Debashish Mazumder

To fit or not to fit: evaluating stable isotope mixing models using simulated mixing polygons

Summary Stable isotope analysis is often used to identify the relative contributions of various food resources to a consumers diet. Some Bayesian isotopic mixing models now incorporate uncertainty in the isotopic signatures of consumers, sources and trophic enrichment factors (e.g. SIAR, MixSIR). This had made model outputs more comprehensive, but at the expense of simple model evaluation, and there is no quantitative method for determining whether a proposed mixing model is likely to explain the isotopic signatures of all consumers, before the model is run. Earlier linear mixing models (e.g. IsoSource) are easier to evaluate, such that if a consumers isotopic signature is outside the mixing polygon bounding the proposed dietary sources, then mass balance cannot be established and there is no logical solution. This can be used to identify consumers for exclusion or to reject a model outright. This point-in-polygon assumption is not inherent in the Bayesian mixing models, because the source data are distributions not average values, and these models will quantify source contributions even when the solution is very unlikely. We use a Monte Carlo simulation of mixing polygons to apply the point-in-polygon assumption to these models. Convex hulls (‘mixing polygons’) are iterated using the distributions of the proposed dietary sources and trophic enrichment factors, and the proportion of polygons that have a solution (i.e. that satisfy point-in-polygon) is calculated. This proportion can be interpreted as the frequentist probability that the proposed mixing model can calculate source contributions to explain a consumers isotopic signature. The mixing polygon simulation is visualised with a mixing region, which is calculated by testing a grid of values for point-in-polygon. The simulation method enables users to quantitatively explore assumptions of stable isotope analysis in mixing models incorporating uncertainty, for both two- and three-isotope systems. It provides a quantitative basis for model rejection, for consumer exclusion (those outside the 95% mixing region) and for the correction of trophic enrichment factors. The simulation is demonstrated using a two-isotope study (15N, 13C) of an Australian freshwater food web.

Trophic relationships between itinerant fish and crab larvae in a temperate Australian saltmarsh

Comparisons of zooplankton inputs and outputs for a temperate Australian saltmarsh demonstrate a substantial contribution of crab larvae to the ebbing tide water, particularly during the cooler months. Few crab larvae were present in the incoming tide (mean abundance 4 m −3 ), whereas many crab larvae were present in the outgoing water (mean abundance 2124.63 m −3 ). Stomach content analysis of itinerant fish exiting the saltmarsh with the ebbing tide demonstrated extremely high proportions of crab larvae in the gut of glassfish (Ambassis jacksoniensis), as well as flat tail mullet (Liza argentea) and blue eye (Pseudomugil signifer). The results suggest a direct trophic link between secondary production of saltmarsh and itinerant fish, and a significant ecological role for burrowing crabs occupying saltmarshes in temperate Australia in the trophic food web of saltmarsh-estuarine systems.

Trophic importance of a temperate intertidal wetland to resident and itinerant taxa: evidence from multiple stable isotope analyses

Juveniles of commercially important fish species congregate in shallow vegetated estuarine habitats during high tides. Considerable debate has centred on whether the significance of these habitats lies in their provision of greater feeding opportunities, or shelter from predation afforded by greater structural complexity. We tested the hypothesis that an inundated mangrove and saltmarsh wetland provided feeding opportunities for itinerant species, and that the contribution of wetland primary producers and grazing herbivores could be identified in their diet, using stable isotopes of carbon and nitrogen. Potential sources of dietary carbon included mangrove, saltmarsh, seagrass, seagrass epiphytic material and benthic organic material. Saltmarsh plants (mostly Sporobolus virginicus and Juncus kraussii) and fine benthic organic material appeared to be the primary sources of dietary carbon for the resident grazing herbivores in the wetlands, based on IsoSource mixing models. During high tide, species of itinerant fish enter the mangrove and, when inundated, the saltmarsh, and feed primarily on crab larvae and copepods. Fine benthic organic matter, seagrass epiphyte, and C3 and C4 plant materials also supplement the diet of some fish. The crab larvae therefore provide a significant source of nutrition and an important link between the intertidal wetlands and the adjacent estuarine ecosystem. The carnivorous fish Acanthopagrus australis, at the highest trophic level, hunted within or adjacent to the mangrove–saltmarsh wetland and fed on several lower-order consumers within the wetland. The present study highlights the significance of mangrove and saltmarsh wetlands as a feeding habitat for resident grazers and itinerant nekton.

Mangrove Leaves are Not an Important Source of Dietary Carbon and Nitrogen for Crabs in Temperate Australian Mangroves

The carbon and nitrogen stable isotope ratios of the mangrove sesarmid crabs Parasesarma erythrodactyla, Paragrapsus laevis, and Helograpsus haswellianus were measured from sites in arid and humid temperate Australia, and compared with potential food sources. Crab stable isotope ratios were uniform and consistent with published values from subtropical Queensland. Fresh and decomposed leaves of the dominant mangrove Avicennia marina were discounted as a potential food source, being too depleted in their δ13C and δ15N signatures compared to crabs to have made a substantial contribution to the diet. A three-source mixing model indicated that fine benthic organic material was the primary food source. Mangrove epiphyte and highly decomposed mangrove leaf material were too depleted in δ15N to be contributing to crab diet.

