Study of the Effects of Fresh Water on the Plankton, Benthos, and Nekton Assemblages of the Lavaca Bay system, Texas
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This study was typified by above-normal freshwater inflow. The average inflow into the Lavaca Bay system during this 30-month study (1944 cfs) was about 59% above normal (1,254 cfs). Inflow was greater than 4000 cfs during 10% (3months) of the study and daily inflow ranged from 100 to 94,949 cfs. A total of 156 phytoplankton taxa representing 6 divisions were identified. Phytoplankton taxa diversities generally declined with increasing freshwater inflow. Minimum phytoplankton density was associated with high river discharge (above 2000 cfs) while maximum standing crops occurred with blooms of small microflagellates and diatoms as the bay salinity began to stabilize afte rhigh inflow. Chlorophyll a values seemed negatively related to high river discharge (above 2000 cfs). A total of 201 zooplankton taxa representing 14 phyla wer identified. Barnacle nauplii, Acartia tonsa, and Oithona spp. populations comprised 80% of the total zooplankton standing crop. No significant correlations were found between zooplankton standing crops or taxa diversity and freshwater inflow; however, taxa diversity generally increased when river inflow increased to above 2000 cfs. Standing crops were inversely related to water temperature and directly related to salinity. A total of 169 benthos taxa representing 9 phyla were identified. The numerically most abundant benthos taxa included Littoridina sphinctostoma, Mediomastus californiensis, Rangia cuneata, Mulinia lateralis, and Streblospio benedicti. Benthos taxa diversity was positively related to bottom salinity and negatively related to bottom turbidity and nutrients. Taxa diversity declined from the high salinity lower bay to the low salinity upper bay and river areas. Benthos standing crops were not significantly correlated to freshwater inflow; however, standing crops were related to salinity, turbidity, total carbon, organic nitrogen, and nitrate. No relation between benthos populations and bottom sediment types was found. Benthos populations were generally lowest at dredged channel sites. A total of 70 nekton taxa representing 3 phyla were identified from trawl samples. The five numerically dominant species were Anchoa mitchilli, Micropogon undulatus, Brevoortia patronus, Penaeus setiferusm and Leiostomus xanthurus. Nekton populations appeared to be affected more by water temperature than by freshwater inflow.