Phytoplankton community structure in the river‑influenced continental margin of the northern Gulf of Mexico

Abstract

Phytoplankton community composition was characterized over varying seasonal and river discharge conditions during 5 research cruises across the continental margin of the northern Gulf of Mexico (NGOM). The spatial and temporal patterns of variation in the composition of the algal community were examined using high performance liquid chromatography (HPLC) analyses of phytoplankton pigments in conjunction with classification using the CHEMTAX software (v.1.95). Cluster analysis and principal component analysis were used to identify different regimes and to understand the relationship of the phytoplankton community to biological and environmental variables. The large river dominated margin of the NGOM was characterized by (1) an estuarine and inner shelf regime dominated by diatoms, cryptophytes, cyanobacteria, and chlorophytes; (2) midshelf waters, a transition zone between coastal river-influenced and oligotrophic slope waters, associated with a mixed phytoplankton composition; and (3) a slope water regime typical of oligotrophic ocean conditions with a surface community dominated by cyanobacteria, haptophytes, and prochlorophytes. A chlorophyll fluorescence maximum (CFM) was a regular feature at offshore stations and showed significant differences from that of surface waters in seasonal variability of phytoplankton pigment ratios and community composition. Our findings support the view that large river outflows, along with other environmental variables such as wind forcing and stratification, have a profound influence on phytoplankton communities over a large regional extent in the continental margin waters of the NGOM.

Biological hot spots and the accumulation of marine dissolved organic matter in a highly productive ocean margin

“…A large number of hot spots were identified in margin surface waters and they exhibited substantial spatial and temporal variability, reflecting the productive and dynamic nature of this ecosystem. Hot spots frequently occurred at mid‐salinities (26–29) on the inner shelf, where high phytoplankton biomass and production are often observed during most seasons (Redalje et al ; Chakraborty and Lohrenz ). Interestingly, unlike neutral sugar hot spots, amino acid‐rich hot spots were mostly identified in waters collected during low‐ and no‐light time periods, suggesting diel variability in the source of labile DOM.…”

“…Hot spots with labile DOM enriched in amino acids showed distinct spatial distributions from those with carbohydrate‐rich labile DOM, indicating compositional variability in labile plankton DOM. Amino acid‐rich hot spots were found at relatively few locations, and they mostly occurred on the inner shelf where concentrations of nutrients and chlorophyll were high (Chakraborty and Lohrenz ). A few amino acid hot spots appeared further south near the shelf‐edge in March, coinciding with a widespread nutrient‐rich plume that resulted in elevated concentrations of chlorophyll across outer shelf and slope waters (Huang et al ; Chakraborty and Lohrenz ).…”

“…It appears that nutrient limitation promotes the accumulation of marine DOM on the Louisiana margin as it enhances phytoplankton production of carbohydrates and limits bacterial utilization of C‐rich DOM (Myklestad and Haug ; Chin‐Leo and Benner ; Thinstad et al ; Skoog et al ). There were relatively low concentrations of nutrients on the shelf and slope during summer (Chakraborty and Lohrenz ), when large accumulations of carbohydrate‐rich DOM were observed. Concentrations of nutrients were elevated on the outer shelf and slope during March (Chakraborty and Lohrenz ), and this could have resulted in the relatively low concentrations of accumulated DOC at mid‐ to high‐salinities at this time.…”

“…There were relatively low concentrations of nutrients on the shelf and slope during summer (Chakraborty and Lohrenz ), when large accumulations of carbohydrate‐rich DOM were observed. Concentrations of nutrients were elevated on the outer shelf and slope during March (Chakraborty and Lohrenz ), and this could have resulted in the relatively low concentrations of accumulated DOC at mid‐ to high‐salinities at this time. Our results indicate there are multiple controls on the accumulation of DOC on the Louisiana margin, which is consistent with observations in other coastal ecosystems (Zweifel et al ).…”

Photophysiological and light absorption properties of phytoplankton communities in the river‐dominated margin of the northern Gulf of Mexico

“…Discharge from both Mississippi and Atchafalaya Rivers strongly influences the distribution of dissolved and particulate material [ D'Sa and DiMarco , ; D'Sa et al ., ], nutrients [ Lehrter et al ., ; Turner et al ., ], and the availability and spectral properties of light [ Schaeffer et al ., ]. In conjunction with the large environmental gradients, our prior research has demonstrated substantial variations in phytoplankton communities across different water mass types and over seasonal time scales in these continental shelf waters [ Chakraborty and Lohrenz , ]. A recent study [ Zhao and Quigg , ] in a shallow inner shelf region of the NGOM also highlighted the importance of phytoplankton community composition in understanding the diel patterns of photosynthesis and photoacclimation under the complicated light and nutrient conditions.…”