Final Report Summary - HABIT (Harmful Algal Bloom species in Thin Layers)
The project HABIT researched the development and dispersion of HAB populations in sub-surface micro-layers. It focused on the genus of phytoplankton, Dinophysis, that has the most serious impact on the economic development of the European coastal zone through contamination of filter feeding shellfish with Diarrhoeic shellfish poisons (DSP). Populations of this genus frequently occur in sub-surface, thin micro-layers. The overall objective of HABIT was therefore to resolve fundamental patterns in the occurrences of Dinophysis and quantify the processes that are important in governing their distribution. To this end, the approach of the project HABIT was:
(1) to investigate the maintenance and persistence of high density thin layers through studying interactions between fine scale physical diffusion and net growth and trophic relationships within them;
(2) to investigate the precise role of small scale structures on the coastal shelf as incubators for accumulations of Dinophysis; and
(3) to utilise physical models to examine the formation and persistence of gyres and other small scale structures on the shelf, to predict their transport, and as a consequence HAB events at the coast.
Approach used and significant findings
The negative impact of harmful algal blooms can only be prevented through resolving fundamental patterns in their occurrences and quantifying the processes that are important in governing their distribution. To this end, the project HABIT from the outset was to focus on the harmful genus Dinophysis through:
(1) 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;
(2) investigating the precise role of small scale structures on the coastal shelf (small gyres, pycnoclines) as incubators for accumulations of Dinophysis spp.; and
(3) 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. HABIT dealt with this challenging task by measuring organic compounds in fractions based on molecular weight and investigating their relative influence on growth. This was the first attempt to study this aspect for Dinophysis, a genus for which information is non-existent due to a historical 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? Are they usually in the steepest part of the pycnocline? Do they vertically migrate, or change vertical position by swimming behaviour?
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 three-dimensional (3D) baroclinic models and meteorological models. Recent advances in technology (principally microelectronics) have led to affordable high frequency instrumentation suitable for measuring physical processes at smaller scales. Achievement of a profile of the microstructure of turbulence can be constructed using free-fall probes via techniques using ADCPs to measure shear stress or via high frequency three axis current metres. Estimates of horizontal mixing are more difficult to measure and are best obtained by the use of tracers.
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 were investigated as incubators for, and transporters of, thin layers of Dinophysis. 3D physical models were utilised at a high enough resolution such that the persistence and movement of these structures can be modelled. In this way, HAB dynamics were shown to depend on the hydrodynamic regime of the coastal ocean.
Significant findings beyond state of the art
The most significant findings of the work output from HABIT can be summarised as follows:
- High density sub-surface layers of a variety of potentially harmful species exist. These can be extremely difficult to sample using conventional sampling methodologies.
- The scale of populations of Dinophysis, which can be considered a rare species, is very small, occurring in thin layers (0.5-5 m thickness) in patch size of less than 10 km x 10 km.
- Sub-surface layers of Dinophysis arise offshore but do not always impact on the coast.
- The mixotrophic nutrition of Dinophysis is complex, and in the case of particulate nutrition seems to be highly opportunistic.
- The processes involved in transporting these populations can now be modelled using high quality (and resolution) 3D physical models.
In addition to the traditional shipboard sampling techniques (e.g. CTD rosette samplers), three key equipments were used during the programme for studying thin layers: the IFREMER profiler, the fine-scale sampler, and Scanfish. These are outlined in more detail below. High vertical resolution measurements of shear were made using high frequency ADCP.
Thin layers of phytoplankton were observed on all collaborative field exercises. High density thin layers of Dinophysis were not observed until the cruise off the south coast of Ireland in 2007. Detailed summaries of the phytoplankton composition can be found in D07 and in the data files currently located at the project ftp site (please see http://www.toxicblooms.com online). A number of in situ and laboratory experiments were carried out in an effort to determine the relative importance of the balance between physical and biological processes in maintaining thin layers of Dinophysis. Relevant biological processes included growth, migration, nutrition and mortality whereas substantial effort was made to look at the effects of shear in controlling thin layer distribution.
