How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations. Introduction Seagrass meadows are ecologically and economically important ecosystems that provide numerous crucial services for society (Constanza et al. 1997; Waycott et al. 2009). Seagrasses stabilize sediments and reduce the water flow, thus contributing to coastal protection (Orth et al. 2006). They recycle nutrients and produce a large amount of organic carbon, providing a critical supply of organic matter to the deep sea and significantly adding to the sequestration of carbon in the biosphere (Duarte et al. 2005). Furthermore, they provide the habitat for a diverse community of plants and animals and serve as nursery ground for many important finfish and shellfish species (Heck et al. 2003). Seagrass beds also have important linkages to other habitats like coral reefs or mangroves, facilitating trophic transfer and cross-habitat utilization of invertebrates and fish (Beck et al. 2001; Heck et al. 2008). Coastal development, growing human population and climate change threaten these valuable ecosystems (Harley et al. 2006; Orth et al. 2006). One-third of the known seagrass meadows has disappeared since the first records in 1879, and the rate of loss has accelerated in recent decades (Waycott et al. 2009). One of the most severe anthropogenic stressors of coastal submerged vegetation is eutrophication (Howarth et al. 2000). This process has the potential to initiate shifts in coastal and freshwater ecosystems from high-diversity to low-diversity status that reduce the ecological and economical functioning and value of these systems (Smith et al. 1999; Howarth et al. 2000). Excessive nutrient inputs have been linked to increasing occurrence of harmful algae blooms, fish kills caused by toxins or hypoxia, and the destruction or degradation of highly productive valuable coral reefs, kelp beds and seagrass ecosystems (Anderson et al. 2002; Kemp Communicated by F. Bulleri. S. Jaschinski (&) U. Sommer IFM-GEOMAR Leibniz Institute of Marine Sciences, Düsternbrooker Weg 20, 24105 Kiel, Germany e-mail: [email protected] 123 Mar Biol (2011) 158:193–203 DOI 10.1007/s00227-010-1551-0