Academic literature on the topic 'Estuarine invertebrates'

The importance of different sources of carbon (energy) supporting secondary productivity in estuaries and coastal waters is still rather uncertain, despite decades of research on this topic. One of the major uncertainties is the relative importance of carbon from macrophyte detritus and microalgae, particularly diatoms and cyanobacteria within sediments (microphytobenthos, MPB). Existing methods have been unable to determine their relative contributions to consumer nutrition. This thesis addresses some of the limitations of existing methods, and then uses an innovative combination of techniques to determine the relative contributions of detritus and MPB to the nutrition of consumers within a mangrove forest in southeast Queensland, Australia. To ensure wide applicability, techniques were developed for both mud and sand, encompassing the range of sediments encountered in estuaries and protected coastal waters. Stable isotope analysis is commonly used to resolve food web issues. Difficulty extracting MPB from sediment to obtain a pure carbon isotopic signature (δ13C), however, has hampered efforts to determine the importance of MPB as a carbon source for consumers. I showed that compound-specific isotope analysis of phytol, part of the chlorophyll molecule, can be used to estimate MPB δ13C with enough precision for most food web studies: within 1-1.8‰ of actual values (95% CI) at normal levels of replication (n = 5 or 10). Although the majority of phytol in sediments is produced in situ by MPB, part of the phytol pool is derived from detritus (e.g. 33% for mud, 17% for sand in the current study). In some situations, this could lead to errors in estimating MPBδ13C, realistically by about 1‰ (in sand) or 2‰ (in mud), but an adjustment can be made where necessary. Compound-specific isotope analysis of phytol (the phytol method) can therefore be confidently used to estimate MPB δ13C values in estuarine environments. Where δ13C values of detritus and MPB are similar, natural abundance stable isotopes are unable to resolve their contribution to consumer nutrition. Enrichment of producers with the heavy 13C isotope generates distinct δ13C values that can be traced into consumers. Studies aiming to enrich MPB using 13C-enriched sodium bicarbonate have, however, observed simultaneous enrichment of bacteria. This may simply be due to use of MPB-derived carbon by bacteria, but if bacteria acquire the label directly, enrichment of consumers would indicate use of bacteria, rather than MPB, as a primary carbon source. Application of 13C-enriched bicarbonate resulted in enrichment of MPB in both mud and sand and subsequent secondary enrichment of bacteria. Direct bacterial uptake was trivial in comparison to MPB uptake. Any labelling of animals in subsequent enrichment experiments could therefore be attributed to use of MPB as a primary carbon source. I used a combination of the phytol method, stable isotope enrichment, and compartment modelling within a mangrove forest to examine use of detritus and MPB as carbon sources for common consumers; crabs (Australoplax tridentata and Parasesarma erythrodactyla) and foraminifera (Ammonia beccarii and Trochammina inflata). Compartment modelling quantified the extent of use through comparison of producer and consumer enrichment over time. The main carbon source for T. inflata was unable to be established, but all other species acquired virtually all of their carbon locally (within a 1 m radius), from MPB and/or mangrove detritus. The majority of carbon for A. tridentata was derived from MPB (99%), whereas P. erythrodactyla and A. beccarii predominantly utilised mangrove detritus (88 and 84%, respectively). For the latter two species, the remainder of carbon was derived from MPB. The different strategies in carbon utilisation by the crab species indicate feeding selectivity that may be a strategy to avoid interspecific competition for food resources. The high abundance of benthic invertebrates and their rapid assimilation of local carbon sources highlight their potential importance in influencing carbon cycling and productivity in estuaries. Consumption of benthic invertebrates by more mobile, higher consumers may also allow for transport of locally-produced carbon to adjacent waters via trophic relay. The methods demonstrated in this thesis have potential application in resolving food web issues in a variety of habitats. Further application of these techniques at a variety of temporal and spatial scales would help to develop a broader understanding of the importance of macrophyte detritus and MPB, and establish general patterns in utilisation of carbon sources in estuarine systems.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment
Faculty of Environmental Sciences
Full Text

Between 1988 and 2014, otter trawls, seine nets, and plankton nets were deployed along the salinity gradients of 18 estuaries by the University of South Florida and the Florida Fish and Wildlife Research Institute (FWRI, a research branch of the Florida Fish and Wildlife Conservation Commission). The purpose of these surveys was to document the responses of aquatic estuarine biota to variation in the quantity and quality of freshwater inflows that were being managed by the Southwest Florida Water Management District (SWFWMD). In the present analyses, four community types collected by these gears were compared with a diversity of habitat factors to identify the factors with the greatest influence on beta diversity, and also to identify the factors that were most influential to important prey species and economically important species. The four community types were (1) plankton-net invertebrates, (2) plankton-net ichthyoplankton, (3) seine nekton, and (4) trawl nekton. The habitat factors were (1) vertical profiles of salinity, dissolved oxygen, pH, and water temperature taken at the time of the biological collections, (2) various characterizations of local habitat associated with seine and trawl deployments, (3) chlorophyll a, color, and turbidity data obtained from the STORET database (US Environmental Protection Agency), and (4) data that characterize the effects of freshwater inflow on different estuarine zones, including factors for freshwater inflow, freshwater turnover time, and temporal instability in freshwater inflow (flashiness). Only 13 of the 18 estuaries had data that were comprehensive enough to allow habitat-factor analysis. An existing study had performed distance-based redundancy analysis (dbRDA) and principle component analysis (PCA) for these data within 78 estuarine survey zones that were composited together (i.e., regardless of estuary of origin). Based on that study’s