Abstract
AbstractUnderstanding 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.
Highlights
Both resource availability and predation pressure are fundamental ecological processes determining community structure and biodiversity (Hairston et al 1960; Hillebrand et al 2007)
Empirical work on the structure of seagrass food webs has been widespread (e.g., Vizzini et al 2002; Hoshika et al 2006; Vafeiadou et al 2013), but only a few studies have provided mechanistic insights by combining stable isotope analysis with experimental manipulations of nutrients and/or predation pressure (e.g., Armitage and Fourqurean 2009, Howard et al 2016), and none in the Baltic region. We explored these topics to address the following specific research questions: 1. What is the trophic structure of the northern Baltic eelgrass food web and what are the main energy flow pathways between primary producers, grazers, and higher-level consumers?
We focused on crustacean mesograzers, as they are effective grazers of filamentous algae and more commonly affected by trophic cascades caused by the overexploitation of top predators than gastropod mesograzers (Baden et al 2012; Riera et al 2020)
Summary
Both resource availability (bottom-up) and predation pressure (top-down) are fundamental ecological processes determining community structure and biodiversity (Hairston et al 1960; Hillebrand et al 2007). It is well established that human activities increase nutrient loading (through eutrophication) and food web alterations (through the overexploitation of top predatory species), which can significantly modify the strength and relative importance of bottom-up and top-down forcing in coastal vegetated ecosystems. Specific loss mechanisms involve reductions in water clarity, along with increased epiphytic loading and shading, caused by micro- and macro algal blooms, that reduce seagrass growth and increase mortality (Thomsen et al 2012; Gustafsson and Boström 2014; Bittick et al 2018)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
