Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

John W. Day, Jr

Abstract

Water column primary production and chlorophyll were sampled between 1986-1991 in Fourleague Bay, LA, a shallow (1.5 m), river-dominated estuary that is extremely turbid (K$\sb{\rm D}$ = 4.44 m$\sp{-1}$). A high speed system for continuous flow-through sampling, Dataflow$\sp\copyright$, was developed to measure physicochemical variables and in vivo chlorophyll fluorescence at high temporal (1 s) and spatial (5 m) resolution from a small boat. Phytoplankton net primary production (NPP) was measured using an incubator which rotated bottles to prevent settling of the contents. NPP was found to be artificially increased by 10-83% at high light levels in non-rotated bottles when cells and sediments settled, reducing photoinhibition. Chlorophyll (17-27 $\mu$g L$\sp{-1}$) and NPP (0-4.5 g m$\sp{-2}$ d$\sp{-1}$) distribution varied with season, and was correlated with K$\sb{\rm D}$ and temperature, but not with nutrients. Spatially, chlorophyll was lower in the upper bay, increasing toward the middle estuary and laterally toward shores, especially in bayous, where concentrations were up to 42% higher than open bay waters. Bayous may tidally export chlorophyll to the bay. Turbidity from SPM (64 mg L$\sp{-1}$) was generated by river flow in spring and wind and current resuspension in summer and fall. Minimum water column NPP occurred in the upper estuary during spring, coincident with maximum turbidity. Annually, NPP averaged about 400 g C m$\sp{-2}$, peaking in fall in the upper estuary. Phytoplankton photosynthetic parameters were adapted to a high light regime: P$\sp{\rm B}\sb{\rm max}$ averaged 10.99 $\mu$g C $\mu$g Chla$\sp{-1}$ h$\sp{-1}$, I$\sb{\rm K}$ ranged from 150-400 $\mu$E m$\sp{-2}$ s$\sp{-1}$, and $\alpha\sp{\rm B}$ averaged 0.05 $\mu$g C $\mu$g Chl a$\sp{-1}$ h$\sp{-1}$ $\mu$E m$\sp{-2}$ s$\sp{-1}$. Frequent vertical circulation of phytoplankton in the shallow water column exposes them briefly to high light, sufficient to establish high photosynthetic capacity for the community, and prevent photoadaptation to lower light at depth. Parameters were not correlated with subsurface light, but integrated water column NPP was, indicating light control of NPP. In a simulation model, construction of a levee across the bay entrance had little impact on productivity, but shell dredging activity increased turbidity and reduced primary production nearly 50%, extinguishing the zooplankton population.

Pages

259

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