δ13C and δ15N of particulate organic matter in the Santa Barbara Channel: drivers and implications for trophic inference (original) (raw)
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Assessing the importance of land and marine sources of organic matter to kelp forest food webs
Marine Ecology Progress Series, 2008
We used stable isotope analysis to investigate the relative importance of marine and terrestrial sources of organic matter to the food web of 4 nearshore reefs in the Santa Barbara Channel (California, USA) over a 4 yr period that included substantial variability in the availability of marine and terrestrial organic matter. We measured stable C and N isotope values of ocean-and landderived sources of organic matter and of different types of consumers on reefs that varied in exposure to freshwater runoff and in biomass of giant kelp. δ 13 C values of suspended particulate organic matter (POM) on reefs tended to decrease following periods of significant rainfall at the reef most influenced by freshwater runoff, to increase with phytoplankton standing crop at all reefs, and generally to decline in both wet and dry years in late fall-early winter at all reefs. Stable isotope values of reef consumers indicated little direct use of terrestrially-derived POM. However, a pattern of 15 N-enrichment in 2 common benthic-feeding species, the sea urchin Strongylocentrotus purpuratus and the annelid Diopatra ornata, with increasing influence of runoff indicated that terrestrially-derived nitrogen may enter the food web indirectly through a trophic intermediate (e.g. microbes, algae). The importance of giant kelp to the reef food web varied with consumer feeding mode. In contrast to previous studies, δ 13 C values suggested little use of kelp-derived material by suspension-feeders, based on the similarity in isotope values of these consumers among reefs when kelp biomass was low, and the absence of a directional shift in isotope values that would indicate the use of more 13 C-enriched kelp when kelp biomass was higher at 2 of the reefs. However, isotope values for herbivores were generally 13 C-enriched relative to suspension-feeders, reflecting the use of local giant kelp or other 13 C-enriched benthic algal production. Thus, spatial and temporal fluctuations in the biomass of giant kelp would be least likely to impact the food resources of suspension-feeders and have greatest effect on benthic herbivores.
Frontiers in Marine Science, 2021
Trophic contributions of diverse OM sources to estuarine and coastal food webs differ substantially across systems around the world, particularly for nekton (fish, cephalopods, and crustaceans), which utilize basal resources from multiple sources over space and time because of their mobility and feeding behaviors at multiple trophic levels. We investigated the contributions of putative OM sources to fish food webs and assessed the spatiotemporal patterns, structures, and trophic connectivity in fish food webs across four seasons from three closely spaced (10–15 km) sites: an estuarine channel (EC), a deep bay (DB), and an offshore (OS) region in Gwangyang Bay, a high-productivity, low-turbidity estuarine embayment off the Republic of Korea. While nearly all previous studies have focused on few representative species, we examined δ13C and δ15N values of whole nekton communities along with dominant benthic macro-invertebrates, zooplankton, and their putative primary food sources. The ...
Estuaries and Coasts, 2009
Complex links between the top-down and bottom-up forces that structure communities can be disrupted by anthropogenic alterations of natural habitats. We used relative abundance and stable isotopes to examine changes in epifaunal food webs in seagrass (Thalassia testudinum) beds following six months of experimental nutrient addition at two sites in Florida Bay (USA) with different ambient fertility. At a eutrophic site, nutrient addition did not strongly affect food web structure, but at a nutrient-poor site, enrichment increased the abundances of crustacean epiphyte grazers, and the diets of these grazers became more varied. Benthic grazers did not change in abundance, but shifted their diet away from green macroalgae + associated epiphytes and towards an opportunistic seagrass (Halodule wrightii) that occurred only in nutrient addition treatments. Benthic predators did not change in abundance, but their diets were more varied in enriched plots. Food chain length was short and unaffected by site or nutrient treatment, but increased food web complexity in enriched plots was suggested by increasingly mixed diets. Strong bottom-up modifications of food web structure in the nutrient-limited site and the limited top-down influences of grazers on seagrass epiphyte biomass suggest that in this system, the bottom-up role of nutrient enrichment can have substantial impacts on community structure, trophic relationships, and, ultimately, the productivity values of the ecosystem.
