Intra- and interannual variability in seagrass carbon and nitrogen stable isotopes from south Florida, a preliminary study

δ13C and δ15N values were determined for the seagrassThalassia testudinum at four permanent seagrass monitoring stations in southFlorida, USA, through a quarterly sampling program over 3-years (1996–1998). All sites are seagrass beds with water depths of less than 6 m. Two sites are located on the Florida Bay side of the Florida Keys, and the other two sites are on the Atlantic side. The data analyzed over the 3 year study period display unique patterns associated with seasonal changes in primary productivity and potentially changes in the N and C pools. The mean carbon and nitrogenisotope values of T. testudinum from all four stations vary from −7.2 to −10.4‰ and 1.1 to 2.2‰, respectively. However, certain stations displayed anomalously depleted nitrogenisotope values (as low as −1.2‰). These values may indicate that biogeochemical processes like N fixation, ammonification and denitrification cause temporal changes in the isotopic composition of the source DIN. Both δ13C and δ15N values displayed seasonal enrichment-depletion patterns, with maximum enrichment occurring during the summer to early fall. The intra-annual variations of δ13C values from the different stations ranged from about 1 to 3.5‰; whereas variations in δ15N ranged from about 1 to 4.9‰. Certain sites showed a positive relationship between isotope values and productivity. These data indicate δ13C values display a high degree of seasonal variability as related to changes in productivity. δ15N values show clear intra-annual variations, but the observed changes do not necessarily follow a distinct seasonal cycle, indicating that changes in DIN will need further investigation.

Stable isotopes reveal complex changes in trophic relationships following nutrient addition in a coastal marine ecosystem

Complex links between the top-down and bottomup 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 6 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.

Fatty acids and stable isotopes as indicators of early-life feeding and potential maternal resource dependency in the bull shark Carcharhinus leucas

The degree of reliance of newborn sharks on energy reserves from maternal resource allocation and the timescales over which these animals develop foraging skills are critical factors towards understanding the ecological role of top predators in marine ecosystems. We used muscle tissue stable carbon isotopic composition and fatty acid analysis of bull sharks Carcharhinus leucas to investigate early-life feeding ecology in conjunction with maternal resource dependency. Values of δ13C of some young-of-the-year sharks were highly enriched, reflecting inputs from the marine-based diet and foraging locations of their mothers. This group of sharks also contained high levels of the 20:3ω9 fatty acid, which accumulates during periods of essential fatty acid deficiency, suggesting inadequate or undeveloped foraging skills and possible reliance on maternal provisioning. A loss of maternal signal in δ13C values occurred at a length of approximately 100 cm, with muscle tissue δ13C values reflecting a transition from more freshwater/estuarine-based diets to marine-based diets with increasing length. Similarly, fatty acids from sharks >100 cm indicated no signs of essential fatty acid deficiency, implying adequate foraging. By combining stable carbon isotopes and fatty acids, our results provided important constraints on the timing of the loss of maternal isotopic signal and the development of foraging skills in relation to shark size and imply that molecular markers such as fatty acids are useful for the determination of maternal resource dependency.

Applying stable isotopes to examine food-web structure: an overview of analytical tools

Stable isotope analysis has emerged as one of the primary means for examining the structure and dynamics of food webs, and numerous analytical approaches are now commonly used in the field. Techniques range from simple, qualitative inferences based on the isotopic niche, to Bayesian mixing models that can be used to characterize food-web structure at multiple hierarchical levels. We provide a comprehensive review of these techniques, and thus a single reference source to help identify the most useful approaches to apply to a given data set. We structure the review around four general questions: (1) what is the trophic position of an organism in a food web?; (2) which resource pools support consumers?; (3) what additional information does relative position of consumers in isotopic space reveal about food-web structure?; and (4) what is the degree of trophic variability at the intrapopulation level? For each general question, we detail different approaches that have been applied, discussing the strengths and weaknesses of each. We conclude with a set of suggestions that transcend individual analytical approaches, and provide guidance for future applications in the field.

