Contributed Session.
Lead Organizer: Rita Franco-Santos, CSIRO (rita.franco-santos@outlook.com)
Presentations
04:30 PM
Do Algal Blooms Reduce Grazing by Having High Oxygen or High pH conditions? (9678)
Primary Presenter: Frieda Taub, University of Washington (taub@uw.edu)
Daphnia magna mortalities were associated with elevated oxygen and pH during experiments with high initial nitrate concentrations, resulting in high algae concentrations (Chl a) in Closed Ecological Systems. With lower initial nitrate concentrations, Daphnia populations survived much longer and grazed the algae to low abundance (Chl a), with lower oxygen and pH levels. These simple experimental systems do not allow exchange with the open atmosphere, and conditions probably exceed those observed in natural systems. Given the increased frequency of reported algal blooms, are oxygen and pH increased, and are there any observations that zooplankton are avoiding edible algal blooms, reducing grazing pressure? Our Closed Ecological Systems are 250 ml to 1-liter systems containing 3 species of green algae (Ankistrodesmus, Scenedesmus, and Selenastrum) that are presumed non-toxic and suitable foods for the grazer, Daphnia magna. Unidentified microbes are also present. The medium is chemically defined. Given high nitrate, pH can exceed 10, and oxygen can exceed 28 mg/l (the limit of our instrumentation). In naturally occurring blooms, unusually high pH and oxygen could be irritating to zooplankton, even if nonlethal, and might interfere with normal zooplankton diurnal migration behavior. In lakes with automated pH, oxygen, and Chl a monitoring, are measurements during algal blooms available, and is there evidence that grazer behavior is modified during edible algal blooms?
04:45 PM
Diet of Lepidophthalmus louisianensis Determined by Fecal DNA Metabarcoding (9296)
Primary Presenter: Julian Venable, Jackson State University (jvenable82@icloud.com)
Lepidophthalmus louisianensis is a common prey to many species including stingrays, shorebirds, sturgeon, and catfish and plays a critical role in estuarine communities both through the bioturbation and aeration of sediments, as well as the creation of habitat for a number of commensal species. However, despite their importance in the estuarine environments of the northern Gulf of Mexico, the diet of L. louisianensis is currently unknown. While the literature suggests, based largely on the morphology of the mouth parts, that this species is a filter feeder, reported observations of individuals in aquaria “grazing” on their burrow walls has led some to suggest that they may in fact be “farmers” that use their feces to grow bacterial and fungi in their organic-rich burrow walls that they then “harvest”. As food availability is often a primary driver in determining growth rates, reproductive potential and population densities, knowledge of a species diet is a critical to understanding their ecology. Our primary objective is to use fecal DNA metabarcoding to determine the diet of L. louisianensis. In addition, we propose to use environmental DNA (eDNA) metabarcoding of the burrow walls and surrounding water to determine the diet and feeding ecology of Lepidophthalmus louisianensis by comparing the genetic composition of the feces to those of the burrow walls and the surrounding water. Metabarcoding of 3 genes (i.e., 16S, 18S, and rbCL) for the fecal, burrow wall, and water samples suggests a diet derived primarily by filter feeding.
05:00 PM
CHARACTERIZING THE TROPHIC DYNAMICS OF SMALL PELAGIC FISHES ACROSS THE NORTH IBERIAN UPWELLING SYSTEM BY STABLE ISOTOPE AND FATTY ACID ANALYSES (8852)
Primary Presenter: RITA GARCIA SEOANE, Spanish National Research Council (CSIC) (rita.garcia@ieo.csic.es)
Small pelagic fishes (SPF) are important for the functioning of marine food webs, as they link basal resources with top consumers. Therefore, understanding the feeding ecology and trophic relationships of coexisting SPF at local scales may improve the strategic management of marine resources. This study represents the first effort to investigate the feeding patterns of the most abundant SPF (sardine, anchovy, mackerel and chub mackerel) across different regions in northern Spain (NE Atlantic) using bulk isotope analysis (δ13C, δ15N), compound-specific isotope analysis of amino acids (δ15N), and the fatty acid (FA) composition of lipids. The results imply large zonal variability and diet plasticity across species, largely unrelated to changes in local plankton availability. Furthermore, low mobility of East Cantabrian fishes was indicated by their C and N isotopic baselines. However, low resource competition among co-occurring species across zones can be deduced from the comparison of trophic position (TP) estimates and FA profiles, with mackerels occupying the highest TP. Overall, this work shows the importance of: i) integrating multiple biochemical tracers, and ii) the use of more sensitive trophic indicators (amino acid δ15N and neutral lipid FA), instead of traditional methods (bulk δ15N and total lipid FA), to strengthen ecological interpretation of the fish dietary preferences and TP estimates. Finally, this contribution advances our understanding of the pelagic food web dynamics and nutrient cycling in highly productive systems.
