Background and approach

The key objectives of HABIT are

to quantify the role of small scale physical processes in maintaining harmful phytoplankton in discrete thin layers
to define the essential chemical and biological interrelationships which control the development of harmful populations within these layers

The central theme of this proposal is to research the development and dispersion of HAB populations in sub-surface micro-layers. The physical and biological parameters important in the maintenance of these layers will be measured and quantified. It will focus on a group of phytoplankton, Dinophysis spp., which has the most serious impact on the economic development of the European coastal zone.

The negative impact of these blooms can only be prevented through resolving fundamental patterns in the occurrences of Dinophysis and quantifying the processes that are important in governing their distribution. To this end, the project HABIT follows the sequence:

an investigation of the maintenance and persistence of high density thin layers of Dinophysis through studying interactions between fine scale physical diffusion and net growth and trophic relationships within these layers
investigating the precise role of small scale structures on the coastal shelf (small gyres, pycnoclines) as incubators for accumulations of Dinophysis spp.
utilising physical models to examine the formation and persistence of gyres on the shelf, to predict their transport, and as a consequence, HAB events at the coast.

The accumulation of information regarding the occurrence of HAB species in thin layers has been slow and sporadic. This is mainly because the sub-surface layer can be present at any depth in the water column, but also because measuring instruments now considered as standard (such as in situ fluorometers) are not always able to observe them, and hence cannot target them for sampling. The net result is that this indicative information is poorly disseminated and not available through the normal channels such as the scientific literature.

Very little is known about the nutrition, behaviour and trophic relationships within thin layers of phytoplankton. This proposal deals with this challenging task by measuring organic compounds in fractions based on molecular weight and investigating their relative influence on growth. This is the first attempt to study this aspect for Dinophysis, a genus for which information is non-existent due to our current inability to culture this organism. Several further questions are addressed: are the Dinophysis cells always imbedded within optically detectable layers using spectral optics techniques? Are they associated with layers of dissolved organic matter, as in the case of Pseudo-nitzschia? Are they usually in the steepest part of the pycnocline? Do they vertically migrate, or change vertical position by swimming behaviour? How much are they grazed?

The physical processes relevant to thin layers occur at many length scales. Many of the longer scale processes such as tides, meteorological forcing (solar input, wind etc), inertial waves, and sub tidal internal waves are routinely observed with standard instruments and are well modelled through 3-D baroclinic models and meteorological models. Recent advances in technology (principally microelectronics) have led to affordable high frequency instrumentation suitable for measuring smaller scales such as local turbulence. Achievement of a profile of the microstructure of turbulence can be constructed using free-fall probes (Burchard et al., 2002), via techniques using ADCPs to measure shear stress (Rippeth et al., 2002), or via high frequency 3 axis current meters. Estimates of horizontal mixing are more difficult to measure and are best obtained by the use of tracers. Recent studies in the shallow North Sea using dyes (Fernand et al., 2001) have measured flows along a pycnoline in a high tidal environment. Sundermeyer and Ledwell (2000) and Houghton (2002) have also conducted successful measurements in the deep ocean and shelf edge respectively . High frequency 3 axis current meters, used in moored and profiling mode, and dye studies utilising high performance towed undulating equipment, will be used toNovember 2, 2005

It is recognised that thin layers of dinoflagellates (and other HAB taxa) can require retention zones or other small-scale structures on the coastal shelf for populations to develop with a high-density. These zones will be investigated as incubators for thin layers of Dinophysis. State of the art 3D physical models will be utilised at a high enough resolution such that the formation, persistence and movement of these structures can be modelled and predicted. In this way, potential HAB incubator sites will be shown to depend on the hydrodynamic regime of the coastal ocean. In this way, origins of economically devastating HAB events can be identified and essential information given to policy makers and managers, as the only mitigation action possible for naturally occurring events lies in their prediction.