Kimberly L. Schulz

Nutritional constraints in terrestrial and freshwater food webs

Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earths diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10–30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.

Zooplankton nutrition: recent progress and a reality check

Evidence suggests that marine and freshwater zooplankton generally experience food levels above subsistence values in terms of carbon. However, the quality of this food may be poor due to an insufficiency of other essential nutrients. In this review, we examine recent progress in three main areas of food quality research: (1) elemental (especially P) limitation, (2) digestion resistance, and (3) biochemical (especially fatty acids) limitation. We evaluate laboratory and field evidence in each of these areas, look at new evidence about the life history implications of the elemental limitation hypothesis, and suggest future avenues for research. From a rather large number of seemingly heterogeneous studies, a single consistent picture of food quality emerges: both P and essential fatty acids are predicted to be important dietary factors, but at different places and times. Nevertheless, despite an abundance of valuable laboratory studies, our knowledge of food quality limitation in the field is still poor.

Increased Benthic Algal Primary Production in Response to the Invasive Zebra Mussel (Dreissena polymorpha) in a Productive Ecosystem, Oneida Lake, New York

Increased water clarity associated with zebra mussel (Dreissena polymorpha) populations may favor benthic algal primary production in freshwater systems previously dominated by pelagic phytoplankton production. While zebra mussel-mediated water clarity effects on benthic primary production have been implicated in published reports, few production estimates are available. This study estimates benthic primary production in Oneida Lake, NY before and after zebra mussel invasion (1992), using measured photosynthetic parameters (, alpha(B) and beta) from sampled benthic algal communities. In the summers of 2003 and 2004, primary production was measured as O(2) evolution from algal communities on hard (cobble) and soft (sediment) substrate from several depths. We also backcast estimates of benthic primary production from measurements of light penetration since 1975. Estimates of whole-lake epipelic and epilithic algal primary production showed a significant (4%) increase and exhibited significantly less interannual variability subsequent to the establishment of zebra mussels. We applied our model to two lakes of differing trophic status; the model significantly overestimated benthic primary production in a hypereutrophic lake, but there was no significant difference between the actual and predicted primary production values in the oligotrophic lake. The hypereutrophic lake had higher zebra mussel densities than Oneida (224 vs. 41 per sample respectively). Though total community respiration (measured in total darkness) was factored into our model predictions of production, our model may need modification when heterotrophic respiration is a large portion of total community metabolism.

Toxicity of microcystin-LR, a cyanobacterial toxin, to multiple life stages of the burrowing mayfly, Hexagenia, and possible implications for recruitment

Burrowing mayflies, genus Hexagenia, were extirpated from the major water bodies of North America in the early 1950s, paralleling an increase in eutrophication and organic pollution, and a decrease in dissolved oxygen concentrations. Burrowing mayflies recolonized the western basin of Lake Erie, but remain absent in other former habitats such as Oneida Lake, New York. Eutrophication is commonly associated with a shift in the phytoplankton community toward dominance by cyanobacteria, and therefore, one class of cyanobacterial toxins, microcystins, were investigated as a contributing factor to Hexagenias eradication or as an impediment to recolonization. Laboratory experiments were conducted to determine if microcystin‐LR (MC‐LR) produced negative effects on Hexagenia at three points within its life cycle: egg, hatchling nymph (<24‐h old, <1 mm total length), and pre‐emergence nymph (>17 mm). Treatment concentrations ranged from the guideline set by the World Health Organization for drinking water (0.001 μg mL−1) to 0.1 μg mL−1 for the egg experiment and 10 μg mL−1 for the nymph trials. Eggs showed a delay in hatching and an altered distribution of hatching over the study period when submerged in 0.1 μg mL−1 MC‐LR (an elevated concentration representative of bloom scum). The 72‐h (1.1 μg mL−1) and 96‐h (0.049 μg mL−1) LC50 values for hatchling nymphs exceeded typical bloom concentrations of North American lakes, (0.01 μg mL−1). Large nymphs were more tolerant of the toxin, as indicated by 100% survival over seven days exposure to 10 μg mL−1, suggesting older larvae can withstand brief encounters with high microcystin levels for at least short periods of time. The sensitivity of young nymphs and eggs to MC‐LR may have implications for the recruitment of the genus in water bodies with persistent summer cyanobacterial blooms.

