Water Quality, Phytoplankton, and Microphytobenthos Dynamics of Sediment Borrow Areas on the Louisiana Continental Shelf

Monique Boudreaux


Coastal and continental shelf waters, like northern Gulf of Mexico, are undergoing changes related to natural and human-induced processes causing significant morphological changes. Louisiana, particularly, is experiencing accelerated land loss causing rapid degradation of barrier islands. Restoration of barrier islands employ sediment additions from the continental shelf through dredging. Implications to the ecology of these areas from dredging have been reported in water quality changes, sediment stressors, and contaminant releases, however, research is limited. To understand the implications of dredging on water quality on a seasonal and temporal scale, water samples and sediment cores were taken inside and outside of two borrow pits, Caminada and Sandy Point, located on the Louisiana continental shelf in spring and summer of 2018 and 2019, respectively. Samples were collected from four stations at each borrow pit to quantify inorganic nutrients, organic carbon and nitrogen, chlorophyll a, pigment composition, and dominant phytoplankton and microphytobenthos taxa. Findings suggest that seasonal variability (water column) and sediment substrate and microbial activity (sediments) were the dominant factors driving the differences in primary production inside vs outside of the borrow pits. Caminada borrow pit, a low-flow environment, experienced introduction of nutrients from the Mississippi River that promoted spring algal bloom in the surface waters (2.12 ± 0.85 μg chl a L-) that later fueled organic matter deposition in summer in the bottom waters within the pit (4.87 ± 0.15 μg chl a L-1). Sandy Point borrow pit, high-flow environment, experienced an introduction of nutrients from the Mississippi River during a period of prolonged high river discharge that allowed development of a larger spring algal bloom (7.49 ± 0.52 μg chl a L-1) and even greater a secondary summer bloom (14.32 ± 1.3 μg chl a L-1) in the surface waters but not at depth. Both borrow sites had a unique microphybenthos community of mainly centric diatom Coscinodiscus spp. with differences in biomass, while outside the pit microphybenthos was mainly composed of pennate diatoms. This study provides new information of borrow pits and the surrounding water column and a comprehensive analysis of two differing borrow pit environments.