findings, the communities of primarily spring-fed and primarily surface-fed estuaries were analyzed separately in the present study. Analysis was also performed with the habitat factors grouped into three categories (water management, restoration, and water quality) based on their ability to be directly modified by different management sectors. For an analysis of beta diversity interactions with habitat factors, dbRDA (called distance-based linear modeling (DistLM) in the PRIMER software) was performed using PRIMER 7 software (Quest Research Limited, Auckland, NZ). The dbRDA indicated pH, salinity, and distance to the Gulf of Mexico (distance-to-GOM) usually explained the most variation in the biotic data. These results were compared with partial dbRDA using the Akaike Information Criterion (AIC) as the model selection criterion with distance-to-GOM held as a covariate to reduce the effect of differences in the connectivity of marine-derived organisms to the different estuaries; distance-to-GOM explained between 8.46% and 32.4% of the variation in beta diversity. Even with the variation from distance-to-GOM removed, salinity was still selected as most influential factor, explaining up to an additional 23.7% of the variation in beta diversity. Factors associated with the water-management sector were most influential (primarily salinity), followed by factors associated with the restoration sector (primarily factors that describe shoreline type and bottom type). For the analysis of individual species, canonical analysis of principal coordinates (CAP) was performed to test for significant difference in community structure between groups of sites that represented high and low levels of each factor. For those communities that were significantly different, an indicator value (IndVal) was calculated for each species for high and low levels of each factor. Among species with significant IndVal for high or low levels of at least one factor, emphasis was given to important prey species (polychaetes, copepods, mysids, shrimps, bay anchovy juveniles, and gammaridean amphipods) and to species of economic importance, including adults, larvae and juveniles of commercial and recreational fishes, pink shrimp, and blue crab. Shrimps, copepods and mysids were all associated with estuarine zones that had low percentages of wooded or lawn-type shoreline, a factor that may serve as a proxy for flood conditions, as lawns or trees were usually only sampled with seines at high water elevations and in the freshwater reaches of the estuaries. Many copepod and shrimp species were strongly associated with high flushing times, which suggests that if flushing times were too short in an estuarine zone, then these species or their prey would be flushed out. Multiple regression analysis was performed on each of the selected indicator species, using AIC as a selection criterion and distance-to-GOM as a covariate. As might be expected, the apparent influences of different habitat factors varied from species to species, but there were some general patterns. For prey species in both spring-fed and surface-fed estuaries, pH and flushing time explained a significant amount of variation. In surface-fed estuaries, the presence of oysters on the bottom also had a positive effect for many prey species. For economically important species, depth was important in both spring-fed and surface-fed estuaries. This suggested the importance of maintaining large, shallow areas, particularly in surface-fed estuaries. Another important factor in spring-fed estuaries was the percent coverage of the bottom with sand; however, a mixture of positive and negative coefficients on this factor suggested the importance of substrate variety. In surface-fed estuaries, flashiness also often explained substantial variation for many economically important species, usually with positive coefficients, possibly due to the importance of alternation between nutrient-loading and high-primary-productivity periods. When comparing the three management sectors, the restoration sector was the most explanatory. Several factors were averaged over entire estuaries due to data scarcity or due to the nature of the factors themselves. Specifically, the STORET data for chlorophyll, color, and turbidity was inconsistently distributed with in the survey areas and was not collected at the same time as the biological samples. Moreover, certain water-management factors such as freshwater-inflow rate and flashiness are inherently less dimensional than other factors, and could only be represented by a single observation (i.e., no spatial variation) at any point in time. Due to concern that reduced spatiotemporal concurrence/dimensionality was masking the influence of habitat factors, the community analysis was repeated after representing each estuary with a single value for each habitat factor. We found that far fewer factors were selected in this analysis; salinity was only factor selected from the water-management factors. Overall, the factor that explained the most variation most often was the presence of emergent vegetation on the shoreline. This factor is a good proxy for urban development (more developed areas have lower levels of emergent vegetation on the shoreline). Unlike the previous analysis, the restoration sector overwhelmingly had the highest R2 values compared with other management sectors. In general, these results indicate the seeming importance of salinity in the previous analysis was likely because it had a higher resolution compared with many other factors, and that the lack of resolution homogeneity did influence the results. Of the habitat factors determined to be most influential with the analysis of communities and individual species (salinity, pH, emergent vegetation and lawn-and-trees shoreline types, oyster and sand bottom types, depth, flashiness, and flushing time) most were part of an estuarine gradient with high values at one end of the estuary with a gradual shift to low values at the other end. Since many of the analyzed species also showed a gradient distribution across the estuary, the abundance and community patterns could be explained by any of the habitat factors with that same gradient pattern. Therefore, there is a certain limitation to determining which factors are most influential in estuaries using this type of regression-based analysis. Three selected factors that do not have a strong estuarine gradient pattern are the sand bottom type, depth, and flashiness. In particular, flashiness has a single value for each estuary so it is incapable of following the estuarine gradient. This suggests that flashiness has an important process-based role that merits further investigation of its effect on estuarine species.