Regional Surface Turbidity and Hydrographic Variability on the South Texas Continental Shelf

Abstract

ABSTRACT Regional surface-water turbidity patterns and associated hydrography were monitored on the South Texas Continental Shelf during an 18-month period (November 1975-May 1977) as part of an environmental study. On six monitoring cruises, quasi-synoptic surface measurements were made of water transmissivity, suspended-sediment concentrations, temperature, salinity, and drifter trajectories. Time-sequence patterns of these parameters illustrate substantial spatial and temporal variability; temporal variations take place at both the seasonal and annual time scales. Turbidity and hydrographic patterns suggest a surface-sediment dispersal system regulated by a lateral shelf-water exchange process, driven by both wind and thermohaline forcing agents. Relatively turbid inner-shelf waters reflect the offshore and alongshore transport of sediment derived from coastal sources. Turbidity patterns along the tuner shelf are characterized by a regional gradient of shoreward-increasing turbidity with superimposed local gradients established at major tidal inlets that serve as prominent sediment point sources and dispersal centers. Turbidity variability along the inner shelf is jointly attributed to variations in coastal runoff, wind-driven currents, and relative flux from individual tidal inlets. Relatively nonturbid outer-shelf waters suggest the shelfward incursion of an open-ocean water mass regulated by deep-Gulf circulation; the extent of incursion appears to vary spatially and temporally, resulting in outer-shelf turbidity variations. The overall shelf turbidity patterns reflect the degree of lateral interchange between the gulfward movement of turbid inner-shelf waters and the shelfward incursion of clear open-ocean waters.