Marine habitat dynamics: A case study of habitat suitability for a temperate flatfish (winter flounder, <i>Pseudopleuronectes americanus</i>, Walbaum) on an estuarine nursery ground
Estuaries throughout the world serve nurseries for many fish and invertebrates. Biotic and abiotic factors affecting critical early life history processes are extremely variable in space and time in temperate estuaries where nursery habitat suitability is likely be spatially dynamic. A series of mensurative experiments was performed throughout a New Jersey USA estuary to describe the effects of habitat dynamics on larval settlement, early juvenile growth, and juvenile predation risk for a model flatfish; winter flounder, Pseudopleuronectes americanus. All three early life history processes were spatially dynamic because of the effects of physico-chemical gradients variable at coarse spatial (≈10 km) but fine temporal scales (days-weeks), and factors nested within coarse scale spatial gradients that varied at fine spatial (≤1 km) but fine to coarse temporal scales (days-decades). Specifically, larvae settled earlier on depositional substrata located upstream in the estuary where waters warmed more rapidly than downstream areas during the spring. Early juvenile growth was also most rapid at upstream sites where prey were abundant and optimal temperatures and salinities coincided in space for a limited period following settlement. Sizes of juveniles were consistently larger upstream than downstream as a result of the dynamics of habitat suitability for settlement and growth. Comparison of settlement and juvenile distributions indicated that post-settlement processes modified settlement more slowly in upstream areas. Predation studies demonstrated that predation risk was, indeed, low in shallow structurally complex habitats with salinities lower than those preferred by dominant fish predators and typical of upstream habitats. Thus, during most years, habitat effects on settlement, growth and predation risk produced a high quality nursery upstream in the estuary. This finding was consistent with upstream juvenile distributions measured in estuaries throughout the species range. However, abnormally warm and dry conditions in the spring appeared to cause the volume of the upstream predation refuge to contract and high mortality in the primary nursery. This result suggests that climatic forcing which controls the structure of physico-chemical gradients and the location and volume of habitats optimal for survivorship and other critical processes probably regulates local year class strength in estuaries.