High quality physical models have been employed to simulate movement of small scale physical structures which can act as incubators or transport thin layers of Dinophysis. Physical modelling in relation to blooms of Dinophysis were confined to Biscay and the Celtic sea for operational regions.
Physical models are now available which have a satisfactory resolution that incorporates, realistically, the small scale features such as gyres and coastal jets that are important in the transport of populations of harmful algae onto the coastline. Importantly, even if these are present as sub-surface thin layers, the populations can still be incorporated into the models.
Prediction of DSP events
Prediction of harmful events, including DSP events derived from the presence of Dinophysis, requires an implementation of the models described above in operational mode. Unfortunately, this was not possible during the lifetime of HABIT. However, it was possible to use the approach used for the bay of Biscay in hind cast mode.
For the southwest of Ireland, there is not enough historical data to do such a hind cast with the model approach used. Instead, during HABIT, the implementation of a bloom prediction logic model based on the weather forecast was applied. This is due to the advection of HABs into the bays where shellfish culture is carried out is wind forced (see HABIT submission Raine et al., 2008). If the wind changes are correct and the time of year correct, then there is a high probability of a harmful bloom. The idea was developed and put into operational mode.
From a scientific viewpoint, a significant added value of the programme has been the discovery of two new species, one of Fragilidium and an as yet undesignated species of Chrysochromulina. Furthermore, cultures of a second suspected new species of Fragilidium have been isolated from the 2007 Explorer cruise off the south coast of Ireland.
Another finding that occurred during the course of the HABIT studies was the observation of digestive vacuoles full of phycoerithrin-like pigments in the obligate heterotroph Dinophysis rotundata and in its co-occurring ciliate prey Tiarina cf fusus. This, along with toxin analyses of single cell isolates of D. rotundata and other accompanying species of Dinophysis spp., has led us to propose that D. rotundata does not produce toxins de novo, but rather acts as a secondary vector of DSP toxins, produced by co-occuring phototrophic species of Dinophysis, that are eaten by its ciliate prey.
Recommendations for future work
Origin of Dinophysis blooms: The precise origins of the Dinophysis blooms in the three regions of concern of HABIT remain elusive. This has mainly arisen from the absence of established blooms in the Bay of Biscay and in Galicia during cooperative field exercises, and due to the motile nature of the established thin layer bloom off the south coast of Ireland. There were, however, clues to the origin of the Irish bloom in that viable populations of reasonable density close to the seabed. This is not, in the global sense, the first time populations have been observed in this environment and their association with the benthic boundary layer should be investigated as a possible origin.
Offshore observatories: It is clear that Dinophysis blooms are transported physically into sites used for shellfish aquaculture. In some cases, the transport system is weather driven and as a consequence blooms may be predicted using weather forecast and time of year approach. The range of this prediction is however linked to that of the weather forecast, i.e. a maximum of only five days. Species of Dinophysis are relatively large (50-70 microns in length) and most, if not all that are found along Europe west coast toxic. It should therefore be possible to establish offshore observatories that are capable, in real time, of sending back information on the presence of Dinophysis, therefore increasing considerably the forecast range. Given the size of Dinophysis, these observatories could be optically based, as opposed to the rather complex ones currently being trialled based on molecular biology reactions such as the Environmental sample processor (ESP) methodology.
Incorporation of models into monitoring regimes: A significant outcome of the HABIT project has been the ability of high resolution 3D models to reproduce the transport of Dinophysis populations in thin layers. Given their accuracy, and the current rate of incorporation of computing power into smaller and more manageable units, and thus easier and more user friendly to operate, the feasibility of transferring the models into management systems operated by key stakeholders such as monitoring agencies and hence providing industry with valuable data on prediction.