielki Zalew behave differently. While the first seems strongly influenced by internal lagoon processes, the latter is much stronger driven by external Oder/Odra river loads.
The smoothened data of the last 30–40 years for both parts of the lagoon indicate a strong decline of nutrient concentrations in the water that reflect the decline in external nutrient loads (Fig. 8b, d). In Wielki Zalew, a slight decline of chl.a is visible during the last 30 years, while chl.a-concentrations in Kleines Haff remain stable.
Most important for macrophytes are changes in water transparency. From the 1990’s, summerly water transparency has slightly increased in the Kleines Haff from 0.5m to 0.6 m and in Wielki Zalew from 0.85 m to 1.1 m (Fig. 9). However, the reliability of these trends is limited by the strong data variability and the non-homogeneous water transparency developments during other seasons. Reasons for different water transparencies between the two parts of the lagoon could be the Oder/Odra river water, which has a higher transparency, the lower water depth of the Kleines Haff, that favours sediment resuspension, and the availability of resuspendable organic material. The artificially deepened and regularly dredged shipping channel in the Wieki Zalew additionally serves as trap for organic matter (Minning 2004) and in a longer perspective reduces the amount of available organic material. On the other hand, ship induced turbulence may even increase resuspension, at least locally.
In late winter and autumn the chl.a concentrations in the Kleines Haff seem to have increased during the last three decades. This could result from a climate warming. Higher temperatures in autumn and in winter, with less ice coverage, potentially enable a higher primary production during these seasons.
Obviously, the Kleines Haff is still light limited and changes in nutrient concentrations do not affect primary productions. Wielki Zalew shows a tendency to shift from a light limited towards a N controlled system. However, the chl.a-concentrations are still very high and one can hardly speak of a lasting N limitation. Shallow, turbid systems, such as the lagoon, enable a fast cycling of nutrients within days. Further, cyanobacteria are dominating in summer and have the potential to make atmospheric N accessible. A prove of N-fixation by cyanobacteria, that would indicate a real N shortage, is still lacking for the Szczecin Lagoon. This is very different in comparable lagoons, such as the Curonian Lagoon (Zilius et al. 21).