Underway Hyperspectral Bio-Optical Assessments of Phytoplankton Size Classes in the River-Influenced Northern Gulf of Mexico

Neeharika Verma,Steven Lohrenz,Sumit Chakraborty,Cédric G Fichot

doi:10.3390/rs13173346

Neeharika Verma, Steven Lohrenz + Show 2 more

Open Access

https://doi.org/10.3390/rs13173346

Copy DOI

Abstract

High inflows of freshwater from the Mississippi and Atchafalaya rivers into the northern Gulf of Mexico during spring contribute to strong physical and biogeochemical gradients which, in turn, influence phytoplankton community composition across the river plume–ocean mixing zone. Spectral features representative of bio-optical signatures of phytoplankton size classes (PSCs) were retrieved from underway, shipboard hyperspectral measurements of above-water remote sensing reflectance using the quasi-analytical algorithm (QAA_v6) and validated against in situ pigment data and spectrophotometric analyses of phytoplankton absorption. The results shed new light on sub-km scale variability in PSCs associated with dynamic and spatially heterogeneous environmental processes in river-influenced oceanic waters. Our findings highlight the existence of localized regions of dominant picophytoplankton communities associated with river plume fronts in both the Mississippi and Atchafalaya rivers in an area of the coastal margin that is otherwise characteristically dominated by larger microphytoplankton. This study demonstrates the applicability of underway hyperspectral observations for providing insights about small-scale physical-biological dynamics in optically complex coastal waters. Fine-scale observations of phytoplankton communities in surface waters as shown here and future satellite retrievals of hyperspectral data will provide a novel means of exploring relationships between physical processes of river plume–ocean mixing and frontal dynamics on phytoplankton community composition.

Highlights

Oceanic phytoplankton communities are major contributors to the global biogeochemical cycles given their role in photosynthetic carbon fixation and net primary production.These single-celled autotrophs span multiple taxonomic divisions and have a wide range of classifications based on their size, physiological traits, and ecological functions [1,2]
An intercomparison of remote sensing reflectance (Rrs) determined using the shipboard sensors (HyperSAS and HyperPro) and Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua-derived Rrs was possible at selected stations and revealed reasonable agreement, with some differences for some mid- and inner shelf stations (Figure 6 and Figure S2)
Retrieval of hyperspectral measurements allow for greater precision in the application of methods such as the QAA, and we contend that they have the potential to provide improved estimates of phytoplankton community composition, especially in coastal waters

Summary

Introduction

Oceanic phytoplankton communities are major contributors to the global biogeochemical cycles given their role in photosynthetic carbon fixation and net primary production. These single-celled autotrophs span multiple taxonomic divisions and have a wide range of classifications based on their size, physiological traits, and ecological functions [1,2]. This diversification in phytoplankton life traits is a consequence of dynamic physical processes and environmental factors [3]. Several remote sensing techniques have been developed that take advantage of distinct bio-optical signatures of various phytoplankton size classes (PSCs) to characterize phytoplankton communities across differing water types [5,6,7,8,9]

Methods

Results

Discussion

Conclusion