Small-scale patterns of nekton use among marsh and adjacent shallow nonvegetated areas of the Galveston Bay Estuary, Texas (USA)

We quantified and compared nekton and infaunal densities among vegetated (e dge Spartina alterniflora, inner Spartina alterniflora, Scirpus maritimus, Juncus roemerianus, and Spartina patens marsh) and shallow nonvegetated (ma rsh pond, marsh channel, cove, and shallow bay) areas of upper Galveston Ba y and East Bay, Texas. in 2 seasons (spring and fall) of high nekton abunda nce, and over 2 yr, we collected 267 quantitative samples (upper Galveston Bay, 1993 = 127 and East Bay, 1994 = 140) using a 1 m(2) drop sampler. The vegetated marsh surface consistently contained more species (i.e, higher sp ecies richness) and total numbers of decapod crustaceans than nonvegetated areas. In contrast, fish species richness and densities of total fishes on the marsh and in nonvegetated areas were not significantly different in mos t comparisons. Most numerically dominant species of nekton seemed to exhibi t at least some degree of habitat selection. Within vegetation, 2 factors, elevation and proximity to open water, were most important in influencing t he distribution of nekton. Low marsh edge dominated by Spartina alterniflor a or Scirpus maritimus was apparently selected by most species that used th e marsh surface including brown shrimp Farfantepenaeus aztecus, blue crab C allinectes sapidus, and daggerblade grass shrimp Palaemonetes pugio. White shrimp Litopenaeus setiferus and striped mullet Mugil cephalus also were co ncentrated in low edge marsh; although in one comparison, densities of thes e 2 species in edge and inner S, alterniflora were not significantly differ ent. In contrast, gulf killifish Fundulus grandis and sheepshead minnow Cyp rinodon variegatus were most abundant on inner S. alterniflora or S, patens marsh. Other fishes (gulf menhaden Brevoortia patronus, spot Leiostomus xa nthurus, bay anchovy Anchoa mitchilli, blackcheek tonguefish Symphurus plag iusa, and Atlantic croaker Micropogonias undulatus) had higher densities ov er nonvegetated bottoms than on the marsh surface. Specific habitat types t hat these pelagic species seemed to favor were marsh channels (gulf menhade n, bay anchovy), marsh ponds (spot), and coves (Atlantic croaker, blackchee k tonguefish). Overall, marsh-surface and adjacent nonvegetated habitat typ es contained much higher densities of most nekton than the shallow bay. Inf aunal densities were estimated from sediment cores, and taxa (mainly anneli ds, crustaceans, molluscs, and insects) were most abundant in nonvegetated areas contiguous with marsh in the spring. Factors that influenced infaunal abundance are complex and may include predation, flooding patterns, elevat ion, and distance to edge. Our study has important implications for designi ng marsh-creation projects. Based on our results, we recommend creating a v ariety of marsh and contiguous shallow-water areas to enhance nekton biodiv ersity. To maximize fishery habitat, priority should be given to constructi ng low marsh edge by creating large areas of low marsh interspersed with a dense network of shallow channels and interconnected ponds.