Benjamin D. Walther

Context: Geochemical proxies in bivalve shells can reconstruct past environmental conditions in dynamic estuaries, yet field-based validation of how quickly these proxies respond to ambient water fluctuations remains limited. Aims: This study evaluated eastern oyster (Crassostrea virginica) shell stable isotope ratios of carbon (δ13C) and oxygen (δ18O) and barium:calcium (Ba:Ca) ratio across a salinity gradient and quantified proxy equilibration times by using a field transplant experiment. Methods: Transplanted and natural shells from three estuarine sites in the north-western Gulf of Mexico were analyzed using high-resolution microsampling and laser ablation. All three proxies were compared within and between transplanted and natural shells to verify transplant-associated geochemical shifts, and Ba:Ca ratios were used to estimate equilibration timing following transplantation. Key results: Clear directional shifts in δ13C, δ18O and Ba:Ca ratios were observed. Following transplantation, Ba:Ca ratios indicated geochemical equilibrium within 31.2 ± 20.9 days. Whereas salinity appeared to be a major driver of temporal patterns recorded by all three proxies in transplanted and natural shells, additional fluctuations driven by other exogenous and endogenous drivers were present. Conclusions: Eastern oyster shells record rapid and subseasonal changes in water properties in subtropical estuaries where freshwater inflow varies spatiotemporally. Implications: These findings support the utility of oyster shells as bioindicators that archive past estuarine conditions.

Organism movement is a key process in the transfer of individuals, genes, functional traits, matter, and energy among habitat patches, at sea and across the land–sea interface. The resulting fluxes, collectively termed marine functional connectivity (MFC), underpin planetary health and an array of ecosystem services. The ecological and economic impacts of rapid environmental change, including climate change, overexploitation, habitat loss and fragmentation, and the global transport of nonindigenous species make accurate estimation and prediction of MFC patterns paramount. However, estimating MFC is challenging given the relative inaccessibility of the oceans and the small size of many of the organisms and life stages with the highest dispersal potential. Here, we provide a methodological roadmap to help researchers and stakeholders understand, use, and integrate different tools to estimate organism movement and connectivity, focusing on (1) tagging and telemetry, (2) analysis of chemical markers in body tissues and structures, (3) genetics, and (4) numerical modeling. We describe method strengths and weaknesses, and the spatiotemporal resolution and scale of resulting connectivity estimates. Ancillary and emerging methods to estimate MFC are also reviewed. We then present case studies that have successfully applied or integrated different methods, particularly to support (1) marine protected area design, (2) global change predictions, focusing on climate change and bioinvasions, and (3) fisheries management. Finally, we highlight methodological innovations and concepts that promise to transform MFC research in the future.

American eel (Anguilla rostrata) life history strategies and food web interactions are highly variable in temperate climates but not well understood in sub-tropical hydrologic systems. Intraspecific variability in isotopic niche breadth and overlap can have major implications for species of conservation concern by revealing dietary plasticity of populations across varied habitats. Muscle tissue stable isotope ratios of carbon (δ¹³C), nitrogen (δ¹⁵N), and sulfur (δ³⁴S) were analyzed to assess geographic variance in recent trophic histories within and among American eels collected in tributaries of the northwestern Gulf of Mexico and to quantify isotopic niche breadths and overlaps of capture groups. A total of eight specimen groupings were designated according to the riverine and coastal basins where they were collected, with one group comprised of only individuals caught within a wastewater treatment plant. Univariate analyses revealed highly variable isotopic ratios within and between groups. The probability of groups being found in the niche region of another group was low, with only six instances in which the probability of overlap was over 10%. The Colorado (C) group showed the highest probability of overlap of any group, being found in 86.57% of the Brazos-San Jacinto (B-SJ) group’s niche region. Collectively, these results illustrate the high degree of geographic variation in isotope ratios within and between regional habitats in an understudied portion of the species’ range.