Joint variation of zooplankton and seston stoichiometry in lakes and reservoirs

Summary We tested several stoichiometric hypotheses for thejoint dependence in element ratios in zooplanktonand their resources using direct chemical measure-ments of zooplankton (>80 µm) and seston. Our fivefield sites included three small boreal lakes, a warmwater reservoir and Lake Superior. Zooplankton ele-ment pools were generally not large compared to thetotal seston. We found a tight homeostasis in bulkzooplankton element ratios compared to theirresources. Weak correlations consistent with stoichi-ometric hypotheses were seen. Zooplankton C:P andC:N were very slightly positively correlated withseston ratios, but the zooplankton N:P ratio wasnegatively correlated with the same ratio in theseston. Acknowledgements Financial support came from the National ScienceFoundation, NOAA, and the University of Minne-sota. Fig. 3. Expansion of the vertical scale of Fig. 2A.Horizontal dotted lines indicate reported values ofC:P for a high and a low C:P zooplankton taxon(from S TERNER et al. 1992). The wavy line is a dis-tance-weighted least squares regression. The solidinclined line represents an hypothetical region ofexclusion (high seston C:P precludes dominance bylow C:P zooplankton).

Initial genetic diversity enhances population establishment and alters genetic structuring of a newly established Daphnia metapopulation

When newly created habitats are initially colonized by genotypes with rapid population growth rates, later arriving colonists may be prevented from establishing. Although these priority effects have been documented in multiple systems, their duration may be influenced by the diversity of the founding population. We conducted a large‐scale field manipulation to investigate how initial clonal diversity influences temporal and landscape patterns of genetic structure in a developing metapopulation. Six genotypes of obligately asexual Daphnia pulex were stocked alone (no clonal diversity) or in combination (‘high’ clonal diversity) into newly created experimental woodland ponds. We also measured the population growth rate of all clones in the laboratory when raised on higher‐quality and lower‐quality resources. Our predictions were that in the 3 years following stocking, clonally diverse populations would be more likely to persist than nonclonally diverse populations and exhibit evidence for persistent founder effects. We expected that faster growing clones would be found in more pools and comprise a greater proportion of individuals genotyped from the landscape. Genetic composition, both locally and regionally, changed significantly following stocking. Six of 27 populations exhibited evidence for persistent founder effects, and populations stocked with ‘high’ clonal diversity were more likely to exhibit these effects than nonclonally diverse populations. Performance in the laboratory was not predictive of clonal persistence or overall dominance in the field. Hence, we conclude that although laboratory estimates of fitness did not fully explain metapopulation genetic structure, initial clonal diversity did enhance D. pulex population establishment and persistence in this system.

Zooplankton food quality in large lakes — growth of Daphnia on high P content seston from Lake Superior

Many recent studies have documented that laboratory cultures of phytoplankton with a low elemental phosphorus (P) content are poor food for the anomopod herbivore Daphnia (HESSEN 1992, URABE & WATANABE 1992, STERNER 1993, STERNER et al. 1993, URABE et al. 1997). Daphnia fed such low P content phytoplankton exhibit reduced growth, delayed age at first reproduction, and overall decreased fecundiry compared with Daphnia fed phytoplankton replete in phosphorus. To our knowledge, however, only one study has examined the growth of Daphnia on naturallake seston relative to seston P content (MACKAY & ELSER 1998).

Sediment and water quality limit mayfly survivorship in an urban lake undergoing remediation

Abstract Onondaga Lake is a highly perturbed system undergoing a large-scale remediation effort. The benthic macroinvertebrate community in the lake is degraded and lacks mayfly diversity. The objective of this study was to assess the effects of Onondaga Lake water and sediment on survivorship of indicator mayfly species to determine the potential for colonization before and after remediation. We performed a survivorship experiment in situ by incubating mayfly nymphs, Stenonema femoratum, (n = 20) that were given either Onondaga Lake or reference lake sediment in Onondaga Lake or the reference lake. We repeated this study in the laboratory using burrowing mayflies, Hexagenia spp. (n = 100). S. femoratum that were incubated in Onondaga Lake with Onondaga Lake sediment yielded 0% survival after 24 h. On higher quality substrate, survival after 24 h increased to 80%, indicating that Onondaga Lakes sediment significantly decreased survival and that survivorship probability after sediment remediation may be increased. Water quality also negatively affected survival, however. Even when incubated on higher quality sediment in Onondaga Lake, no mayflies survived for 9 d. Similarly, in the lab, survivorship of Hexagenia nymphs after 15 d was significantly lower (52%) when exposed to both Onondaga Lake sediment and water compared to nymphs given reference sediment and water (88% survival). Results suggest that areas of the lake not included in sediment remediation plans may still be unsuitable for colonization of more sensitive benthic taxa and that areas undergoing sediment remediation may still not be suitable for sensitive species if water quality does not improve.