Dina M. Leech

Warming and Resource Availability Shift Food Web Structure and Metabolism

Experimental warming of a marine food web suggests that ocean warming can lead to greater consumer abundance but reduced overall biomass, providing a potentially species-independent response to environmental warming.

IS TOLERANCE TO UV RADIATION IN ZOOPLANKTON RELATED TO BODY SIZE, TAXON, OR LAKE TRANSPARENCY?

Solar ultraviolet radiation (UVR) has been demonstrated to have damaging effects on zooplankton, but little is known about what factors influence UVR tolerance in nature. Here we examined the relationship between UVR tolerance (the sum of photopro- tection and photorepair processes) and zooplankton taxon, body size, and source lake UVR transparency. Zooplankton of various sizes and taxa from lakes of different UV transparency were exposed to different intensities of a constant artificial UVR source. UVR tolerance was expressed as the UVR dose at which 50% mortality was observed for a given species. Smaller zooplankton species showed a uniformly high UVR tolerance, while larger zoo- plankton varied in their UVR tolerance both among and within species. The smaller rotifers, Keratella in particular, showed a high UVR tolerance while the larger, more transparent rotifer (Asplanchna) showed an intermediate UVR tolerance. Both cyclopoid and calanoid copepod adults were more highly tolerant of UVR than nauplii. Late-instar larvae of the predatory insect Chaoborus were more UVR tolerant than earlier instars. UVR tolerance showed no relationship to the UVR transparency of the source lake. Differential UVR tolerances among zooplankton taxa may alter community and ecosystem structure and function during anticipated changes in underwater UVR environments.

Natural organic matter and sunlight accelerate the degradation of 17ß-estradiol in water

Nanomolar concentrations of steroid hormones such as 17beta-estradiol can influence the reproductive development and sex ratios of invertebrate and vertebrate populations. Thus their release into surface and ground waters from wastewater facilities and agricultural applications of animal waste is of environmental concern. Many of these compounds are chromophoric and susceptible to photolytic degradation. High intensity UV-C radiation has been demonstrated to degrade some of these compounds in engineered systems. However, the degradation efficacy of natural solar radiation in shallow fresh waters is less understood. Here photolytic experiments with 17beta-estradiol demonstrated modest photodegradation (~26%) when exposed to simulated sunlight between 290 and 720 nm. Photodegradation significantly increased (~40-50%) in the presence of 2.0-15.0 mg/l of dissolved organic carbon (DOC) derived from humic acids of the Suwannee River, GA. However, rates of photodegradation reached a threshold at approximately 5.0 mg/l DOC. Observed suppression of photolysis in the presence of a radical inhibitor (i.e. 2-propanol) indicated that a significant proportion of the degradation was due to radicals formed from the photolysis of DOC. Although photodegradation was greatest in full sunlight containing UV-B (290-320 nm), degradation was also detected with UV-A (320-400 nm) and visible light (400-720 nm) alone.

Effects of nutrients and zooplankton on an estuary's phytoplankton: inferences from a synthesis of 30 years of data

Phytoplankton biomass is commonly used as a water quality metric in the management of anthropogenic nitrogen and phosphorus loading, yet interpretation of the long-term response of phytoplankton biomass to nutrient regulation may be confounded by changes in zooplankton grazing pressure. Zooplankton community structure may be affected by planktivorous fish whose populations are subject to losses through fishing and gains through restoration efforts. We investigated temporal changes in phytoplankton biomass, nitrogen and phosphorus concentrations, zooplankton, and planktivorous fish in the tidal fresh and oligohaline Chowan River, NC, over a 30-year period in order to compare long-term trends in these parameters and infer their relative influence on phytoplankton biomass. Data were compiled from the records of two state agencies and several academic studies and supplemented by our own two year monitoring of water quality and zooplankton in the Chowan River mainstem and two tributaries. Seasonal trend decomposition using locally weighted regression was applied to chlorophyll a (a proxy for phytoplankton biomass), nitrogen, and phosphorus, and showed that chlorophyll a declined more quickly than did nutrient concentrations over the 30-year period. Despite the long-term decline in nitrogen and to a lesser extent phosphorus, the long-term trend in phytoplankton growth rate (predicted using an empirical model involving irradiance and nutrients) remained relatively constant. Zooplankton abundance increased from the period 1981–1982 to 2008–2010, as did the predicted zooplankton community water clearance rate (a proxy for zooplankton grazing). River herring, historically the dominant planktivore, declined dramatically over the 30-year period. The data indicate that the most parsimonious explanation for the long term decrease in chlorophyll a is an increase in zooplankton and not a decrease in nutrients. This inference leads us to hypothesize that the historically large river herring population exerted a positive, indirect influence on phytoplankton biomass in the Chowan River system, and that the decline in river herring has lead to higher zooplankton abundance and subsequently lower phytoplankton biomass.

Alterations in the photomineralization of allochthonous DOM related to elevated atmospheric CO2

Abstract Climate-related shifts in forest composition and chemistry will likely affect the quantity and quality of dissolved organic matter (DOM) entering inland waters, and consequently, carbon dioxide (CO2) evasion. We examined the photodegradation of DOM derived from 2 common riparian plant species (Populus tremuloides and Salix alba) grown at ambient (360 ppm) and elevated (720 ppm) atmospheric CO2 concentrations. Rates and total photolytic CO2 production were determined for sterilized leachates ranging from 5 to 100 mg L−1 dissolved organic carbon (DOC). Based on multiple regression analysis, DOC concentration, followed by plant species, best predicted the rate and total flux of CO2. Photolytic CO2 production increased linearly with DOC concentration; however, the 5 mg L−1 treatment had the greatest rate per unit carbon, suggesting a self-shading effect of increasing DOC. The atmospheric CO2 conditions under which the plants were grown had no statistically significant effect, despite observed differences in CO2 fluxes between ambient and elevated Populus leachates. Fluorescence data suggest differences in photolytic CO2 production among treatments are related to differences in plant chemistry within the humic fraction. Thus, the magnitude of the photolytic CO2 flux from fresh waters in the future will depend primarily on climate-related changes in the quantity of terrestrial DOM inputs and secondarily by DOM source.