MACROBENTHIC MONITORING IN THE LOWER ST. JOHNS RIVER, FLORIDA

The macrobenthos in the oligohaline 132 km reach of the Lower St. John s River, Florida, is an unusual blend of freshwater and marine organis ms within the annelid, mollusk, aquatic insect, and crustacean groups. During 1993-1994, the community composition was freshwater-oriented i n the 47 km (seg I) and estuarine-dominated in the lower 85 km (seg II ). Of the total 146 taxa collected, 89% were euryecious 'eutrophic' an d 'pollution-tolerant' organisms. Densities ran;Sed between 5000 and 2 0 000 individuals/m(2) ann av and maximum densities (85 000 individual s/m(2)) occurred in the muck substrate at the confluence of major trib utaries and embayments. Throughout the lake-like seg LI, benthic hypox ia existed during much of summer through fall. Biotic index values for grab, artificial substrate, and sled trawl samples reflected greatest stresses to the macrobenthos then. Both low taxonomic diversity and d ensities of organisms at the most downriver stations nearest the port of Jacksonville indicated that toxic substances also adversely affecte d the macrobenthos. Maintenance of the narrow band of littoral submerg ed aquatic vegetation (SAV) and adjacent shoreline riparian vegetation is important to sustain macrobenthic communities and other kinds of a quatic life in the river.