Cheryl L. Podemski

Whole-ecosystem study shows rapid fish-mercury response to changes in mercury deposition

Methylmercury contamination of fisheries from centuries of industrial atmospheric emissions negatively impacts humans and wildlife worldwide. The response of fish methylmercury concentrations to changes in mercury deposition has been difficult to establish because sediments/soils contain large pools of historical contamination, and many factors in addition to deposition affect fish mercury. To test directly the response of fish contamination to changing mercury deposition, we conducted a whole-ecosystem experiment, increasing the mercury load to a lake and its watershed by the addition of enriched stable mercury isotopes. The isotopes allowed us to distinguish between experimentally applied mercury and mercury already present in the ecosystem and to examine bioaccumulation of mercury deposited to different parts of the watershed. Fish methylmercury concentrations responded rapidly to changes in mercury deposition over the first 3 years of study. Essentially all of the increase in fish methylmercury concentrations came from mercury deposited directly to the lake surface. In contrast, <1% of the mercury isotope deposited to the watershed was exported to the lake. Steady state was not reached within 3 years. Lake mercury isotope concentrations were still rising in lake biota, and watershed mercury isotope exports to the lake were increasing slowly. Therefore, we predict that mercury emissions reductions will yield rapid (years) reductions in fish methylmercury concentrations and will yield concomitant reductions in risk. However, a full response will be delayed by the gradual export of mercury stored in watersheds. The rate of response will vary among lakes depending on the relative surface areas of water and watershed.

Direct and indirect responses of a freshwater food web to a potent synthetic oestrogen

Endocrine-disrupting chemicals (EDCs) in municipal effluents directly affect the sexual development and reproductive success of fishes, but indirect effects on invertebrate prey or fish predators through reduced predation or prey availability, respectively, are unknown. At the Experimental Lakes Area in northwestern Ontario, Canada, a long-term, whole-lake experiment was conducted using a before-after-control-impact design to determine both direct and indirect effects of the synthetic oestrogen used in the birth control pill, 17α-ethynyloestradiol (EE2). Algal, microbial, zooplankton and benthic invertebrate communities showed no declines in abundance during three summers of EE2 additions (5–6 ng l−1), indicating no direct toxic effects. Recruitment of fathead minnow (Pimephales promelas) failed, leading to a near-extirpation of this species both 2 years during (young-of-year, YOY) and 2 years following (adults and YOY) EE2 additions. Body condition of male lake trout (Salvelinus namaycush) and male and female white sucker (Catostomus commersonii) declined before changes in prey abundance, suggesting direct effects of EE2 on this endpoint. Evidence of indirect effects of EE2 was also observed. Increases in zooplankton, Chaoborus, and emerging insects were observed after 2 or 3 years of EE2 additions, strongly suggesting indirect effects mediated through the reduced abundance of several small-bodied fishes. Biomass of top predator lake trout declined by 23–42% during and after EE2 additions, most probably an indirect effect from the loss of its prey species, the fathead minnow and slimy sculpin (Cottus cognatus). Our results demonstrate that small-scale studies focusing solely on direct effects are likely to underestimate the true environmental impacts of oestrogens in municipal wastewaters and provide further evidence of the value of whole-ecosystem experiments for understanding indirect effects of EDCs and other aquatic stressors.

ASSIMILATION OF FRESHWATER SALMONID AQUACULTURE WASTE BY NATIVE AQUATIC BIOTA

An experimental finfish aquaculture farm was operated in a small lake at the Experimental Lakes Area in northwestern Ontario, Canada. In this study, we used the distinct and enriched carbon and nitrogen isotope ratios (δ13C and δ15N) of rainbow trout (Oncorhynchus mykiss) feed and waste to determine whether the operation provided a novel source of energy for native biota. For 1 year prior to and for 3 years during the cage culture, we collected littoral, pelagic, and profundal invertebrates and minnows from the experimental and reference lakes. In both the second and third years of aquaculture, there was a significant increase in δ15N of all organisms sampled in the experimental lake; mean δ15N values of littoral, pelagic, and profundal invertebrates and minnows shifted towards the signature of the fish feed by up to 4.2‰. Significant increases in δ13C of up to 2.6‰ were observed in Mysis, profundal chironomids, and minnows but not in littoral invertebrates or zooplankton. Aquaculture waste became a progr...

Freshwater rainbow trout (Oncorhynchus mykiss) farming affects sediment and pore-water chemistry

Marine aquaculture has come under scrutiny, whereas little is known about the nature and extent of the effects of cage aquaculture in freshwater. We describe the development of changes in sediment and pore-water chemistry caused by an experimental Oncorhynchus mykiss farm located in freshwater Lake 375 (Experimental Lakes Area, north-western Ontario, Canada) during its first two production cycles along a distance transect from the cage. Significant changes in sediment chemistry were quick to develop; pore-water ammonia was elevated under the cage after 1 month and sediment nutrients were elevated 1 month later. The effects on the benthic environment were spatially localised, although variables responded to different extents. Within 16 months, nutrient concentrations in surface sediment reached an asymptote, whereas concentrations of metals and ammonia continued to increase. Copper (Cu) and zinc (Zn) concentrations under the cage reached levels that exceed sediment quality guidelines and may cause adverse biological impacts. An anti-fouling coating applied to the net pen was the major source of Cu, whereas Zn originated also from fish feed. Ammonia and pH are recommended for inclusion in monitoring programs because they were sensitive to fish farming and are biologically relevant. We also suggest inclusion of Cu and Zn in monitoring programs at farms with treated nets.

Incorporation of wastes by native species during and after an experimental aquaculture operation

Freshwater aquaculture increases dissolved and particulate nutrients near fish cages, but the degree to which they are incorporated into tissues of native animals is uncertain. At the Experimental Lakes Area in northwestern Ontario, Canada, a cage culture of Rainbow Trout was operated seasonally for 5 y, and invertebrates and fishes were collected before, during, and after culturing from the experimental lake and a reference lake to assess changes in stable isotopes of C and N in their tissues. The feed contained marine fishmeal and was higher in C and N isotope values (δ13C and δ15N) than lake biota by ≥4‰ (all taxa) and 3‰ (all invertebrates), respectively. During the aquaculture operation, δ15N of littoral and pelagic invertebrates, profundal chironomids, minnows, and Lake Trout (Salvelinus namaycush) increased by 2 to 5‰ relative to before aquaculture values. In the 1st and 2nd years after aquaculture, δ15N of several invertebrate taxa and all fishes continued to increase 1 to 2‰/y. In contrast, during aquaculture, only minnows and trout had significant increases (up to 3‰) in δ13C. In the period after aquaculture, the δ13C of fishes, plankton, and profundal chironomids declined to below values measured before or during aquaculture. Isotopic analysis of native biota can be used to monitor assimilation of cage culture wastes in freshwater ecosystems.