05:15 PM
An Updated Analysis on the Diet of Lemon Sharks (Negaprion brevirostris) (8888)
Primary Presenter: Owen Fleischer, University of Rhode Island (fleischerowen1@uri.edu)
Although many studies on the diet of juvenile lemon sharks (Negaprion brevirostris) have been published, publications on the diet of sub-adult and adult lemon sharks are lacking. An analysis of the diet of lemon sharks collected from 2011 - 2018 in the South Atlantic and Gulf of Mexico was conducted on 67 lemon sharks, ranging from 179 to 270 cm in fork length (L). Of the 67 stomachs, 53 (79.1%) had prey items present. To quantify the diet of lemon sharks, the Index of Relative Importance (IRI) (a common method) was chosen which quantifies diet numerically (N), gravimetrically (W), and considers the frequency of occurrence (O). The use of these three variables eliminates biases that less comprehensive indices may fail to account for. The Index of Relative Importance was further expressed as a percentage value to allow for comparison among studies. Based on %IRI, lemon sharks are mainly piscivorous (60.2%), primarily Lutjanus campechanus and Caranx hippos but also predate heavily on both molluscs (35.8%) and other elasmobranch species (4.21%). Other prominent prey items include octopodidae sp. (35.4%). These findings indicate a clear ontogenetic shift in lemon shark diet, as molluscs, specifically octopodidae sp., become more important in the diet of adults and sub-adults than in the diet of juveniles. Overall, the diet of adult and sub-adult lemon sharks is more evenly distributed across fish, molluscs and elasmobranchs whereas the diet of juveniles contains higher proportions of fish and crustaceans when compared with other prey, especially molluscs.
05:30 PM
THE PREDICTIVE POWER OF AQUATIC FOOD WEBS UNDER CHANGE: ATLANTIFICATION OF THE BARENTS SEA (8995)
Primary Presenter: Katalin Patonai, Université de Montréal (kati.patonai@gmail.com)
Food webs provide a snapshot of a community with potential insight into the spatial-temporal dynamics of the investigated community. The trophic structure of aquatic communities is changing with climate change. One aspect is atlantification, where species from the Atlantic Ocean move north (e.g. to the Barents Sea). We present a recent case study from the Barents Sea showcasing the predictive power of network analysis in better understanding the ecological transformations happening within these aquatic ecosystems. Food webs for the Barents Sea are assembled based on spatial and temporal data sets in relation to atlantification. Spatio-temporal subsystems are assessed at three levels: the identity of key organisms, critically important interactions, and the entire food web. Additionally, we propose two novel indices for identifying key species based on loop analysis. The indices quantify the role a species plays in passing the impacts to other species, and their role as sinks of the perturbations. The Barents Sea shows that the appearance of an invasive fish species altered the predominantly bottom-up control (pre-invasion) to mixed community effects (post-invasion). Food webs can provide insight and prediction into community dynamics using these novel indices.
05:45 PM
Geometric ecosystem complexity regulates food chains (9132)
Primary Presenter: Akira Terui, University of Tokyo (hanabi0111@gmail.com)
Since Charles Elton's time, ecologists have been intrigued by the variation in food chain length across natural communities. A widely held belief is that the size of an ecosystem significantly influences food chain length, known as the ecosystem size hypothesis. However, discussions around food webs have often overlooked the potential role of spatial ecosystem complexity—a geometric feature that operates independently of ecosystem size. In this study, we combine theoretical modeling and meta-analysis to show that ecosystem complexity, rather than size, is a key factor in regulating food chains in rivers. Our spatial theory suggests that the intricate branching of tributaries helps to buffer downstream areas from disturbances like floods, thus supporting longer food chains. However, our theory suggests a vague relationship between food chain length and ecosystem size (measured as the total river length within a watershed), a prediction that contrasts sharply with the ecosystem size hypothesis. A meta-analysis of global food chain data strongly confirmed these theoretical insights. The complexity of river networks consistently had a positive impact on riverine food chains worldwide, while the total river length showed no consistent relationship. This study offers a conceptual framework for understanding the structure of food webs in ecosystems with complex spatial configurations.
CS01 - Aquatic Food Webs
Description
Time: 4:30 PM
Date: 27/3/2025
Room: W201CD