Application of biomarkers and compound specific stable isotopes for the assessment of hydrology as a driver of organic matter dynamics in the Everglades ecosystem

The Everglades is a sub-tropical coastal wetland characterized among others by its hydrological features and deposits of peat. Formation and preservation of organic matter in soils and sediments in this wetland ecosystem is critical for its sustainability and hydrological processes are important divers in the origin, transport and fate of organic matter. With this in mind, organic matter dynamics in the greater Florida Everglades was studied though various organic geochemistry techniques, especially biomarkers, bulk and compound specific δ13C and δD isotope analysis. The main objectives were focused on how different hydrological regimes in this ecosystem control organic matter dynamics, such as the mobilization of particulate organic matter (POM) in freshwater marshes and estuaries, and how organic geochemistry techniques can be applied to reconstruct Everglades paleo-hydrology. For this purpose organic matter in typical vegetation, floc, surface soils, soil cores, and estuarine suspended particulates were characterized in samples selected along hydrological gradients in the Water Conservation Area 3, Shark River Slough and Taylor Slough. ^ This research focused on three general themes: (1) Assessment of the environmental dynamics and source-specific particulate organic carbon export in a mangrove-dominated estuary. (2) Assessment of the origin, transport and fate of organic matter in freshwater marsh. (3) Assessment of historical changes in hydrological conditions in the Everglades (paleo-hydrology) though biomarkes and compound specific isotope analyses. This study reports the first estimate of particulate organic carbon loss from mangrove ecosystems in the Everglades, provides evidence for particulate organic matter transport with regards to the formation of ridge and slough landscapes in the Everglades, and demonstrates the applicability of the combined biomarker and compound-specific stable isotope approach as a means to generate paleohydrological data in wetlands. The data suggests that: (1) Carbon loss from mangrove estuaries is roughly split 50/50 between dissolved and particulate carbon; (2) hydrological remobilization of particulate organic matter from slough to ridge environments may play an important role in the maintenance of the Everglades freshwater landscape; and (3) Historical changes in hydrology have resulted in significant vegetation shifts from historical slough type vegetation to present ridge type vegetation. ^

Using stable isotopes to investigate trophic interactions of bottlenose dolphins (Tursiops truncatus) in the Florida Coastal Everglades

Top predators are known for their ability to 1) affect their communities through predation and 2) induce behavioral modifications. Recent research suggests that they may also play “bottom-up” roles in ecosystems, including transporting materials within and across habitat boundaries. The Florida Coastal Everglades (FCE) is an “upside-down” oligotrophic estuary where productivity decreases from the mouth of the estuary to freshwater marshes. Therefore, movements of predators may be important in ecosystem dynamics. While other estuarine predators in the FCE have been shown to potentially move nutrients among ecosystems, the potential for bottlenose dolphins (Tursiops truncatus) to play a similar role in the systems has not been investigated. Stable isotope analysis of biopsy samples were used to investigate spatial variation in trophic interactions of dolphins to see if they might transport nutrients. Values of δ15 N suggest dolphins feed at a trophic level similar to other top predators in the ecosystem while δ13 C suggest that dolphins forage largely within food webs where they were sampled rather than transporting nutrients across ecosystem boundaries. The exception may be dolphins foraging in rivers, which may transport nutrients downstream; a pattern opposite to that of bull sharks and alligators in these habitats. Further research is necessary to predict how future changes occurring due to restoration and climate will affect the ecological roles of dolphins.

Using Stable Isotopes to Investigate Trophic Interactions of Bottlenose Dolphins (Tursiops truncatus) in the Coastal Everglades

Top predators are best known for their ability to affect their communities through inflicting mortality on prey and inducing behavioral modifications (e.g. risk effects). Recent scientific evidence suggests that predators may have additional roles in bottom-up processes such as transporting materials within and across habitat boundaries. The Florida Coastal Everglades (FCE) is an “upside-down” oligotrophic estuary where productivity decreases from the mouth of the estuary to freshwater marshes. Research in the FCE suggest that predators can act as mobile links between disparate habitats and can potentially affect nutrient and biogeochemical dynamics through localized behaviors (e.g. American alligators and juvenile bull sharks). To date, little is known about bottlenose dolphins (Tursiops truncatus) in the FCE beyond broad-scale patterns of abundance. Because they are highly mobile mammals commonly found in coastal waters, bottlenose dolphins are an interesting case study for investigating the influence of ecology on the evolution of local adaptations. Within this influence lies the potential for investigation of the related roles those adaptations play in coastal ecosystems due to their high metabolic rates, movement capabilities, and tendency to display specialized foraging behaviors. Stable isotope analysis of biopsy samples were used to investigate habitat use, trophic interactions, and patterns of individual specialization in bottlenose dolphins to gain functional insights into ecosystem dynamics. δ13 C isotopic values are used to differentiate the relative importance of a food web to the diet of an organism, while δ15 N values are used to evaluate the relative trophic position of an organism. Dolphin δ13 C isotopic values seem to suggest that dolphins are foraging within single ecosystems and may not be moving nutrients across ecosystem boundaries while their δ15 N isotopic values appear to be of a top predator, at a similar level to bull sharks and alligators in FCE. Further research is necessary to provide vital insight into the large predators’ role in affecting the evolution of local adaptations. Conducting this research should also provide information for predicting how future changes occurring due to restoration dynamics (see CERP: evergladesplan.org) and climate change will affect the ecological roles of these animals.

