Seasonality of floc strength in the southern North Sea

Abstract

The suspended particulate matter (SPM) concentration in the high turbidity zones of the southern North Sea is inversely correlated with chlorophyll (Chl) concentration. During winter, SPM concentration is high and Chl concentration is low and vice versa during summer. This seasonality has often been associated with the seasonal pattern in wind forcing. However, the decrease in SPM concentration corresponds well with the spring algal bloom. Does the decrease of SPM concentration caused by changing wind conditions cause the start of algae bloom, or does the algae bloom decrease SPM concentrations through enhanced flocculation and deposition? To answer the question, measurements from 2011 of particle size distribution (PSD), SPM, and Chl concentrations from the southern North Sea have been analyzed. The results indicate that the frequency of occurrence of macroflocs has a seasonal signal, while seasonality has little impact upon floc size. The data from a highly turbid coastal zone suggest that the maximum size of the macroflocs is controlled by turbulence and the available flocculation time during a tidal cycle, but the strength of the macroflocs is controlled by the availability of sticky organic substances associated with enhanced primary production during spring and summer. The results highlight the shift from mainly microflocs and flocculi in winter toward more muddy marine snow with larger amounts of macroflocs in spring and summer. The macroflocs will reduce the SPM concentrations in the turbidity maximum area as they settle faster. Consequently, the SPM concentration decreases and the light condition increases in the surface layer enhancing algae growth further.

Spatial and Seasonal Variation of Biomineral Suspended Particulate Matter Properties in High-Turbid Nearshore and Low-Turbid Offshore Zones

“…The aggregates occurring during algae bloom periods or in the OSZ have a lower settling velocity as a larger fraction is composed of organic matter and sticky bio-polymers organized in a fluffy structure [16,48]. Previous studies, carried out in the same TMZ, reported that large and settleable biomineral aggregates were dominant SPM species during bio-enriched spring and summer periods [31,32]. These large, settleable biomineral aggregates are contrary to less settleable biological aggregates observed in this current study.…”

“…These sticky polymeric substances can not only enhance flocculation, but also reduce the erosion and resuspension of muddy deposits from the seabed to the water column [49]. Previous studies, carried out in the same TMZ, reported that large and settleable biomineral aggregates were dominant SPM species during bio-enriched spring and summer periods [31,32]. These large, settleable biomineral aggregates are contrary to less settleable biological aggregates observed in this current study.…”

“…Regarding the seasonality, the annual cycle of SPM concentration in the high turbidity area off the Belgian coast is mainly caused by the seasonal biological cycle, rather than wind and waves. Wind strengths and wave heights have a seasonal signal, but these do not explain the large differences observed in SPM concentration [31,32]. This seasonality is linked with the seasonal changes in aggregate size and thus settling velocity due to biological effects.…”

“…Measurements have indicated SPM concentrations of 20-70 mg/L in the nearshore area; reaching 100 to more than a few g/L near the bed; lower values (<10 mg/L) occur in the offshore [31]. As shown in Figure 1, the MOW1 measurement site is located in the TMZ.…”

Seasonal Variation in Flocculation Potential of River Water: Roles of the Organic Matter Pool

“…It was initially hypothesized that EPS production from algae could enhance the flocculation potential of the river water based on previous studies that reported high linkage between algal population dynamics (or EPS production) and flocculation potential [11,14,46]. The peak concentration of chlorophyll-a (i.e., phytoplankton) was followed by the maxima of EPS production and flocculation potential [19,47,48].…”