Fine-scale variability in the dietary sources of grazing invertebrates in a temperate Australian saltmarsh

Saltmarsh floristic diversity declines with increasing latitude on the Australian east coast, with the dominant tropical C4 grass Sporobolus virginicus being replaced progressively by a suite of mostly succulent C3 species. The temperate Towra Point saltmarsh consists of a mosaic of vegetation communities, including stands of the C4 saltmarsh grass Sporobolus virginicus, and the C3 succulents Suaeda australis and Sarcocornia quinqueflora. The contrasting stable isotope signatures of these plants provide an opportunity to determine the extent to which plant material is contributing to the diet of grazing invertebrates inhabiting these communities. The grazing crabs Parasesarma erythrodactyla and Helograpsus haswellianus, and the snail Littoraria luteola, were sampled for their carbon and nitrogen isotope signatures. In the Sarcocornia communities, crab and snail δ13C signatures could not be matched to the signature of dominant plants, but were close to the fine benthic material on the marsh surface. In the Sporobolus community, the δ13C signatures of the same species were enriched and closer to that of the dominant plant. Results suggest that grazing herbivores feed over very small spatial ranges within mosaics of vegetation on locally sourced benthic material, with S. virginicus plant material making a contribution to dietary carbon where present.

Fish Assemblages in Three Tidal Saltmarsh and Mangrove Flats in Temperate NSW, Australia: A Comparison Based on Species Diversity and Abundance

Fish assemblages in three geographically distinct saltmarsh meadows and mangrove forests in the Sydney region were examined using fyke nets over three common sampling periods. The saltmarshes at Towra Point and Allens Creek, though of contrasting geomorphic setting, showed similar fish assemblages during spring tides, with relatively high diversity and abundance. The saltmarsh at Bicentennial Park, reclaimed from dredge spoil in the early 1960’s, showed significantly lower diversity and abundance. Fish assemblages in the three mangrove settings were each distinct, though with no significant differences in diversity and abundance between sites. The result raises questions about the efficacy of created saltmarsh as a compensatory habitat for fish.

Effect of acidification on elemental and isotopic compositions of sediment organic matter and macro-invertebrate muscle tissues in food web research.

Stable isotope techniques in food web studies often focus on organic carbon in food sources which are subsequently assimilated in the tissue of consumer organisms through diet. The presence of non-dietary carbonates in bulk samples can affect their δ(13)C values, altering how their results are interpreted. Acidification of samples is a common practice to eliminate any inorganic carbon present prior to analysis. We examined the effects of pre-analysis acidification on two size fractions of sediment organic matter (SOM) from marine and freshwater wetlands and pure muscle tissue of a common freshwater invertebrate (Cherax destructor). The elemental content and isotopic ratios of carbon and nitrogen were compared between paired samples of acidified and control treatments. Our results showed that acidification does not affect the elemental or isotopic values of freshwater SOM. In the marine environment acidification depleted the δ(13)C and δ(15)N values of the fine fraction of saltmarsh and δ(15)N values of mangrove fine SOM. Whilst acidification did not change the elemental content of invertebrate muscle tissue, the δ(13)C and δ(15)N values were affected. We recommend to researchers considering using acidification techniques on material prepared for stable isotope analysis that a formal assessment of the effect of acidification on their particular sample type should be undertaken. Further detailed investigation to understand the impact of acidification on elemental and isotopic values of organic matter and muscular tissues is required.

Zooplankton inputs and outputs in the saltmarsh at Towra Point, Australia

The contribution made by saltmarsh to the production of estuarine zooplankton was examined through a comparison of inputs and outputs of tidal water at a site on Towra Point, NSW, Australia. Saltmarsh proved to be a net exporter of crab and gastropod larvae, although it functioned as a sink for copepods and amphipods. Further, the highest density of zooplankton in estuarine nearshore habitats (saltmarsh, mangrove, seagrass, and open water) during a high tide event was found in the saltmarsh. The presence of high concentrations of zooplankton, predominantly crab and gastropod larvae, in the saltmarsh and lesser extent in the mangrove represents a source of food for estuarine fish.

Grazing kangaroos act as local recyclers of energy on semiarid floodplains

On Australian semiarid floodplains, large herbivores such as kangaroos have a role in the cycling of energy (carbon) through the mechanism of feeding and defaecation of vegetative material. The degree to which kangaroos are vectors of energy within this system is not fully understood. This study describes the stable carbon isotope signature of floodplain plants and kangaroo scats at two close study sites. Kangaroos were found to deposit scats that mirrored the forage composition at each particular feeding site. Scats were 3.94‰ higher in δ13C values at the site where C4 grasses were available, indicating that this grass contributed ~25–30% of the diet of these kangaroos. The difference in diet due to the relative availability of C3 and C4 forage, detectable in the carbon stable isotope signature of scats, is used to demonstrate that kangaroos are recycling and redistributing energy locally, rather than transporting it more broadly across the floodplain.

Trophic Shifts Involving Native and Exotic Fish During Hydrologic Recession in Floodplain Wetlands

Stable isotopes (δ13C and δ15N) and gut contents were analysed for two species of co-occurring native and exotic fish in three shallow water bodies within an Australian riparian wetland system. During a period of hydrologic recession we found depletions in δ13C of up to −25‰ for algae and −2‰ for sediment organic matter (SOM). The native Hypseleotris sp. (carp gudgeon) and the exotic Cyprinus carpio (common carp) were depleted in δ13C up to −3.2‰, indicating that the SOM was the dominant dietary source of carbon for the two species of fish in both high- and the receded low-water conditions. In the low-water conditions, however, there was a five-fold increase in the occurrence of insects in the gut of the exotic C. carpio and the trophic positions of C. carpio and Hypseleotris sp. were more similar in all three water bodies than at high-water conditions. Our results indicate that there were shifts in dietary sources and trophic positions during hydrologic recession and provide evidence that flow reductions in wetland systems can increase the dietary overlap between native and exotic fishes.