Hyperspectral-based assessment of phytoplankton communities in a shallow estuary by means of model simulations

Abstract

The presence of several harmful algal species responsible for different types of toxicity in bivalves have made the Alfacs Bay, an estuarine embayment of the Ebro Delta (NW Mediterranean Sea, maximum depth of 6.5 m), one of the most studied coastal sites in Catalonia. Several studies have demonstrated the advantages offered by hyperspectral optical measurements for characterizing phytoplankton biodiversity (Chang et al., 2004; Lubac et al., 2008), suggesting that the effectiveness of this information should be further explored. In fact, there is a need to test whether hyperspectral observations, which have proven useful in open ocean waters (Torrecilla et al., 2011), can also be effective for the identification of phytoplankton communities in shallow estuarine waters like the Alfacs Bay. In this contribution, as a proofof- concept study, the feasibility of a hyperspectral-based approach for monitoring phytoplankton biodiversity in a complex shallow estuary scenario is examined. Using a simulation-based framework, a set of remote-sensing reflectance spectra are simulated (Hydrolight 5.0). The sensitivity of remote-sensing reflectance to different phytoplankton communities is assessed under different conditions in terms of bottom type and abundance of suspended matter. Additionally, a sensitivity analysis of Rrs is also performed under different conditions of vertical structure of phytoplankton along the water column. In this case, a thin layer condition characterized by the presence of a sub-surface chlorophyll maximum at a specific depth is simulated based on field observations and theoretical estimates. The depth and amplitude of the idealized thin layer emulating similar conditions to those found in the Alfacs Bay is varied to estimate their influence on the Rrs. Our results from this optical modelling exercise illustrate that scenarios dominated by different phytoplankton communities are clearly discriminated regardless the abundance of phytoplankton and suspended matter, and the bottom type and vertical structure of the phytoplankton along the water column