Carbon stable isotopes as indicators of coastal eutrophication
Ecological Applications, 2014
Coastal ecologists and managers have frequently used nitrogen stable isotopes (d 15 N) to trace and monitor sources of anthropogenic nitrogen (N) in coastal ecosystems. However, the interpretation of d 15 N data can often be challenging, as the isotope values fractionate substantially due to preferential retention and uptake by biota. There is a growing body of evidence that carbon isotopes may be a useful alternative indicator for eutrophication, as they may be sensitive to changes in primary production that result from anthropogenic nutrient inputs. We provide three examples of systems where d 13 C values sensitively track phytoplankton production. First, earlier (1980s) mesocosm work established positive relationships between d 13 C and dissolved inorganic nitrogen and dissolved silica concentrations. Consistent with these findings, a contemporary mesocosm experiment designed to replicate a temperate intertidal salt marsh environment also demonstrated that the system receiving supplementary nutrient additions had higher nutrient concentrations, higher chlorophyll concentrations, and higher d 13 C values. This trend was particularly pronounced during the growing season, with differences less evident during senescence. And finally, these results were replicated in the open waters of Narragansett Bay, Rhode Island, USA, during a spring phytoplankton bloom. These three examples, taken together with the pre-existing body of literature, suggest that, at least in autotrophic, phytoplankton-dominated systems, d 13 C values can be a useful and sensitive indicator of eutrophication.
Marine Biology, 1993
In a study to assess qualitatively the importance of organic matter derived from kelp production in the Aleutian Islands of subarctic Alaska, replicated samples of autotrophic sources and primary and secondary consumer organisms were sampled for 613C among sources, sites, (treatment) islands, and years. Unanticipated variation in the ~ 3C of kelps occurred among overtly similar sites at different islands. Variation in the 6~3C of the surface canopy-forming kelp Alariafistulosa was particularly extreme, ranging from -15.5 to -28.0 %0 compared to the understory kelps, Laminaria spp. A.fistulosa 6~3C varied by as much as 6 to 7%o among similar sites at a given island within years, and by as much as 3 to 4%o between years at the same sampling site. In several cases, ~3C variation was weakly tracked by some consumer organisms, suggesting that even detritus pathways through the food web can be localized and tightly coupled. Dynamic cycles in the concentration and 6~3C of dissolved inorganic carbon (DIC) and aqueous CO2 concentration ([CO2]aq) were measured at three sites on one island. The 613C or organic carbon fixed by A.fistulosa, calculated from diurnal DIC concentration and 613C measurements, varied by 15%o and varied inversely with [COz]aq concentrations. Local DIC variability, probably resulting from high productivity and decreased turbulence in dense kelp habitats, provides a possible mechanism of variation in kelp 6 ~ 3C. The short-term variability in the 6~3C of organic carbon fixed by kelps indicates that sampling methodology and design must assess this potential variation in marine macrophyte 013C before making assumptions about the transfer of 613C_invariat e organic matter to higher trophic levels. On the positive side, a predictable relationship between [CO2Lq concentration and kelp 613C offers a potentially robust means to assess productivity effects on CO 2 limitation in kelps and other complex aquatic macrophyte habitats,
Marine Ecology Progress Series, 2013
Studies that use biomarkers to elucidate consumer diets often must assume that these signatures are relatively invariant in space and time. We tested this assumption for multiple stable isotopes (MSI: δ 13 C, δ 15 N, δ 34 S) and fatty acids in 10 marine macrophytes (macroalgae and seagrass) on 3 different dates, and also quantified MSI at 3 sites in the coastal northeast Pacific. For all comparisons, we found significant variation in biomarkers among species, sites, and dates; furthermore, there were always significant site × species and date × species interaction terms, indicating that biomarkers do not change consistently across species among dates or sites. Despite this variation within and among species, biomarkers could readily distinguish macrophyte phyla. To observe how variation could affect conclusions about diets, we used a Bayesian mixing model to evaluate scenarios for a theoretical consumer given diverse diets, and with varying assumptions about the way it integrates foods over sites and seasons. Accuracy of the model runs (predicted diet versus simulated diet) increased with the number of biomarker variables, and depended strongly on initial assumptions about diets. The lowest accuracy occurred when biomarker values were based on macrophytes sampled only from 1 season but the consumer integrated foods across multiple seasons, a situation commonly seen in biomarker literature. Contrary to the notion that natural biomarker variation reduces insight into food web structure, exploring the potential mechanisms behind this variation should provide a more realistic view of coastal ecosystem dynamics.
