Analysis of Secchi Depths and Light Attenuation Coefficients in the Louisiana-Texas Shelf, Northern Gulf of Mexico

We analyzed Secchi depths (Zs), scalar downwelling irradiance, and transmissometer data collected during 10 cruises that spanned two and one-half years (May 1992-Nov. 1994) over the Louisiana-Texas shelf. The Zs increases from the inner shelf (depths ≤50 m) to the outer shelf and from nonsummer (Zs =10m) to summer (Zs = 14m). These changes in Zs are induced by the shelf\u27s circulation and freshwater input, both of which vary seasonally, i.e., nonsummer to summer. Optical parameters (e.g., Zs), like the circulation, divide the shelf into an inner and outer shelf along the 50-m isobath. The inner shelf contains turbid waters with low levels of light penetration and large attenuation coefficients that decrease from nonsummer to summer. The outer shelf contains clearer water with high levels of light penetration and small attenuation coefficients year round. Linear regressions between the attenuation coefficients and Zs conform to theory and vary seasonally. The scalar downwelling attenuation coefficient and Zs are inversely related with coefficients of 1.36 in summer and 1.22 in nonsummer. The beam attenuation coefficient and Zs are also inversely related with coefficients of 6.16 in summer and 5.54 in nonsummer