Role of food web interactions in promoting resilience to nutrient enrichment in a brackish water eelgrass (Zostera marina) ecosystem

Abstract

Understanding the ecological interactions that enhance the resilience of threatened ecosystems is essential in assuring their conservation and restoration. Top-down trophic interactions can increase resilience to bottom-up nutrient enrichment, however, as many seagrass ecosystems are threatened by both eutrophication and trophic modifications, understanding how these processes interact is important. Using a combination of approaches, we explored how bottom-up and top-down processes, acting individually or in conjunction, can affect eelgrass meadows and associated communities in the northern Baltic Sea. Field surveys along with fish diet and stable isotope analyses revealed that the eelgrass trophic network included two main top predatory fish species, each of which feeds on a separate group of invertebrate mesograzers (crustaceans or gastropods). Mesograzer abundance in the study area was high, and capable of mitigating the effects of increased algal biomass that resulted from experimental nutrient enrichment in the field. When crustacean mesograzers were experimentally excluded, gastropod mesograzers were able to compensate and limit the effects of nutrient enrichment on eelgrass biomass and growth. Our results suggest that top-down processes (i.e., suppression of algae by different mesograzer groups) may ensure eelgrass resilience to nutrient enrichment in the northern Baltic Sea, and the existence of multiple trophic pathways can provide additional resilience in the face of trophic modifications. However, the future resilience of these meadows is likely threatened by additional local stressors and global environmental change. Understanding the trophic links and interactions that ensure resilience is essential for managing and conserving these important ecosystems and the services they provide.

Mesopredator‐mediated trophic cascade can break persistent phytoplankton blooms in coastal waters

“…Furthermore, decreased macrophyte biomass lowers direct (nutrients, Kufel and Kufel 2002) and indirect competition (allelopathy, Vanderstukken et al 2011) with phytoplankton, leading ultimately to a decreased habitat for phyto‐ and bacterioplankton grazers, like sessile rotifers (Miracle et al 2007). An increased grazing pressure on gammarids would still allow growth control of epiphytic algae by gastropods (Gagnon et al 2021), which are very abundant in the form of Hydrobia sp. in the coastal waters of the southern Baltic Sea (Paar et al 2023).…”

“…We used the gobiid species Pomatoschistus microps and P. minutus , as well as the shrimp species Palaemon elegans . These species are abundant within the DZLS, and graze as mesopredators on larger zooplankton and mesograzers (amphipods, isopods) (Pihl et al 2006, Pasquaud et al 2010, Gagnon et al 2021). P. microps/minutus were 2–3 cm long at the start of each experiment, which is a normal length during spring (Zander and Hartwig 1982).…”

Mesopredator-mediated trophic cascade can break persistent phytoplankton blooms in coastal waters

“…We used the gobiid species Pomatoschistus microps and P. minutus, as well as the shrimp species Palaemon elegans. These species are abundant within the DZLS, and graze as mesopredators on larger zooplankton and mesograzers (amphipods, isopods) (Gagnon et al, 2021; Pasquaud et al, 2010; Pihl et al, 2006). For simplicity, those mesocosm are labelled as fish mesocosm throughout, whereas mesocosms without added fish and shrimp are labelled as no-fish mesocosm.…”

“…We used the gobiid species Pomatoschistus microps and P. minutus , as well as the shrimp species Palaemon elegans . These species are abundant within the DZLS, and graze as mesopredator on larger zooplankton and mesograzer (amphipods, isopods) (Gagnon et al, 2021; Pasquaud et al, 2010; Pihl et al, 2006). P. microps/minutus were 2 – 3 cm long at the start of each experiment, which is a normal length during spring (Zander and Hartwig 1982).…”

“…Furthermore, decreased macrophyte biomass lowers direct (nutrients, Kufel & Kufel 2002) and indirect competition (allelopathy, Vanderstukken et al 2011) with phytoplankton, leading ultimately to a decreased habitat for phyto-and bacterioplankton grazers, like sessile rotifers (Miracle et al 2007). An increased grazing pressure on gammarids would still allow growth control of epiphytic algae by gastropods (Gagnon et al 2021), which are very abundant in the form of Hydrobia sp. in the coastal waters of the southern Baltic Sea (Paar et al 2022).…”