Zones of coastal hypoxia revealed by satellite scanning have implications for strategic fishing

Abstract

Hypoxic (oxygen concentrations <2.5mg l⁻¹) and occasionally anoxic bottom waters occur along the inner continental shelf of Texas and Louisiana^1–4. Little is known, however, about the spatial and temporal scales of these hypoxic areas. Boesch⁵ concludes the nearshore continental shelf west of the Mississippi River Delta has characteristics potentially favourable for large-scale bottom water oxygen depletion. These characteristics include influences from river and marsh discharges of freshwaters, rich in nutrients and sediments, The river effluent flows over the higher salinity shelf waters, creating two-layer vertical density stratification. When surface waters are heated during the summer, vertical stability increases, especially during calm wind periods, and vertical eddy diffusion is limited. Large-scale phytoplankton blooms stimulated by nutrients from the freshwater runoff can then produce large amounts of organic matter which sink and, through respiration and decomposition, deplete oxygen concentrations in the bottom layer. The hypoxia is normally limited to several metres of the water column next to the bottom⁶ in water depths usua <50m. Because hypoxia appears to be related to surface chlorophyll and temperature, which can both be measured with the Coastal Zone Color Scanner (CZCS) on board the Nimbus 7 spacecraft^7,8, we have tried to determine if conditions favourable for formation of hypoxia could be detected and monitored from space. A linear discriminant function successfully identified areas of bottom water hypoxia detected by research vessels up to 10 days after satellite overpass. The discriminant function also successfully predicted hypoxic areas in June 1983 without resort to research vessel data. Shrimp and finfish were absent in the hypoxic zones; hence, the mapping from space of conditions favourable for hypoxia development may have significant marine resource implications for both strategic fishing and management.