Inland Waters, 2014
Studies designed to assess the resources supporting aquatic consumers using stable isotope analysis require measurements of the potential end members (basal resources). While some basal resources are easily measured, it is often difficult to physically separate phytoplankton (one potential end member) from other components in seston. Further, terrestrial materials entering aquatic ecosystems undergo diagenetic change, potentially altering isotope composition and making it difficult to assign end member values. We tested techniques for determining the isotopic hydrogen (δ 2 H), carbon (δ 13 C), and nitrogen (δ 15 N) values of terrestrial and phytoplankton end members in seston. Long term in situ leaf decomposition experiments were performed. No appreciable change was found in the isotope values of degraded material (mean change 3.6‰ for δ 2 H, 0.0‰ for δ 13 C, and −0.1‰ for δ 15 N). We conclude that the isotope values of terrestrial plant material can be used to assign end members for terrestrial detritus. Using samples collected from 10 lakes with phytoplankton-dominated seston, we compared 3 published methods for estimating the δ 13 C and δ 15 N of phytoplankton. One method, which corrected bulk particulate organic matter (POM) isotope values based on a δ 2 H mixing model, accurately predicted measured phytoplankton δ 13 C. Another method, which used a C:N mixing model to correct bulk POM, also performed well. A new method, proposed here, modified seston isotope values using the difference in C:N of phytoplankton and terrestrial material in a δ 2 H mixing model and correctly predicted measured phytoplankton δ 15 N. We recommend estimating phytoplankton δ 13 C and δ 15 N by correcting bulk POM using a δ 2 H mixing model, with the C:N modification proposed here for δ 15 N.
Marine Ecology Progress Series, 2013
Owing to their productivity, kelps may be the main primary carbon source for consumers in coastal areas. Their contribution has often been compared to that of phytoplankton, but the potential involvement of the red algae associated with kelp forests has been overlooked. All these 3 primary sources have distinct life cycles and may contribute to the particulate organic matter pool differently according to season. In the present study, we characterised the trophic structure of a pristine Laminaria hyperborea forest off the coast of Brittany (France) in terms of the organic carbon biomass of the main primary producers and consumer trophic groups on 4 sampling dates over a 1 yr period. Senescence of many red algae species occurred during their resting period (i.e. before November), whereas the kelp regrowth period (i.e. before March) was accompanied by the fragmentation of old lamina. During these periods, when phytoplankton biomass is comparable, stable isotope analyses (δ 13 C and δ 15 N) were conducted in an attempt to link structure to trophic functioning. By combining analyses of temporal variability in primary source and consumer isotopic ratios and using mixing models, we inferred changes in the trophic significance of macroalgae in the associated food web. Decaying kelp laminae were a major contributor to the particulate organic matter pool, and the fragmentation of old lamina promoted their contribution to the diet of deposit-and suspension-feeders in March. Growth of red algae enhanced direct grazer consumption in March, while their senescence contributed significantly to primary consumer diets in November via indirect consumption.