Using Stable Isotopes to Investigate Trophic Interactions of Bottlenose Dolphins (Tursiops truncatus) in the Coastal Everglades”

Top predators are best known for their ability to affect their communities through inflicting mortality on prey and inducing behavioral modifications (e.g. risk effects). Recent scientific evidence suggests that predators may have additional roles in bottom-up processes such as transporting materials within and across habitat boundaries. The Florida Coastal Everglades (FCE) is an “upside-down” oligotrophic estuary where productivity decreases from the mouth of the estuary to freshwater marshes. Research in the FCE suggest that predators can act as mobile links between disparate habitats and can potentially affect nutrient and biogeochemical dynamics through localized behaviors (e.g. American alligators and juvenile bull sharks). To date, little is known about bottlenose dolphins (Tursiops truncatus) in the FCE beyond broad-scale patterns of abundance. Because they are highly mobile mammals commonly found in coastal waters, bottlenose dolphins are an interesting case study for investigating the influence of ecology on the evolution of local adaptations. Within this influence lies the potential for investigation of the related roles those adaptations play in coastal ecosystems due to their high metabolic rates, movement capabilities, and tendency to display specialized foraging behaviors. Stable isotope analysis of biopsy samples were used to investigate habitat use, trophic interactions, and patterns of individual specialization in bottlenose dolphins to gain functional insights into ecosystem dynamics. δ13 C isotopic values are used to differentiate the relative importance of a food web to the diet of an organism, while δ15 N values are used to evaluate the relative trophic position of an organism. Dolphin δ13 C isotopic values seem to suggest that dolphins are foraging within single ecosystems and may not be moving nutrients across ecosystem boundaries while their δ15 N isotopic values appear to be of a top predator, at a similar level to bull sharks and alligators in FCE. Further research is necessary to provide vital insight into the large predators’ role in affecting the evolution of local adaptations. Conducting this research should also provide information for predicting how future changes occurring due to restoration dynamics (see CERP: evergladesplan.org) and climate change will affect the ecological roles of these animals.

A molecular and stable isotopic approach to investigate the importance of algal and detrital energy pathways in a freshwater marsh

The relative importance of algal and detrital energy pathways remains a central question in wetlands ecology. We used bulk stable isotope analysis and fatty acid composition to investigate the relative contributions of periphyton (algae) and floc (detritus) in a freshwater wetland with the goal of determining the inputs of these resource pools to lower trophic-level consumers. All animal samples revealed fatty acid markers indicative of both microbial (detrital) and algal origins, though the relative contributions varied among species. Vascular plant markers were in low abundance in most consumers. Detritivory is important for chironomids and amphipods, as demonstrated by the enhanced bacterial fatty acids present in both consumers, while algal resources, in the form of periphyton, likely support ephemeropteran larvae. Invertebrates such as amphipods and grass shrimp appear to be important resources for small omnivorous fish, while Poecilia latipinna appear to strongly use periphyton and Ephemeroptera larvae as food sources. Both P. latipinna and Lepomis spp. assimilated small amounts of vascular plant debris, possibly due to unintentional ingestion of floc while foraging for invertebrates and insect larvae. Physid snails, Haitia spp., were characterized by considerably different fatty acid compositions than other taxa examined, and likely play a unique role in Everglades’ food webs.

