EXPERIMENTS WITH LARGE ENCLOSED ECOSYSTEMS

Three of the major advantages of enclosure experiments are that they ensure (1) that the same populations are sampled over a long period; (2) that populations of at least three trophic levels are initially enclosed in naturally occurring proportions and that they are self sustaining over a long experimental period; and (3) that replicate enclosed populations can be experimentally manipulated. There are two disadvantages which must be mentioned. These are (1) that vertical mixing, which may be reduced by as much as an order of magnitude compared to the open sea, will undoubtedly affect the sinking rates of phytoplankton and may influence the structure of the population; and (2) that as a general rule the larger and therefore more expensive the enclosures become, the more difficult it is to run sufficient replicates. An experiment is described in which 1 microgram Hg/l was added to two 95 m3 bags (3 mdiameter by 17 m deep) and the response of the pelagic population monitored over the following 20 days. A further 10 micrograms Hg/l was then added to each enclosure and the response measured for a further 20 days. The results indicated that: (i) inorganic mercury added to the water column is very rapidly transformed into 'bound' or 'non-reactive' mercury and that about 25% of the mercury added was recovered associated with the organic material settling to the bottom of the bags; (ii) the response of the biological population to 1 microgram Hg/l was very limited and in fact a transient reduction in photosynthetic carbon uptake per unit chlorophyll was the only noticeable effect and there were no changes in population size or structure that could be attributed to mercury; (iii) at 10 micrograms Hg/l the zooplankton population was reduced markedly and this did produce changes in the structure of both the zooplankton and phytoplankton populations. These results are similar to the results of a comparable experiment carried out in Vancouver Island (Cepex) and point to the conclusion that the levels of mercury found in surface waters around the coast of the U.K. (0.001--0.022 microgram Hg/l) are one or two orders of magnitude below the levels at which a response of the biological population can be demonstrated. The usefulness of large scale enclosed ecosystems for further pollution research is discussed and it is concluded that those facilities that provided a link between the water column and the sediments would be most useful since they would (1) enable estimates to be made of the flux rates of pollutants from the water column to the sediments; and (2) allow experiments to be carried out with the pollutant in contact with sediment in its natural form.