Maternal provisioning of embryos in Gambusia (Poeciliidae) entails both production of large, yolky eggs and mother-to-embryo transfer of nutrients, the latter of which is readily quantified using injection of radiolabeled nutrients. We assayed patterns of nutrient transfer in broods of 26 Gambusia geiseri and 23 Gambusia affinis females, using injection of tritiated leucine. We examined maternal and embryo characteristics affecting the instantaneous rate of transfer and characterized the pattern of transfer to individual embryos within broods. Maternal (female size and condition) and brood characteristics (mean embryo size, developmental stage, brood size) did not predict the mean level of nutrient transfer to embryos in a brood for either species. Within broods, individual provisioning of embryos was not related to developmental stage, but was related to embryo mass in G. affinis with nutrient transfer higher to larger embryos. In addition, overall within-brood variation in nutrient transfer, measured as coefficient of variation in embryo radioactivity, was higher in G. affinis than in G. geiseri.

We developed 4 simple numerical models of plankton dynamics to explore how nutrient enrichment and habitat variability might influence the efficiency by which phytoplankton (P) production is transferred to growth of zooplankton (Z) consumers in coastal ecosystems. The 4 models range in complexity from 2 (P and Z) to 5 state variables (including detritus, nutrients, and 2 algal size-groups). The models employ generic equation formulations, which are generally well supported by empirical studies and are widely used in coastal ecosystem modeling. Simulation experiments revealed that trophic transfer efficiency (TTE = zooplankton growth per unit phytoplankton production) tends to be enhanced with increased variability of resources, particularly at low nutrient levels. Numerical and analytical studies also showed that, regardless of resource variability, these model formulations produce a trend of initial enhancement of trophic efficiency with increasing nutrient levels, followed by a marked reduction in efficiency beginning at moderately eutrophic conditions. This precipitous drop in trophic efficiency is attributable to a saturation of the ability of zooplankton to utilize the increased primary production associated with nutrient enrichment. Under these conditions, an increasing fraction of the primary production is shunted to microbial food chains and associated respiratory losses. The steepness of this reduction in trophic efficiency with nutrient enrichment is related to the strength of predation (or disease) control at upper trophic levels. Model formulations simulating more intense top-down control (i.e. increasing mortality rates with increasing Z abundance) resulted in sharper declines in TTE with increasing nutrients. We speculate that these model results may help to explain how observed reductions in relative fish yield (per unit primary production) in many shallow nutrient-enriched estuaries and lakes are related to interacting effects of cultural eutrophication and intense fisheries exploitation. Furthermore, we surmise that these relationships are robust characteristics of most existing aquatic ecosystem models.

1. We tested the hypothesis that indirect food web interactions between some common, invertivorous fishes and their prey would positively affect growth of an algivorous fish species. Specifically, we predicted that orangethroat darter (Etheostoma spectabile) would increase periphyton biomass via a top-down pathway, indirectly enhancing growth of the algivorous central stoneroller minnow (Campostoma anomalum). Moreover, we predicted that sand shiner (Notropis stramineus) would increase periphyton biomass via a bottom-up pathway and indirectly enhance growth of the stoneroller minnow. 2. In an 83-day experiment in large, outdoor, stream mesocosms, we stocked two fish species per mesocosm (stoneroller and either darter or shiner), estimated the effects of the invertivorous and grazing fishes on periphyton biomass and estimated growth of the algivorous fish. 3. The darter consumed grazing invertebrates, indirectly increasing periphyton biomass. The shiner consumed terrestrial insects as predicted, but it did not affect periphyton biomass. 4. In support of our hypothesis, the darter indirectly enhanced stoneroller growth. As predicted, stonerollers consumed the increased periphyton in streams with darters, resulting in greater growth, condition and gut fullness compared to streams without darters. No indirect interaction was observed between stonerollers and shiners. 5. Our study suggests that some invertivorous fish species can positively affect growth of algivorous fishes through indirect food web interactions. Thus, in stream communities, it is possible that the loss of a single, invertivorous fish taxon could have negative consequences on algivorous fish populations via the removal of positive indirect food web interactions.