Optical imaging devices such as the Video Plankton Recorder (VPR) harness unique capabilities to perform in situ observations and observe planktonic organisms in their natural environmental context. However, applying this technology in shallow and turbid coastal waters comes with a number of challenges. Depending on the research goal, methodological choices need to be made regarding the appropriate towing procedure and instrument settings, like magnification or field of view. In addition, limitations can be expected related to the physical characteristics of the water column, more specifically regarding suspended matter concentration and turbidity. To inform VPR users on the possibilities and limitations of the device in shallow and turbid coastal waters, this paper evaluates a number of specific deployment procedures in the Belgian part of the North Sea (BPNS). For three different towing procedures the practical feasibility, characteristics and output are assessed and the assets and liabilities for each of the tow types are discussed. A Z-shaped and a clover-shaped tow type are seen as best fit for detailed characterization of the plankton community of a limited geographical area. A straight tow type is more suitable for plankton studies over a larger area, with the potential to capture local plankton abundance peaks and to determine the relation with the spatial variation of the environmental conditions. The capacity of the various VPR magnification settings to capture specific plankton taxa or size groups, was tested during four straight line transects with different magnifications. The highest magnification can be used for organisms from 0.3 to 0.7 mm while the low magnification allows to observe larger organisms within the size range of 1.0 to 3.8 mm. Finally, the boundary conditions for the deployment of the VPR related to the turbidity of the water column were defined and the implications for deployment within the study area were investigated. This study shows that high turbidity values over 6.2 NTU inhibit the collection of useable data, complicating the VPR's application in many coastal and transitional waters. |
