Frontiers in Marine Science (Nov 2015)
Changes in assimilation of C3 marsh plants by resident fishes in estuarine systems with distinct hydrogeomorphology features.
Abstract
Although saltmarshes are widely recognized as important habitats providing shelter for estuarine organisms and protection against predators, there is still no consensus on the trophic value of marsh plants for estuarine food webs. We employed stable isotopes to evaluate differences in assimilation of nutrients derived from marsh plants with C3 (Juncus acutus, Scirpus maritimus, Scirpus olneyi) and C4 (Spartina densiflora) photosynthetic pathways by resident fishes in three estuaries with contrasting hydrogeomorphology characteristics. Carbon (δ13C) and nitrogen (δ15N) stable isotope ratios of basal food sources (C3 and C4 marsh plants, macroalgae, seagrass and seston) and estuarine resident fishes (Achirus garmani, Atherinella brasiliensis, Genidens genidens, Ctenogobius shufeldti, Jenynsia multidentata, Odonthestes argentinensis) were analyzed in two choked lagoons (Tramandai-29°S, Patos-30°S) and a coastal river (Chui-33°S). Average δ13C values of consumers were statistically significant higher in the two choked-type estuaries (Tramandaí: -16.11; Patos: -15.82) than in the coastal river (Chui: -24.32) (p0.292). SIAR mixing models revealed that the most assimilated basal food sources by consumers in the choked-type lagoon estuaries were a pool of 13C enriched food sources (macroalgae, C4 marsh and seagrass) and seston (95% credibility interval: 0.38 to 0.80 and 0.00 to 0.54, respectively). In contrast, nutrients derived from C3-marsh plants were the main basal food source assimilated by estuarine resident fishes at the coastal river (0.33 to 0.87). These findings could be explained by the absence of extensive shallow embayments and a steeper slope at the coastal river that could promote higher transport of C3-marsh detritus and, consequently, higher assimilation by estuarine fishes. In contrast, detritus derived from C3 marsh plants could be trapped in the upper intertidal zone of choked-typed estuaries and, consequently, be less available for aquatic estuarine consumers.
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