Restructuring of benthic communities in eutrophic estuaries: lower abundance of prey leads to trophic shifts from omnivory to grazing

Abstract

Anthropogenic nutrient loading to coastal waters has increased producer biomass, leading to more frequent hypoxic events particularly in estuarine systems. To examine how eutrophication and hypoxia might alter consumer assemblages, we surveyed benthic communities in 2 subestuaries of Waquoit Bay, Massachusetts, representing a eutrophic-hypoxic regime and an oligotrophic-oxic regime. The number of consumer species and abundance of organisms were lower in the eutrophic estuary. In particular, there were fewer primary consumers, mainly small crustaceans. These differences in consumer community structure also alter trophic interactions. To examine changes in food web structure that might result from lower prey abundance, we sampled organisms from the 2 sub-estuaries and determined their trophic relationships based on nitrogen stable isotope ratios. Reduced numbers of primary consumers, and hence lower prey availability, led to changes in food web linkages. Specifically, omnivores shifted their diets from an omnivorous diet that is mainly carnivorous in the oligotrophic estuary to feeding mainly as herbivores in the eutrophic estuary, where prey were scarce and macroalgae were abundant. These shifts in trophic structure may have consequences for higher trophic levels.KEY WORDS: Eutrophication · Hypoxia · Benthic community · Crustacean · Food web · Trophic structure · MacroalgaeResale or republication not permitted without written consent of the publisher

Seasonal variation in carbon sources of ­mesograzers and small predators in an eelgrass community: stable isotope and fatty acid analyses

“…switched to the qualitatively inferior macroalgae if the preferred amphipods became scarce (Jephson et al 2008, Persson et al 2008. The cause of the limitation of preferred food sources can be natural prey-consumer cycles as seen in our study, seasonal nutrient limitation, overfishing, which can promote competitors via a trophic cascade, or even hypoxia as a consequence of eutrophication, which reduces the number of small invertebrates available as potential prey for omnivores (Jephson et al 2008, Fox et al 2009). …”

Top-down and bottom-up regulation in a high nutrient–high herbivory coastal ecosystem

“…Moreover, bacterial decomposition of macroalgal biomass consumes oxygen (Viaroli et al 1996), which can cause periods of extended and persistent anoxia. These events of hypoxia and anoxia affect the survival of many organisms such as fishes, shellfishes, and other consumers, including macroalgal grazers (D'Avanzo & Kremer 1994, Fox et al 2009). Thus, environments with high nutrient loads are characterized by the dominance of fast-growing seaweeds that leads to hypoxic and anoxic events in sediments and near-bottom waters and, consequently, lower abundance and diversity of organisms.…”

δ15N and δ13C reveal differences in carbon flow through estuarine benthic food webs in response to the relative availability of macroalgae and eelgrass

“…Previous studies in Waquoit Bay have argued that although there may be differences among the estuaries, the differences in N loading received from land overwhelm the influence of other potential controls on nutrient and oxygen concentrations, phytoplankton, microphytobenthos, macrophyte biomass, and higher trophic levels and food webs. Comparisons among the subestuaries of Waquoit Bay have previously been used to examine the effects of different N loads at the watershed -estuary scale, both on abundances and diversity of estuarine flora and fauna (Valiela et al 1992, Deegan et al 2002, 2009) and on their stable isotopic signatures and food web relationships (McClelland et al 1997, McClelland & Valiela 1998, Martinetto et al 2006, Fox et al 2009). Today, eelgrass Zostera marina is present in only 3 Waquoit Bay sub-estuaries (see Table 1, Fig.…”