Relative importance of trophic interactions and nutrient enrichment in seagrass ecosystems: A broad‐scale field experiment in the Baltic−Skagerrak area

Abstract

The interaction of eutrophication and predation in structuring seagrass Zostera marina L. ecosystems was assessed in a field experiment in three regions along an estuarine salinity gradient, from southern Finland to the Skagerrak area of the Swedish west coast. All regions are considered to be affected by eutrophication and overfishing but differ in the abundance of intermediate predators (e.g., small fish, shrimp, and crabs), mesograzers, and the biomass of epiphytic algae. Using transplanted Zostera (eelgrass), nutrient levels and intermediate predator abundance were manipulated in a full-factorial cage experiment. On the Swedish west coast, where ambient densities of mesograzers are very low, epiphytic algae responded strongly to nutrient enrichment, resulting in significantly reduced growth of eelgrass. At the Baltic sites however, where ambient densities of mesograzers are high, no significant growth of epiphytic algae was detected, and only grazer biomass responded to nutrient enrichment. Predation from small fish, shrimp, and crabs decreased the biomass of mesograzers by . 98% on the Swedish west coast, but natural predators had no significant effect on mesograzers biomass at the Baltic sites. Predation and nutrient enrichment interacted to affect the growth of eelgrass by controlling the biomass of mesograzers and nuisance algae. The differing effect of nutrient enrichment and grazing in the three regions may therefore be a result of the prevailing low and high predation pressure on mesograzers in Zostera. This absence or presence of predation may derive from interregional changes in trophic interactions, possibly caused by a combination of eutrophication and overfishing.

Effects of Altered Offshore Food Webs on Coastal Ecosystems Emphasize the Need for Cross-Ecosystem Management

“…3; Moksnes et al 2008;Baden et al 2010). Field experiments using cages show that predation by local mesopredators decrease the biomass of potential mesograzers by more than 95% in this system Baden et al 2010). Cage experiments also demonstrate that the black goby indirectly increases the biomass accumulation of ephemeral algae in seagrass patches up to five times by controlling the most efficient grazers: adult ([9 mm) individuals of the amphipod Gammarus locusta .…”

“…generated by a combination of increased nutrient supply (via coastal eutrophication) and low grazing pressure, mediated by high predation pressure on functionally important grazers from high densities of mesopredators ( Fig. 3; Moksnes et al 2008;Baden et al 2010). Field experiments using cages show that predation by local mesopredators decrease the biomass of potential mesograzers by more than 95% in this system Baden et al 2010).…”

“…Cage experiments also demonstrate that the black goby indirectly increases the biomass accumulation of ephemeral algae in seagrass patches up to five times by controlling the most efficient grazers: adult ([9 mm) individuals of the amphipod Gammarus locusta . Today, gammarid and isopod mesograsers occur in very low abundances in eelgrass beds, where they were abundant in the 1980s (Jephson et al 2008;Moksnes et al 2008;Baden et al 2010). Seagrass beds decrease turbidity by stabilizing sediments, and act as nursery ground for a number of commercially important fishes in the area (such as cod; Pihl et al 2006).…”

Consumers mediate the effects of experimental ocean acidification and warming on primary producers

“…Z. marina is a clonal angiosperm that forms large underwater meadows, typically at depths of 2-4 m on the Swedish western coast (50). Seagrass meadows support a wide array of trophic interactions between herbivores feeding on epiphytic and free-floating macroalgae, but also function as nursery and feeding grounds for ecologically and commercially important fish (40,51).…”

Essential fish habitats (EFH)

“…In areas where populations of coastal predatory fish such as perch (Perca fluviatilis) and pike (Esox lucius) are weak, mesopredatory fish, such as the three-spined stickleback (Gasterosteus aculeatus), are present in high numbers (Eriksson et al 2009, Byström et al 2015. These mesopredators can have substantial impacts on the community of invertebrate grazers, reducing their numbers and hence the grazing pressure on algae, ultimately leading to eutrophication symptoms and habitat changes with blooms of ephemeral filamentous algae (Korpinen and Jormalainen 2007, Baden et al 2010, Sieben et al 2011, Östman et al 2016. Sticklebacks may also affect the egg and larval stages of many species of predatory fish negatively, further emphasizing this problem Byström et al 2015).…”