Diversity, seasonal abundance, and environmental drivers of chaetognath populations in North Inlet Estuary, South Carolina, USA.

Chaetognaths (Phylum: Chaetognatha) are one of the most abundant phyla of zooplankton worldwide and play an important role in marine trophic interactions. Although the role of chaetognaths in global ecosystems is well understood, the spatial variation and environmental drivers of estuarine chaetognath populations is poorly understood. To provide the first known record of chaetognath species composition in a coastal estuary in the south-eastern USA, chaetognaths were identified and quantified from zooplankton samples collected on a monthly basis in 2019 and 2020 from North Inlet Estuary in South Carolina. Parasagitta tenuis was the most abundant species of the five found, making up 33% of total abundance. The egg presence of these chaetognaths was further analyzed to gauge reproductive cycles. Abundance and egg presence were compared with surface and bottom measurements of temperature, salinity, and dissolved oxygen levels to determine the driving abiotic factors behind chaetognath's seasonal variability and reproductive cycles. Temperature, salinity, and dissolved oxygen all had low (r < ±.29), non-significant correlations with abundance. Chaetognath egg production was most significantly associated with dissolved oxygen (p < .001) and seasonal changes in temperature (p < .001). Our initial findings indicate the continued abundance of chaetognath in a local estuary are dependent on abiotic factors that are strongly influenced by a changing climate.

Cyanobacterial blooms modify food web structure and interactions in western Lake Erie.

With anthropogenic eutrophication and climate change causing an increase in cyanobacterial blooms worldwide, the need to understand the consequences of these blooms on aquatic ecosystems is paramount. Key questions remain unanswered with respect to how cyanobacteria blooms affect the structure of aquatic food webs, the foraging abilities of higher consumers, and the potential for cyanotoxins (e.g., microcystins [MCs]) to accumulate in fish. Toward addressing these uncertainties, physicochemical attributes, water (for MCs), phytoplankton, zooplankton, and epipelagic and benthic age-0 fish were sampled at 75 sites (44 sites for fish) of varying cyanobacteria concentration (0.1-44 µg/L) in western Lake Erie during the cyanobacteria bloom season, 2013-2014. Sites with high cyanobacteria biomass were characterized by Microcystis spp. (84-100% of biomass), detectible levels of MCs (maximum = 10.8 µg/L), and low water transparency (minimum = 0.25 m). Counter to expectations, strong positive relationships were found between cyanobacteria concentration and the biomass of several herbivorous zooplankton taxa (e.g., Daphnia, Diaphanosoma spp., Bosmina (formerly Eubosmina) coregoni, and Calanoida spp.). Expectations regarding fish were partly supported (e.g., diet selectivity varied across a cyanobacteria gradient) and partly not (e.g., consumption of zooplankton did not differ between bloom and non-bloom sites). These findings show that cyanobacterial blooms can strongly affect the distribution, composition, and interactions of zooplankton and fish, sometimes in surprising ways, highlighting the need to further explore their impact on aquatic food webs.

Physiological effects of diet mixing on consumer fitness: a meta-analysis.

The degree of dietary generalism among consumers has important consequences for population, community, and ecosystem processes, yet the effects on consumer fitness of mixing food types have not been examined comprehensively. We conducted a meta-analysis of 161 peer-reviewed studies reporting 493 experimental manipulations of prey diversity to test whether diet mixing enhances consumer fitness based on the intrinsic nutritional quality of foods and consumer physiology. Averaged across studies, mixed diets conferred significantly higher fitness than the average of single-species diets, but not the best single prey species. More than half of individual experiments, however, showed maximal growth and reproduction on mixed diets, consistent with the predicted benefits of a balanced diet. Mixed diets including chemically defended prey were no better than the average prey type, opposing the prediction that a diverse diet dilutes toxins. Finally, mixed-model analysis showed that the effect of diet mixing was stronger for herbivores than for higher trophic levels. The generally weak evidence for the nutritional benefits of diet mixing in these primarily laboratory experiments suggests that diet generalism is not strongly favored by the inherent physiological benefits of mixing food types, but is more likely driven by ecological and environmental influences on consumer foraging.