Interspecific variation in the elemental and stable isotope content of seagrasses in South Florida

The elemental (C, N, and P) and isotope (δ13C, δ15N) content of leaves of the seagrasses Thalassia testudinum, Halodule wrightii, and Syringodium filiforme were measured across a 10 000 km2 survey of the seagrass communities of South Florida, USA, in 1999 and 2000. Trends at local and broad spatial scales were compared to examine interspecific variation in the seagrass characteristics often used as ecological indicators. The elemental and stable isotope contents of all species were variable and demonstrated marked interspecific variation. At broad spatial scales, mean N:P ratios were lowest for T. testudinum (36.5 ± 1.1) and S. filiforme (38.9 ± 1.3), and highest for H. wrightii (44.1 ± 1.8). Stable carbon isotope ratios (δ13C) were highest for S. filiforme (–6.2 ± 0.2‰), intermediate for T. testudinum (–8.6 ± 0.2‰), and lowest for H. wrightii (–10.6 ± 0.3‰). Stable nitrogen isotopes (δ15N) were heaviest for T. testudinum (2.0 ± 0.1‰), and lightest for H. wrightii (1.0 ± 0.3‰) and S. filiforme (1.6 ± 0.2‰). Site depth was negatively correlated to δ13C for all species, while δ15N was positively correlated to depth for H. wrightii and S. filiforme. Similar trends were observed in local comparisons, suggesting that taxon-specific physiological/ecological properties strongly control interspecific variation in elemental and stable isotope content. Temporal trends in δ13C were measured, and revealed that interspecific variation was displayed throughout the year. This work documents interspecific variation in the nutrient dynamics of 3 common seagrasses in South Florida, indicating that interpretation of elemental and stable isotope values needs to be species specific.

Seasonal and spatial variation in the stable isotopic composition (δ18O and δD) of precipitation in south Florida

Precipitation data collected from five sites in south Florida indicate a strong seasonal and spatial variation in δ18O and δD, despite the relatively limited geographic coverage and low-lying elevation of each of the collection sites. Based upon the weighted-mean stable isotope values, the sites were classified as coastal Atlantic, inland, and lower Florida Keys. The coastal Atlantic sites had weighted-mean values of δ18O and δD of −2.86‰ and −12.8‰, respectively, and exhibited a seasonal variation with lower δ18O and δD values in the summer wet-season precipitation (δ18O = −3.38‰, δD = −16.5‰) as compared to the winter-time precipitation (δ18O = −1.66‰, δD = −3.2‰). The inland site was characterized as having the highest d-excess value (+13.3‰), signifying a contribution of evaporated Everglades surface water to the local atmospheric moisture. In spite of its lower latitude, the lower Keys site located at Long Key had the lowest weighted-mean stable isotope values (δ18O = −3.64‰, δD = −20.2‰) as well as the lowest d-excess value of (+8.8‰). The lower δD and δ18O values observed at the Long Key site reflect the combined effects of oceanic vapor source, fractionation due to local precipitation, and slower equilibration of the larger raindrops nucleated by a maritime aerosol. Very low δ18O and δD values (δ18O < −6‰, δD < −40‰) were observed just prior to the passage of hurricanes from the Gulf of Mexico as well as during cold fronts from the north-west. These results suggest that an oceanic vapor source region to the west, may be responsible for the extremely low δD and δ18O values observed during some tropical storms and cold fronts.

Size-based variation in intertissue comparisons of stable carbon and nitrogen isotopic signatures of bull sharks (Carcharhinus leucas) and tiger sharks (Galeocerdo cuvier)

Stable isotopes are important tools for understanding the trophic roles of elasmobranchs. However, whether different tissues provide consistent stable isotope values within an individual are largely unknown. To address this, the relationships among carbon and nitrogen isotope values were quantified for blood, muscle, and fin from juvenile bull sharks (Carcharhinus leucas) and blood and fin from large tiger sharks (Galeocerdo cuvier) collected in two different ecosystems. We also investigated the relationship between shark size and the magnitude of differences in isotopic values between tissues. Isotope values were significantly positively correlated for all paired tissue comparisons, but R2 values were much higher for δ13C than for δ15N. Paired differences between isotopic values of tissues were relatively small but varied significantly with shark total length, suggesting that shark size can be an important factor influencing the magnitude of differences in isotope values of different tissues. For studies of juvenile sharks, care should be taken in using slow turnover tissues like muscle and fin, because they may retain a maternal signature for an extended time. Although correlations were relatively strong, results suggest that correction factors should be generated for the desired study species and may only allow coarse-scale comparisons between studies using different tissue types.