Contrasting seasonality in optical-biogeochemical properties of the Baltic Sea.

Stefan G H Simis,Pasi Ylöstalo,Kari Y Kallio,Tiit Kutser,Kristian Spilling,Steven Arthur Loiselle

doi:10.1371/journal.pone.0173357

Abstract

Optical-biogeochemical relationships of particulate and dissolved organic matter are presented in support of remote sensing of the Baltic Sea pelagic. This system exhibits strong seasonality in phytoplankton community composition and wide gradients of chromophoric dissolved organic matter (CDOM), properties which are poorly handled by existing remote sensing algorithms. Absorption and scattering properties of particulate matter reflected the seasonality in biological (phytoplankton succession) and physical (thermal stratification) processes. Inherent optical properties showed much wider variability when normalized to the chlorophyll-a concentration compared to normalization to either total suspended matter dry weight or particulate organic carbon. The particle population had the largest optical variability in summer and was dominated by organic matter in both seasons. The geographic variability of CDOM and relationships with dissolved organic carbon (DOC) are also presented. CDOM dominated light absorption at blue wavelengths, contributing 81% (median) of the absorption by all water constituents at 400 nm and 63% at 442 nm. Consequentially, 90% of water-leaving radiance at 412 nm originated from a layer (z90) no deeper than approximately 1.0 m. With water increasingly attenuating light at longer wavelengths, a green peak in light penetration and reflectance is always present in these waters, with z90 up to 3.0–3.5 m depth, whereas z90 only exceeds 5 m at biomass < 5 mg Chla m-3. High absorption combined with a weakly scattering particle population (despite median phytoplankton biomass of 14.1 and 4.3 mg Chla m-3 in spring and summer samples, respectively), characterize this sea as a dark water body for which dedicated or exceptionally robust remote sensing techniques are required. Seasonal and regional optical-biogeochemical models, data distributions, and an extensive set of simulated remote-sensing reflectance spectra for testing of remote sensing algorithms are provided as supplementary data.

Highlights

The Baltic Sea is a brackish, semi-enclosed, and relatively shallow coastal sea with large latitudinal gradients in salinity and dissolved organic matter (DOM)
Generic ocean colour algorithms which extract information on phytoplankton pigment concentration from blue-to-green satellite waveband ratios are unsuitable for the Baltic Sea [2,3] due to high concentrations of chromophoric dissolved organic matter (CDOM) and high phytoplankton biomass during productive periods
Chla was always quantified from these samples, whereas CDOM absorption, HPLC pigments, particulate dry weight, and organic fractions of dissolved C and N and particulate C, N and P were included depending on the specific purpose of each cruise

Summary

Introduction

The Baltic Sea is a brackish, semi-enclosed, and relatively shallow (average depth approximately 55 m) coastal sea with large latitudinal gradients in salinity and dissolved organic matter (DOM). Several large watersheds discharge into the sea while exchange with the open ocean is limited through the Danish Straits in the southwest This limited water exchange, combined with high nutrient loads despite ongoing reduction efforts (HELCOM 2009, HELCOM 2013a), negatively affect water quality as evidenced by seasonal high-biomass phytoplankton blooms and extensive low oxygen zones. Generic ocean colour algorithms which extract information on phytoplankton pigment concentration from blue-to-green satellite waveband ratios are unsuitable for the Baltic Sea [2,3] due to high concentrations of chromophoric dissolved organic matter (CDOM) and high phytoplankton biomass during productive periods. Given the poor environmental status of the Baltic Sea ecosystem, efforts into operational optical monitoring and analysis of past and future satellite data are needed to support its costeffective management To this end, remote sensing algorithms that are optimally suited to the unique optical conditions of the Baltic Sea are needed. Detailed and seasonally specific (where required) models, distributions of the underlying biogeochemical parameters, and a set of simulated remote-sensing reflectance (Rrs) are provided with this paper

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Discussion