Abstract
Phytoplankton community structure and phytoplankton size class (PSC) are linked to ecological and biogeochemical changes in the oceanic environment. Many models developed to obtain the fraction of PSCs from satellite remote sensing have only been evaluated in open oceans, and very limited effort has been carried out to report on the performance of these PSC models in productive continental shelf waters. In this study, we evaluated the performance of nine PSC models in the coastal Northwest Atlantic (NWA) by comparison of in situ phytoplankton pigment measurements with coincidental satellite data from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS), Moderate-resolution Imaging Spectroradiometer (MODIS), and the Visible Infrared Imaging Radiometer Suite (VIIRS). Our results show that no PSC model retrieved all three phytoplankton size classes (pico-, nano-, and micro-phytoplankton) with reliable accuracy in the region of interest. In particular, these PSC models showed poor performance for retrieval of the picophytoplankton fraction of total phytoplankton in our study region, which could be related to the under-representation of pico-dominated samples in the productive waters of the NWA. For the accuracy of retrieved microphytoplankton and combined nano–pico phytoplankton fraction, the regional model developed by Devred et al. (2011) yielded the best result, followed by the model of Brewin et al. (2011). The model of Devred et al. (2011) was applied to satellite-derived chlorophyll-a concentration from the Ocean Color Climate Change Initiative (OC-CCI) archive in the NWA from 1998 to 2016. We report solely on the microphytoplankton biomass and fraction given the inverse relationship that exists with the nano–pico class. The multi-decadal trend along with the deseasonalized trend of microphytoplankton fraction was computed and analyzed for six biogeochemical provinces located in the NWA. Over the 19-year time series, there were significant, positive trends for four of the six provinces, with a slope of 0.36%·yr−1 in the Northwest Continental Shelf (NWCS), 0.25%·yr−1 in the Arctic Waters (ARCT), 0.12%·yr−1 in the Slope Waters (SW) and 0.06%·yr−1 in the Gulf Stream (GFST). Strong positive anomalies of microphytoplankton fraction were found in winter months in NWCS between 2009 and 2014, which could be associated with changes in environmental factors.
Highlights
Phytoplankton are the microscopic algae of size ranging from ~0.6 μm to individual and chain forming species greater than 200 μm [1]
The nine models were applied to SeaWIFS, Moderate-resolution Imaging Spectroradiometer (MODIS), and Visible Infrared Imaging Radiometer Suite (VIIRS) datasets, and a comparison of the percentage accuracy was carried out (Table 3 and Figure 5)
Among the models applied to SeaWIFS data, D, E, and F showed relatively high accuracy (>80%) for the microphytoplankton size class, followed by G and H (55.7~63.7%)
Summary
Phytoplankton are the microscopic algae of size ranging from ~0.6 μm to individual and chain forming species greater than 200 μm [1]. This continuum of size is broken down into three size classes: pico- (0.2–2 μm), nano- (2–20 μm), and micro-phytoplankton (20–200 μm) [2]. Phytoplankton cell size has been considered a good indicator of functional role in many ecological and biogeochemical processes [2,3,4,5,6]. It is widely proposed that phytoplankton in different size classes occupy different physical and chemical niches based on their light-harvesting efficiency, nutrient-uptake ability, biogeochemical function, and mobility within the euphotic zone [1]. The phytoplankton community size structure or phytoplankton size class (PSC) is linked to ecological and biogeochemical processes in the oceanic environment
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