The combined use of otolith chemistry and tissue isotopes has the potential to reveal movements, habitat associations and food web interactions at a variety of spatial and temporal scales. Here, a combination of otolith Ba:Ca life‐history transects with muscle tissue δ¹³C and δ¹⁵N values has been used to assess habitat use and oligohaline residence in red drum Sciaenops ocellatus in subtropical estuaries in the north‐western Gulf of Mexico. Tissue isotopes were distinct among capture locations, particularly between bays with differing proximities to freshwater inflow sources. Otolith edge Ba:Ca values and tissue δ¹³C values were not correlated. These results indicated that fish were neither residing in nor feeding in oligohaline waters for significant periods of time within the tissue turnover window of several months prior to capture. Nonetheless, spatial differences in tissue isotope values indicated limited mixing among bays and relatively high site fidelity during estuarine occupancy. Lifetime otolith Ba:Ca transects revealed individual variability in the magnitude of residence in oligohaline waters. Using a mean oligohaline occupancy threshold, an estimated 82% of individuals used oligohaline waters at some point in their life. Nonetheless, 66% of individuals spent <20% of their life histories in oligohaline waters, suggesting intermittent and infrequent excursions into low salinity waters. Finally, a literature survey identified 56 peer‐reviewed publications using combinations of otolith chemistry and tissue stable isotope ratios with a wide range of marker pairings and study aims. The diversity of ecological questions that can be asked with the combined use of these two approaches will provide valuable insight into fish ecology.

Chemical analysis of calcified structures continues to flourish, as analytical and technological advances enable researchers to tap into trace elements and isotopes taken up in otoliths and other archival tissues at ever greater resolution. Increasingly, these tracers are applied to refine age estimation and interpretation, and to chronicle responses to environmental stressors, linking these to ecological, physiological, and life-history processes. Here, we review emerging approaches and innovative research directions in otolith chemistry, as well as in the chemistry of other archival tissues, outlining their value for fisheries and ecosystem-based management, turning the spotlight on areas where such biomarkers can support decision making. We summarise recent milestones and the challenges that lie ahead to using otoliths and archival tissues as biomarkers, grouped into seven, rapidly expanding and application-oriented research areas that apply chemical analysis in a variety of contexts, namely: (1) supporting fish age estimation; (2) evaluating environmental stress, ecophysiology and individual performance; (3) confirming seafood provenance; (4) resolving connectivity and movement pathways; (5) characterising food webs and trophic interactions; (6) reconstructing reproductive life histories; and (7) tracing stock enhancement efforts. Emerging research directions that apply hard part chemistry to combat seafood fraud, quantify past food webs, as well as to reconcile growth, movement, thermal, metabolic, stress and reproductive life-histories provide opportunities to examine how harvesting and global change impact fish health and fisheries productivity. Ultimately, improved appreciation of the many practical benefits of archival tissue chemistry to fisheries and ecosystem-based management will support their increased implementation into routine monitoring.Graphical abstract

Environmental stress can reshape trophic interactions by excluding predators or rendering prey vulnerable, depending on the relative sensitivity of species to the stressor. Classical models of food web responses to stress predict either complete predator exclusion from stressed areas or complete prey vulnerability if predators are stress tolerant. However, if the consumer response to the stress is individually variable, the result may be a distributive stress model (DSM) whereby predators distribute consumption pressure across a range of prey guilds and their trophic niche is expanded. We test these models in one of the largest hypoxic “Dead Zones” in the world, the northern Gulf of Mexico, by combining geochemical tracers of hypoxia exposure and isotope ratios to assess individual‐level trophic responses. Hypoxia‐exposed fish occupied niche widths that were 14.8% and 400% larger than their normoxic counterparts in two different years, consistent with variable displacement from benthic to pelagic food webs. The degree of isotopic displacement depended on the magnitude of hypoxia exposure. These results are consistent with the DSM and highlight the need to account for individually variable sublethal effects when predicting community responses to environmental stress.