Utilization of marsh and associated habitats along a salinity gradient in Galveston Bay

Nursery utilization of estuarine marshes by fishery species was studied in relation to salinity in Galveston Bay. The investigation revealed that effects of salinity on foods may explain distributions of fishery juveniles among marshes. Juvenile shrimp, crab and fish predators follow their prey, and environmental factors, such as the long-term effects of freshwater flows into an estuary, appeared to affect distributions and abundances of prey. Predator and prey abundances varied significantly among marshes in Galveston Bay, thus varying the nursery value of marshes to fishery juveniles. The highest numbers of penaeid shrimps, blue crab and commercial fishes were in marshes of the middle and lower bay. These abundances were associated with high abundances of benthic peracarid crustaceans (amphipods and tanaidaceans) which have been shown to be used as foods through feeding experiments and gut analyses. Other foods such as annelid worms and bivalve mollusks were less utilized and less related to distributions of fishery juveniles. This causeand-effect relationship may partly explains differences in utilization of marshes by fishery species. Habitats in upper Galveston Baywere dominated by long-term effects of low salinity from the Trinity and San Jacinto rivers. Marshes and submerged vegetation at the Trinity River delta were characterized by brackish water plants having highly seasonal growth patterns with complete winter defoliation. This environment was not favorable for development of populations of small estuarine invertebrates, nor to the growth of epiphytic algae. Infauna consisted of a few species of osmoconforming oligochaete worms and bivalve mollusks. Transient species such as juveniles of shrimps, crabs, and fishes had ready access to these marshes but did not use them extensively, despite their own abilities to osmoregulate. This lack of attractiveness was apparently due to the absence of preferred foods, especially epiphytic algae and peracarid crustaceans. Hence the value of upper bay ollgohallne marshes was not throug h direct utilization but was attributed to large quantities of organic detritus exported and utilized downstream. Organic detritus from the upper bay was an apparent energy source for food chains in the middle bay. Here, vascular plant detritus and reformed particles from dissolved organics, colonized and conditioned by bacteria and fungi, provided a nitrogen rich food resource for epibenthic detritivores. Stimulated by favorable food and salinity conditions, large numbers of eplbenthic fauna developed (as indicated by abundances of peracarids and annelids), providing a rich feeding ground in both marsh and subtidal habitats. Importantly, this mid-bay region was the frontal zone where nutrients from the upper bay mixed with immigrating recruits from the lower bay. Detritus in the lower bay was important, but perhaps to a lesser extent than in the middle bay. Reduced turbidities and marine salinities of the lower bay fostered the development of epiphytic algae and grazers as another carbon source. Spartina marshes in the lower bay persisted yeararound and were regularly inundated by marine waters, thus offering perennial substrata for eplphytecolonization. Young shrimp, crabsand fishes were more abundant in marsh habitats, compared to bare subtidal habitat, in the lower bay than in the middle and upper bay. These collective findings revealed how marshes of the Galveston Bay system are utilized by consumers. low salinity (oligohaline) marshes in the upper bay (especially at the Trinity Delta) exported large amounts of organic material to the middle bay. The plants of the river delta defoliate each winter and the entire standing crop is exported downstream. Enriched plant detritus in the middle system increases the productivity of epibenthic detritus feeders (such as peracarid crustaceans) and these were foraged by juveniles of commercially valuable fishes, shrimps and crabs. Because both the marsh and subtidal bottom In the middle bay had high abundances of forage organisms, the entire area was valuable nursery habitat. The moderate influence of mesohaline to polyhaline salinities in the middle bay also encouraged utilization by consumers. In the lower bay, algal carbon was another base for secondary productivity in marsh and seagrass habitats heavily epiphytized by algae. Finally, the interconnections between the different systems of the bay appeared to be critical to maintaining overall fishery productivity.