Regional differences in supply of organic matter from kelp forests drive trophodynamics of temperate reef fish

Abstract

Regional differences in trophic structure and availability of alternate sources of basal organic matter to food webs can affect the volume of organic matter converted into fish biomass. The present study combined stable isotope analyses (δ 13 C and δ 15 N) with estimates of biomass density of 22 common reef fishes to compare supply of organic matter derived from macroalgae versus phytoplankton to reef fish communities among 30 sites distributed across Fiordland and the Marlborough Sounds, 2 contrasting regions in terms of land-based stressors on the South Island, New Zealand. Fish communities in the Marlborough Sounds were supported by food webs that incorporated less organic matter derived from macroalgae compared to those in Fiordland. Contribution of organic matter derived from macroalgae to fish biomass decreased with trophic level in the Marlborough Sounds, while fishes in Fiordland were supported by a more equal mixture of organic matter derived from phytoplankton and macroalgae among trophic levels. Total fish biomass density was 1.72 times higher in Fiordland, yet the fish community converted 2.91 times more organ ic matter to fish biomass, as a result of a higher proportion of biomass at high trophic levels. The observed patterns were consistent with limitation in supply of organic matter derived from macroalgae in the Marlborough Sounds, where extensive losses of kelp forest habitat linked to land-based stressors have been reported. The results highlight the importance of considering regional variability in basal organic matter source pools, particularly those produced from sensitive kelp forest habitats, when applying ecosystem-based approaches to managing coastal resources.

Regional differences in kelp forest interaction chains are influenced by both diffuse and localized stressors

“…, Udy et al. , b). The exploitation of top predators in rocky reef habitats can reduce top‐down control on the structure of marine communities (Frank et al.…”

“…Kelp forests are a vital component of the temperate reef ecosystem, providing habitat structure and an important organic matter source to coastal fishes and invertebrates (Steneck et al 2003, Graham et al 2007, Udy et al 2019a. The exploitation of top predators in rocky reef habitats can reduce top-down control on the structure of marine communities (Frank et al 2006, Leleu et al 2012.…”

Primary production and depth drive different trophic structure and functioning of fish assemblages in French marine ecosystems

“…Trophodynamics, i.e. how energy and nutrients flow between different levels of the food web, largely shapes the organization of marine communities and associated ecosystem services (Petrik et al, 2019;Trebilco et al, 2013;Udy et al, 2019;van Denderen et al, 2018).…”

“…Such mechanisms connect systems and profoundly alter their functioning, as fluxes of nutrients and organic matter or animal migrations influence ecosystem structure and productivity, with a potential impact on ecosystem services, such as carbon storage or fisheries (Croll et al, 2005;Darnaude et al, 2004;Furey et al, 2018;Graham et al, 2018). As organic matter subsidies or the arrival of migrant species increase the amount of matter and energy available in the system, the biomass of high trophic level species are higher in coupled systems; the resulting representation being consequently more top-heavy (Mourier et al, 2016;Trebilco et al, 2016;Udy et al, 2019). This assumption reinforces the importance of considering coupling between ecosystems.…”

“…In addition, comparison between systems with different environmental features is recognized as a convenient framework to identify key environmental and anthropic drivers of trophic functioning (e.g. Graham et al, 2017;Silberberger et al, 2018;Udy et al, 2019;Vander Zanden et al, 2005;Ware and Thomson, 2005). The three French marine ecosystems (Eastern English Channel, Bay of Biscay and Gulf of Lions) are thus well-suited for undertaking such a comparison, since they differ in the environmental parameters, notably the intensity of primary production and average depth (Table 1).…”