Patterns of Hg bioaccumulation and transfer in aquatic food webs across multi-lake studies in the northeast US.

The northeastern USA receives some of the highest levels of atmospheric mercury deposition of any region in North America. Moreover, fish from many lakes in this region carry Hg burdens that present health risks to both human and wildlife consumers. The overarching goal of this study was to identify the attributes of lakes in this region that are most likely associated with high Hg burdens in fish. To accomplish this, we compared data collected in four separate multi-lake studies. Correlations among Hg in fish (4 studies) or in zooplankton and fish (2 studies) and numerous chemical, physical, land use, and ecological variables were compared across more than 150 lakes. The analysis produced three general findings. First, the most important predictors of Hg burdens in fish were similar among datasets. As found in past studies, key chemical covariates (e.g., pH, acid neutralizing capacity, and SO4) were negatively correlated with Hg bioaccumulation in the biota. However, negative correlations with several parameters that have not been previously identified (e.g., human land use variables and zooplankton density) were also found to be equally important predictors. Second, certain predictors were unique to individual datasets and differences in lake population characteristics, sampling protocols, and fish species in each study likely explained some of the contrasting results that we found in the analyses. Third, lakes with high rates of Hg bioaccumulation and trophic transfer have low pH and low productivity with relatively undisturbed watersheds suggesting that atmospheric deposition of Hg is the dominant or sole source of input. This study highlights several fundamental complexities when comparing datasets over different environmental conditions but also underscores the utility of such comparisons for revealing key drivers of Hg trophic transfer among different types of lakes.

Prey selection by the copepod Diacyclops thomasi.

1. Adult females of the predaceous copepod, Diacyclops thomasi, consistently selected for the soft-bodied rotifers Synchaeta pectinata, Polyarthra major and P. remata when presented various combinations of 8 rotifer species and 2 crustacean species as prey. Diacyclops did not select for other small, soft-bodied rotifers such as P. vulgaris and Ascomorpha ecaudis and, for loricate species such as Keratella cochlearis, K. crassa and for large soft-bodied adult Asplanchna priodonta. The small cladocerans, Bosmina longirostris and Chydorus sphaericus also were resistant to predation by this copepod. 2. Increased hunger in Diacyclops increased the clearance rates on both vulnerable and Diacyclops-resistant prey but did not greatly increase mortality of resistant prey relative to vulnerable prey. Sated Diacyclops preferred small, vulnerable prey like P. major over larger-bodied Synchaeta. This effect may be attributed to limited gut space when food is abundant. 3. When Diacyclops was presented different relative proportions of Keratella and Synchaeta at a constant total prey density (500 prey/L), it selected Synchaeta over Keratella in all trial proportions. However, Diacyclops selected more strongly for Keratella (but at a much lower clearance rate than for Synchaeta) when the relative abundance of this predator-resistant species was greatest. These results support optimal foraging in this predator. 4. Predator-prey interactions of the kind reported in this study can help identify important food web pathways and can be used to interpret predator-mediated changes in zooplankton communities in mature.

Body size, ration level, and population growth in Asplanchna.

1.) The daily ration required to maintain a population growth rate, r m, of zero (threshold ration) increased with increasing Asplanchna body mass. This relationship is described by the equation T=0.342 W0.797 where T=threshold ration (µg day-1 dry mass) and W=Asplanchna body mass (µg adult-1 dry mass). 2.) The threshold ration of large campanulate morphs of A. silvestrii was 3.7 times greater than that of conspecific saccate morphs suggesting that campanulates may be restricted to food-rich habitats. 3.) The daily ration required to maintain r m that is half the maximal population growth rate increased with increasing Asplanchna body mass and is described by the equation H=1.107 W1.103 where H=ration level and W=Asplanchna body mass. This population growth characteristic may reflect adaptations of rotifers to resource level. 4.) The relationships between ration level, food concentration, and Asplanchna body mass do not support the predictions of the size-efficiency hypothesis but are consistent with observed patterns of species distribution in nature.