Gerard Duineveld
	Publications
	ALTRAP

 

 

RECRUITMENT OF SUBTIDAL SOFT SEDIMENT BENTHOS

 

The population dynamics of many marine benthic invertebrates is primarily determined during the early life stages. These stages roughly cover the period from spawning and dispersal of planktonic larvae in the water column till the moment that young bottom settlers appear among the macrofauna. When the planktonic stage is ready to settle on the bottom, many species can actively reject or accept suitable sediments using physical and chemical cues. However, the supply of competent planktonic larvae to the habitat of the adult population is dependent on large scale water movement. Especially in large open water - as the southern North Sea - where wind, fronts and stratification play a role - larval supply can be a decisive factor for the recruitment of benthos.

 

Studies on larval supply and settlement of macrobenthos on the open shelf  require samples

1)    to be taken at defined periods of the tide since tidal current strength can affect the number of larvae present near the bottom;

2)    collected with a high frequency over long periods because of unpredictable events such as strong winds in combination with pulsed larval supply;

3)    accompanied by measurements of relevant environmental parameters (currents, chlorophyll etc).

 

Sampling with high frequency in offshore waters is difficult to accomplish because of shiptime availability and costs. A solution are autonomous bottom landers, such as the ALTRAP lander built at Royal NIOZ and funded by NWO-middelgroot. The lander can work autonomously for long periods (weeks, months) at the sea bottom.

 

 

The ALTRAP lander holds a sampler for planktonic larvae and three microcosms for experiments with settling larvae. The larvae sampler consists of a vertically moving barrel. In its upper (open) position, the barrel is released and captures 25 l of water. The captured volume is driven through one of the 12 filters (100 micron) by means of a piston on top of the barrel. Filter selection is non-sequential allowing to make a discrimination between day vs. night or high vs. low tide samples. The larvae retained in the filter are

 

 

Video of Sampler closes

 

preserved in-situ by injecting formaldehyd or alcohol. Barrel and piston are actuated by means of hydraulics offering flexibility to add modules. Energy is stored in rechargeable batteries enclosed in glass spheres. Actions and filter selection are fully programmable through a PC interface. All actions of the sampler and microcosms are logged into a memory. Stored data can be accessed through a GSM-modem in a nearby surface buoy which communicates with the ALTRAP via under water acoustics.

 

 

Three closeable microcosms each with 6 settlement trays are attached to the frame. The trays filled with experimental or natural substrate, are baffled for high current sites. Each microcosm is closed by lids which in open position are flush with the trays in order to reduce turbulence. The exposure time of each

microcosm can be programmed independently.

 

 

 

Video of Opening microcosm

 

 

The ALTRAP system is furthermore equipped with instruments to monitor environmental conditions viz. time lapse video camera, current meter, conductivity, optical back-scatter and fluorescence sensors. In shallow water, the ALTRAP lander is recovered by means of an acoustically released pop-up buoy. In deeper water, an acoustic command releases a ballast weight after which buoyancy spheres bring the lander to the surface.