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1

The impact of tides and waves on near-surface suspended sediment concentrations in the English Channel

Guillou N., Rivier A., Chapalain G., Gohin F.

Oceanologia

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2017

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No. 59 (1)

28--36

EN

Numerous ecological problems of continental shelf ecosystems require a refined knowledge of the evolution of suspended sediment concentrations (SSC). The present investigation focuses on the spatial and temporal variabilities of near-surface SSC in coastal waters of the English Channel (western Europe) by exploiting numerical predictions from the Regional Ocean Modeling System ROMS. Extending previous investigations of ROMS performances in the Channel, this analysis refines, with increased spatial and temporal resolutions, the characterization of near-surface SSC patterns revealing areas where concentrations are highly correlated with evolutions of tides and waves. Significant tidal modulations of near-surface concentrations are thus found in the eastern English Channel and the French Dover Strait while a pronounced influence of waves is exhibited in the Channel Islands Gulf. Coastal waters present furthermore strong SSC temporal variations, particularly noticeable during storm events of autumn and winter, with maximum near-surface concentrations exceeding 40 mg l−1 and increase by a factor from 10 to 18 in comparison with time-averaged concentrations. This temporal variability strongly depends on the granulometric distribution of suspended sediments characterized by local bimodal contributions of silts and sands off coastal irregularities of the Isle of Wight, the Cotentin Peninsula and the southern Dover Strait.

2

Response patterns of phytoplankton growth to variations in resuspension in the German Bight revealed by daily MERIS data in 2003 and 2004

Su J., Tian T., Krasemann H., Schartau M., Wirtz K.

Oceanologia

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2015

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No. 57 (4)

328--341

EN

Chlorophyll (chl a) concentration in coastal seas exhibits variability on various spatial and temporal scales. Resuspension of particulate matter can somewhat limit algal growth, but can also enhance productivity because of the intrusion of nutrient-rich pore water from sediments or bottom water layers into the whole water column. This study investigates whether characteristic changes in net phytoplankton growth can be directly linked to resuspension events within the German Bight. Satellite-derived chl a were used to derive spatial patterns of net rates of chl a increase/decrease (NR) in 2003 and 2004. Spatial correlations between NR and mean water column irradiance were analysed. High correlations in space and time were found in most areas of the German Bight (R2 > 0.4), suggesting a tight coupling between light availability and algal growth during spring. These correlations were reduced within a distinct zone in the transition between shallow coastal areas and deeper offshore waters. In summer and autumn, a mismatch was found between phytoplankton blooms (chl a > 6 mg m−3) and spring-tidal induced resuspension events as indicated by bottom velocity, suggesting that there is no phytoplankton resuspension during spring tides. It is instead proposed here that frequent and recurrent spring-tidal resuspension events enhance algal growth by supplying remineralized nutrients. This hypothesis is corroborated by a lag correlation analysis between resuspension events and in-situ measured nutrient concentrations. This study outlines seasonally different patterns in phytoplankton productivity in response to variations in resuspension, which can serve as a reference for modelling coastal ecosystem dynamics.

3

Monitoring the effect of upwelling on the chlorophyll a distribution in the Gulf of Finland (Baltic Sea) using remote sensing and in situ data

Uiboupin R., Laanemets J., Sipelgas L., Raag L., Lips I.

Oceanologia

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2012

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No. 54 (3)

395-419

EN

The spatio-temporal variability of chlorophyl a (Chl a) caused by a sequence of upwelling events in the Gulf of Finland in July-August 2006 was studied using remote sensing data and field measurements. Spatial distributions of sea surface temperature (SST) and Chl a concentration were examined using MODIS and MERIS data respectively. The MERIS data were processed with an algorithm developed by the Free University of Berlin (FUB) for case 2 waters. Evaluation of MERIS Chl a versus in situ Chl a showed good correlation (r2 = 0.67), but the concentration was underestimated. The linear regression for a 2 h window was applied to calibrate MERIS Chl a. The spatio-temporal variability exhibited the clear influence of upwelling events and related filaments on Chl a distribution in the western and central Gulf. The lowest Chl a concentrations were recorded in the upwelled water, especially at the upwelling centres, and the highest concentrations (13 mg m-3) were observed about two weeks after the upwelling peak along the northern coast. The areas along the northern coast of upwelled water (4879 km2) on the SST map, and increased Chl a (5526 km2) two weeks later, were roughly coincident. The effect of upwelling events was weak in the eastern part of the Gulf, where Chl a concentration was relatively consistent throughout this period.

4

Detecting cyanobacterial blooms in large North European lakes using the Maximum Chlorophyll Index

Alikas K., Kangro K., Reinart A.

Oceanologia

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2010

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No. 52 (2)

237-257

EN

The Maximum Chlorophyll Index (MCI), developed for the MERIS sensor processing scheme, is used to investigate the seasonal dynamics, spatial distribution, and coverage of cyanobacterial blooms over Lake Peipsi (Estonia/Russia) and Lake Vo~rtsjärv (Estonia). In these optically complex waters, the amounts of suspended matter and dissolved organic matter vary greatly and independently of the phytoplankton biomass. We demonstrate that MCI is a useful, new tool for detecting and estimating cyanobacterial biomass (R2 = 0.73), phytoplankton biomass (R2 = 0.70) and chlorophyll a concentration (R2 = 0.64). The MCI-derived results are consistent with known patterns of phytoplankton dynamics in these lakes, whose optical properties are in the same range as in many coastal regions of the Baltic Sea.