Marine fisheries declines viewed upside down: human impacts on consumer-driven nutrient recycling

Abstract

Abstract. We quantified how two human impacts (overfishing and habitat fragmentation) in nearshore marine ecosystems may affect ecosystem function by altering the role of fish as nutrient vectors. We empirically quantified size-specific excretion rates of one of the most abundant fishes (gray snapper, Lutjanus griseus) in The Bahamas and combined these with surveys of fish abundance to estimate population-level excretion rates. The study was conducted across gradients of two human disturbances: overfishing and ecosystem fragmentation (estuaries bisected by roads), to evaluate how each could result in reduced population-level nutrient cycling by consumers. Mean estimated N and P excretion rates for gray snapper populations were on average 456% and 541% higher, respectively, in unfished sites. Ecosystem fragmentation resulted in significant reductions of recycling rates by snapper, with degree of creek fragmentation explaining 86% and 72% of the variance in estimated excretion for dissolved N and P, respectively. Additionally, we used nutrient limitation assays and primary producer nutrient content to provide a simple example of how marine fishery declines may affect primary production. This study provides an initial step toward integrating marine fishery declines and consumer-driven nutrient recycling to more fully understand the implications of human impacts in marine ecosystems.

Ammonium cycling in the rocky intertidal: Remineralization, removal, and retention

“…Analogous to studies of the zebra mussel (Dreissena polymorpha) in freshwater systems (Heath et al 1995;Cecala et al 2008), our findings also suggest that the metabolic activities of California mussels have large ecosystem effects. This is consistent with a broad nontrophic role for animals in aquatic-system nutrient cycles, such as fishes in lakes (Vanni 2002), coral reef systems (Layman et al 2011), and marine mammals in the ocean (Roman and McCarthy 2010). Marine aquaculture enterprises may have a similar feedback between animal nutrient input and productivity (Asmus and Asmus 1991).…”

Introduced lake trout alter nitrogen cycling beyond Yellowstone Lake

“…Higher densities or biomass of fish generally means higher excretion fluxes in lakes (Griffiths 2006) and streams (McIntyre et al 2008). High densities (43,000 fish/ha) of grey snapper (Lutjanus griseus) in the Bahamas excreted large amounts of N and P reducing nutrient limitation (Layman et al 2011). Similarly, a low density of fish, such as cutthroat trout in Yellowstone Lake (51 fish/ha in 1998; Ruzycki et al 2003), supplied little nutrients to primary producers.…”

The Individual and Interactive Effects of Nitrogen and Phosphorus Enrichment on Coral Reefs

“…Likewise, fishes often deliver important limiting nutrients in oligotrophic ecosystems and can be important sources of nutrients on coral reefs . Thus, overfishing on coral reefs, an important driver of change in these ecosystems , could disrupt the critical link between fish excretion and corals and dramatically alter production and coral growth (Layman et al 2011).…”

“…Furthermore, this pattern is likely not unique to reefs and may represent a less recognized threat of overfishing to marine systems. For example, Layman et al (2011) documented an approximately 500% decline in nutrient delivery in fished versus unfished tidal creeks in the Bahamas and subsequent declines in primary production with the removal of fishes. Because many mobile-link organisms, including the grunts in this study, cross system boundaries to forage or shelter (Lundberg & Moberg 2003, Heck et al 2008, conservation must focus not only on the organisms themselves but also on both the donor and recipient ecosystems for the nutrients that they translocate.…”

“…Welsh 2000; Leichter et al2003;Schaffelke et al 2005). However, fishes can be important sources of nutrients, particularly in oligotrophic systems such as tropical seagrass beds and coral reefs (Layman et al 2011;. For example, in the Florida Keys, fishes are one of the most important sources of N on reefs and can impact algal and coral abundance on a reef-wide scale .…”