Final Report Summary - SEAGRASSTIME (Trophic Cascades in Marine Ecosystems) The role of herbivory and hydrodynamics in ameliorating the effects of nutrient enrichment in marine ecosystem Eutrophication is one of the most serious impacts on coastal ecosystems worldwide. It causes a startlingly high loss of seagrass habitats, currently in the order of 110 km2 each year. The implications of this loss are profound given the range of ecosystem services that seagrass meadows provide, including their importance for coastal fisheries. Recent work has identified that among the factors influencing ecosystem susceptibility to nutrient enrichment, is the diversity and abundance of herbivores. Small benthic herbivores such as amphipods and gastropods feed preferentially on epiphytic or benthic algae, facilitating seagrass growth by releasing it from algal competition. Despite the clear indicators that small grazers can control algae accumulation and facilitate seagrass dominance, there is only very limited experimental evidence. A few studies suggests that leaf movement induced by hydrodynamic forces can also ameliorate the effects of eutrophication, directly by controlling the biomass of algae that respond to nutrient availability, and indirectly by controlling the abundance and distribution of mesograzers. Britta Munkes has been funded through the Marie-Curie international outgoing fellowships (MC IOF) to investigate factors, which may mitigate against harmful effects of nutrient pollution in seagrass ecosystems. Through a combination of field experiments, monitoring and laboratory wave tank experiments this research is disentangling how these different processes interact to control the effects of eutrophication in seagrass systems. The research was the first to test the interactive effects of nutrient additions, grazer's abundance and exposure, on seagrass epiphyte loads and seagrass condition. The key findings and their significance were: Nutrient control of epiphyte biomass Nutrient enrichment results in increased epiphytic algal biomass (particularly filamentous brown algae) and a reduction in seagrass biomass. At the loading rate used (50 mmol m-2 d-1 of N and 14 mmol m-2 d-1 of P) it is feasible to expect medium to longer-term reduction in P. australis condition. While not unexpected, this result confirms that fertilisation, at the loading rate used, does not promote seagrass growth, an argument sometimes put forward to suggest that nutrient enrichment may be beneficial for seagrass productivity. Brown algae were the most responsive to nutrient enrichment, with green and red algae showing no significant response, generally consistent with observations for Cockburn sound during the 1970s. This suggests that brown algae may be useful bioindicators of nutrient enrichment while green algae may be less useful, despite a commonly held view that green filamentous algae are strong indicators of N-pollution. Grazing control of epiphyte biomass The presence of amphipod grazers significantly offsets the effect of nutrient additions in promoting algal epiphyte biomass. The removal of amphipods resulted in a 36-50 % increase in algal biomass, depending on the nutrient loading. It is estimated that without this amphipod grazing effect, up to 25 % less light would penetration to the surface of a seagrass leaf. As a reference point, a 12 % reduction in light result in a 15 % reduction in seagrass shoot density, suggesting that the effect of amphipods in reducing epiphytic biomass is likely to have significant benefits for seagrass health. Grazer community affords a level of resilience to seagrass ecosystems ecosystems; therefore it is critical to consider any activity that will negatively impact on the grazer communities. Hydrodynamic control of epiphyte biomass While nutrient additions always resulted in higher algal epiphyte biomass, the degree of exposure at a site had at least an equally significant effect on epiphyte biomass. Under hydrodynamic conditions with high level of wave action (in summer) the abrasive effect of wave movement on leaf epiphytes let to a decrease in epiphyte biomass by up to 31 %. While in autumn, under light wind conditions, the resulting small waves seemed to promote growth of epiphytic algae in comparison to the absolute sheltered sites. Seagrass sites experiencing greater wave action are likely to be more resilient to the negative effects of nutrient enrichment through, at least, the abrasive effect of wave movement on leaf epiphytes. While it is not yet possible to establish threshold levels for these effects of exposure, it is clear that the level of oscillatory water movement and the induced leaf movement need to be considered when predicting the effect of nutrient enrichment on a seagrass meadow. Comparison between different ecosystems Baltic sea Australia In contrast to our previous experiments in western Australia, no significant effects of nutrient enrichment on seagrass biomass or epiphytic algal biomass and Chl a concentration, nor significant differences between grazer treatments were found. Overall epiphytic algal biomass was very low, which might have prevented any strong grazing effect and any significant differences between grazer treatments. Epiphytic algae on Zostera marina leaves in the study area are dominated by diatoms and to a much lesser extent by filamentous algae. This might be one reason for the different response to nutrient enrichment in the Baltic sea in comparison to western Australia. In western Australia increase in epiphytic biomass was particularly due to increase of filamentous algae. Another study of the Baltic sea, at the Swedish coast, also showed that nutrient enrichment let to an increase predominantly in biomass of filamentous algae. One other reason might be the time of the year. In August, epiphytic biomass concentrations are often very low on seagrass plants in the studied area. The period, to run the experiment was restricted due to project limitations. To confirm the outcome of this experiment, it would be necessary to run the experiment in another time of year, preferable in spring, where a higher growth rate of epiphytic algae can be expected. Conclusion: These studies have shown that other factors, specifically grazer abundance and hydrodynamic conditions, can significantly mitigate the effect of nutrient enrichment and are important factors to be considered in management plans and ecosystem models. A depletion of the natural grazing community will make seagrass ecosystems significantly more susceptible to nutrient additions. Conversely, the presence of a healthy grazer assemblage may increase the capacity of seagrass meadows to tolerate nutrient additions. Similarly, seagrass sites subject to high levels of exposure and wave action are more likely to show reduced detrimental effects of nutrient additions than calm sites. Current ecological models generally fail to consider these factors, particularly hydrodynamics, but should do so if they are intended to reflect key drivers of algal biomass and other primary producers.
