Phytoplankton, sediment and optical observations in Netherlands coastal water in spring

Abstract

Factors controlling the dynamics of suspended particulate matter (SPM), its influence on sea-leaving radiance and in-water optical properties, and the consequences of optical variation for phytoplankton growth, were studied at the ‘Processes of Vertical Exchange in Shelf Seas’ (PROVESS) project's southern North Sea site during April 1999. The optical properties of Netherlands coastal water were not unexpectedly found to be primarily determined by suspended sediment (Case 2) and were classified as Jerlov type 7 ‘relatively turbid coastal water’. During the study period, vertical mixing periodically resuspended optically active particles from the bed fluff layer throughout the water column and into the near-surface layer. These particles influenced sea surface radiance reflectance, and the red/green ratio of radiance reflectance, both of which can be observed by remote sensing. Linear relationships between sea surface radiance reflectance and SPM concentration were primarily determined by the inorganic fraction, as organic SPM varied little in concentration throughout the cruise period. The inorganic fraction was an important scatterer of light at all wavelengths, whereas the organic fraction displayed a greater tendency for light absorption at shorter wavelengths. Although the euphotic layer (depth of 1% surface irradiance) was only 8–10 m deep, vertical mixing ensured that phytoplankton throughout the water column (∼18 m) had access to PAR in excess of the estimated compensation illumination. Growth rates of microplankton (which includes pelagic microheterotrophs as well as phytoplankters) were calculated using an algorithm from the PROWQM model. These ranged from 0.1 to 0.3 d −1, and implied loss rates of 3–25% which were mostly attributed to mesozooplankton grazing. Estimated oxygen production, however, was in near equilibrium with oxygen demand observed in dark bottles, and implied a significant oxygen demand due to detrital respiration and nitrification. This was estimated as 3–6 mmol O 2 m −3 d −1. In an order of magnitude timescale analysis, vertical mixing was found to be the single most important factor controlling the dynamics of SPM under mixed or stratified conditions. For a mixed water column microplankton aggregation and fluff layer resuspension also had the potential to redistribute material in the water column several times per day, whilst under stratified conditions horizontal exchange and inorganic particle sinking were more important. Resuspended material in a stratified water column remained below the pycnocline and had little impact on the near-surface layer optics. Other factors varied in importance with the level of stratification, which was recognised as a significant factor in determining the dynamics of SPM in this region of freshwater influence (ROFI).