Investigating marine shallow waters dynamics to explore the role of turbidity on ecological responses

Abstract

The ecological tangible effect of the complex interaction between sediments and water column in shallow waters is represented by turbidity which is a common feature of most aquatic ecosystems: it varies both temporally and spatially; it can cover a huge area and persist for a long period or it can be very localized and temporary. Among many factors able to generate turbidity, wind generated wave action and water mass movements due to tides seem important in causing resuspension of sediments. Although there is much research spent in last decades on this topic and many models to explain the complexity of the wind-water-sediment interaction, some interactive aspects are too site specific and then still poor understood. On the other hand, this interaction involves many physical, chemical and trophic aspects like water flow velocity, turbulence, boundary layer thickness, environmental stresses and, in turn, resuspension, transport, and deposition of particulate matter, mechanical limits to size, larval dispersion, food availability. To get further knowledge on these aspects, we carried out in March 2007 a 5-day-experiment in a Mediterranean shallow area (The Stagnone di Marsala, Western Sicily) by collecting data on wind and water velocities, their directions and the contextual response of the water column in term of turbidity, chlorophyll-a and suspended solids (by ignition). To analyse the interaction, we proceeded step by step. Firstly, we studied data from the two current meters (an acoustic doppler velocimeter 40 ± 2 cm deep, and an electromagnetic current meter 20 ± 2 cm deep). From this data, the water column had the following features: i) during the big semidiurnal tidal transitional phase, the flow field followed a behaviour leading us to hypothesize a logarithmic layer defined by the law of the wall and to obtain friction velocity values with linear regression in good agreement with calculated ones with covariance and TKE method, while ii) during the small tidal transition and at high and low tides, a not-well defined gradient was present (i.e., the mean deviation of the direction of the two water velocities was more than 30° and the flow magnitude at 40 cm was less than that measured at 20 cm implying high values of turbulence intensity). The second step analysed data from multiprobe, ADV, meteorological station and considered turbidity (NTU) as proxy of food availability for consumers. NTU followed a one-day-period and the lower the turbidity, the higher the turbulence (both at 20 and 40 cm).