Grazing pressure and algal biomass dynamics in independent contamination
The highest D. magna abundance was realised after a short time of exposure to high concentration of nutrients, and coincident with reduced algal biomass. High phosphorus stimulates phytoplankton production (Muylaert et al., 2010), as also demonstrated in this study by the positive correlation of algal biomass with nutrient availability(Gusha et al., 2019; Kim et al., 2016; Stevenson et al., 2006). However, in this study, longer exposure period in higher nutrients concentrations resulted in reduced algal biomass, logically explained by the increase in D. magna abundance, and commensurate with increased grazing pressure.
Grazing pressure on phytoplankton varies with zooplankton species composition (Kagami et al., 2002). In our experiments reduced concentrations of nutrients delayed attainment of lowest algal biomass likely because of reduced grazing pressure, while grazer densities responded positively to nutrient enrichment (Roll et al., 2005). Furthermore, as phosphorus is a limiting factor to both phytoplankton and Daphnia populations, high nitrogen to phosphorus ratio in algal diets may limit the growth of grazing zooplankton (Guo et al., 2019). This can explain the reduction of D. magna abundance after longer exposure times at higher N:P ratios.
Pesticide additives reduced the abundance of D. magna over periods of exposure. Increased pesticides concentrations poisons and reduces zooplankton abundance, and therefore the potential grazing pressure on the algae. However, over time, the dynamics of the algal population is limited by the nutrients, thereby reducing the algal biomass. Bengtsson et al. (2004) recorded significant reduction in the grazing rate of D. pulex with exposure to dichlorordiphenyldichloroethylene (DDE) insecticide and enhanced growth of the algae in response to phosphorus and nitrogen glycophosate excreted by the death of the zooplankton Roessink et al. (2008). The pesticide toxicity on D. magna results in higher algal biomass.
Aquatic autotrophic processes in independent contamination
Autotrophic primary production and heterotrophic respiration are influenced by the availability of nutrients (Dodds & Smith, 2016). Within the first 24 hours of exposure, the study resulted in a large increase in algal biomass. Simultaneously, increases in algal productivity along with D. magna abundance, results in high respiration rates, and reduced dissolved oxygen concentrations, and can have a negative effect on grazer abundance(Munn et al., 2010).
Nutrient enhancement led to higher algal biomass in the nutrients-only treatment compared with the control, and the pesticides-only and combined treatments. However, further increase in concentration within the nutrients-only treatment resulted in decreases in algal biomass (Dodds & Smith, 2016; Rabalais, 2009). That the nutrients-only treatment had lower D. magna abundance in comparison with the control, likely reflects the effect of very high algal biomass on inhibiting D. magna grazing through clogging of the filtering apparatus or negative reaction of the Daphnia through possible algal toxicity (Boudry et al., 2020; Sarnelle et al., 2010). The net result was lower food availability for the grazers with increased trophic state of the water (Chislock et al., 2013; Hiltunen et al., 2021; Pinto-Coelho et al., 2006; Rabalais, 2009).
In aquatic ecosystems, availability of nutrients, especially nitrogen and phosphorus, favour primary productivity (FAO, 1996), but high primary producer densities enhance the respiratory demand on aquatic ecosystems limiting primary productivity (Chislock et al., 2013; Koelmans et al., 2001). The pesticides-only treatment showed eutrophication-like effects(Roessink et al., 2008) through poisoning of the grazers and reducing the grazing pressure and increasing algal biomass. This was demonstrated by the increase in algal biomass with increasing concentration in the pesticides-only treatment (Camargo & Alonso, 2006; O´Toole & Irvine, 2006). The high D. magna abundance in the low concentrations of the pesticide-only treatment compared with the nutrients-only and the combined treatment is attributable to pesticides poisoning as a disturbance, triggering proliferation (Hose & Guillette, 1995). However, the higher the concentrations, the lower the abundance, suggesting that the disturbance from poisoning is increased, and the ability of D. magna to recover diminished as a result of increased poisoning (Czub & McLachlan, 2004).
Reducing diversity of flora and fauna in combined contamination
Hegde et al.(2014) established low zooplankton diversity and density when there is a combination of pesticides with fertilisers. The argument being that pesticides cause selective toxicity in algae which reduces invertebrate feeding. Traas et al.(2004) on the other hand, using ecotoxicology models, indicated that nutrient additions alone caused little effects on the fate of the toxicant and the ecological effects were due to the relatively high rate at which pesticides are distributed in the environment. Pesticides can reduce trophic transfer of energy to grazers and predators(Hanazato, 2001) resulting to lower abundance in the higher trophic levels. In contrast, Baker et al.(2016) reported that combination of nutrients and herbicides increased abundance of zooplankton, indicating the possibility of differences between model predictions and field experiments as part of risk assessment in ecotoxicology. In our experiment, a combination of nutrients and pesticides increased algal biomass compared with the control, probably due to the decrease in the D. magna.
Interactions in combined contamination
The combined nutrient-pesticide residue treatment had lower algal biomass compared with the nutrient-only or pesticides-only treatment. This supports the arguments by Kortenkamp et al. (2009) of nutrients-pesticides interaction in aquatic ecosystems. This trend was similar to the pesticides-only treatment; therefore, the effects of combined nutrients and pesticides are attributable more to pesticides, rather than nutrients, in the aquatic ecosystem. Although some studies (SCHER et al., 2012) argue that such interactions are either synergistic or antagonistic, this study concluded that the concentrations of the combined contaminants determine the type of interaction between nutrients and pesticides. Compared with the pesticides-only treatment, an antagonistic interaction was evident at lower concentrations, while synergism developed in higher concentrations. The antagonism at lower concentrations can be attributed to biomass dilution, where nutrients enrichment accelerates the growth of algal biomass which take up or adsorb the pesticides, reducing the expected effect (Skei, 2000). With increasing combined concentration however, eutrophication as a result of the higher nutrients concentrations, and eutrophication-like characteristics mediated by pesticides (Roessink et al., 2008), results in nutrient enrichment reducing algal biomass. Compared with the nutrients-only treatment, the algal biomass was lower in combined treatment irrespective of the concentration. The combined nutrients and pesticides show a synergistic interaction with respect to nutrients-only contamination. While an increase in nutrients concentrations results in a reduction of algal biomass, an increase in combined concentration results in an increase. This shows that combined contamination has a higher effect to algal biomass than nutrients-only treatment and determining nutrients-only would be an under estimation of the effects on algal biomass in an agricultural catchment (Koelmans et al., 2001).
In the combined nutrients and pesticides treatment, the abundance of D. magna was lower compared with the nutrients-only and the pesticides-only treatments. As the combined concentration increased, there was a reduction in the abundance of D. magna. The higher combined concentration resulted in lower food quality and availability for D. magna (Roessink et al., 2008). Coupled with D. magna poisoning, poor food quality and availability results in the low abundance in the combined nutrients and pesticides treatment, compared with the pesticide-only and nutrient-only treatments (Koelmans et al., 2001). Essentially, combined contamination results in a synergistic effect on D. magna abundance compared with the nutrients-only or pesticides-only contamination. As such, effects on D. magna abundance focusing on either pesticides or nutrients underestimate the potential effects to the ecosystem.