Real-time algae monitoring

Introduction

Monitoring and prediction of algal blooms (including harmful algal blooms)and insight in the impact of human activities on the frequency and intensity of blooms are needed to support water managers deciding on mitigating measures.

Multiple methods exist for monitoring algae concentrations and signalling the presence of algal blooms. Field data have the highest confidence level but in most coastal areas, spatial and temporal coverage is rather limited. Optical remote sensing using the new generation ocean colour imaging spectrometers (SeaWiFS, MODIS and MERIS) can provide information on chlorophyll-a concentrations. Because blooms consist of (up to) millions of cells per litre, they generally contain high amounts of chlorophyll-a, and are detectable by this remote sensing method. Remote sensing has the advantage of providing large spatial coverage and resolution however cloud cover can limit the temporal coverage. Computer models can also be used to calculate the temporal and spatial coverage on chlorophyll-a and phytoplankton species but the confidence level is lower than for field data and remote sensing data.

HABs in the Dutch coastal zone

In the Eastern English Channel and the Southern Bight of the North Sea, the spring phytoplankton bloom is dominated by Phaeocystis globosa (Reid et al. 1990 ; Rousseau et al., 2002 ; Seuront et al., 2006 ). The start of the bloom is believed to be determined by the availability of light in the water column (Peperzak et al., 1998 and / or the depletion of ortho-phosphate by diatoms (Lancelot et al., 2005 ). During the bloom rise, once P. globosa dominates the ensemble, the formation of colonies with diameters in excess of 200 μm and chlorophyll-a levels above 20 μg/L may occur (Seuront et al., 2006). During bloom decline, the sedimentation of the large colonies may lead to massive mortality of benthic invertebrates via anoxia (Peperzak, 2002 ).

Real-time algae monitoring system and HAB bulletin

In the Dutch coastal area, harmful algal blooms of Phaeocystis occasionally cause mass mussel mortality in the aquaculture area Oosterschelde. To enable early warnings about future harmful algal blooms to mussel farmers and other end users, an information system is being developed based on the combination of remote sensing data (MERIS sensor on the ENVISAT satellite), field data and model data from WL | Delft Hydraulics’ ecological model GEM for the Dutch Voordelta area. The information system is being developed as a collaboration between WL | Delft Hydraulics, IVM (Free University, Amsterdam) and the National Institute for Coastal and Marine Management (RIKZ).

The field data and remote sensing data give information about the actual status of the spring phytoplankton bloom. The use of the model allows for near real-time forecasting of Phaeocystis blooms. The complementary use of three data sources compensates for the limitations of each of the data sources. The information on the status of the algal blooms is presented to the local coastal water managers in the form of a weekly HAB Bulletin.

In the spring of 2006, a weekly algal bloom information bulletin was started. This service bulletin is based on all data that are available for a given week: field data, remote sensing and operational model computations. If no field data or remote sensing data are available the bulletin is based just on the model data. If remote sensing data with only limited cloud cover are available these data are used as the start situation for the model which is used to forecast further transport of the bloom.

In shallow near-coastal waters in the Netherlands such as the Oosterschelde (Eastern scheldt), turbidity is rather high and wind-induced waves have a strong impact on turbidity. Since the initiation of spring phytoplankton blooms is light-controlled, accurate simulation of turbidity is assumed to be essential for adequate simulation of spring bloom development.

To this end the suspended matter forcing of the model is based on monthly composite total suspended matter data of the MERIS satellite, with a superimposed short term variability due to wind-induced waves. This suspended matter forcing corresponds well with field data on suspended matter.

The system is planned to be operational in 2007, delivering algal bloom information bulletins to government officials and mussel farmers on the present and expected development of algal blooms in the area. The bulletins will be issues twice weekly during the spring period. Phaeocystis blooms generally occur during spring.

GMES-MARCOAST services

Harmful Algae Bloom Monitoring, Evolution and Forecasting Service: The pan-European algae bloom service provides a basic daily detection of algae blooms for the whole European sea region and automated email alert to registered users.

Links

• [1]Water Insight
• [2] Marcoast
• [3] [4]GMES-MARCOAST
• [5] HAB research

Authors

The main authors of this article are Nicki Villars [6] and Hans Roberti [7]