Environmental Statement - Maersk Oil

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Balloch Field Development Environmental Statement
Section 3 Baseline Environment
The composition and abundance of plankton communities vary throughout the year and are
influenced by several factors including depth, tidal mixing, temperature stratification, nutrient
availability and the location of oceanographic fronts. Species distribution is directly influenced by
temperature, salinity, water inflow and the presence of local benthic communities (Robinson, 1970).
Plankton also includes the eggs, larvae and spores of non‐planktonic species (fish, benthic
invertebrates and algae). This meroplankton population may have a very different seasonal cycle
depending on the life cycle strategy of the fish species and benthic organisms which inhabit the area.
The plankton community, although vulnerable to chemical or hydrocarbon releases to the sea, is less
vulnerable to one‐off incidents than the benthos, because most phytoplankton have rapid maximum
doubling times and there is a continual exchange of individuals with the surrounding waters (North
Sea Task Force, 1993). A consequence of rapid doubling times is that when light and nutrient
conditions are favourable, “blooms” of these organisms can develop. Although they are sometimes
caused by anthropogenic pollution, plankton blooms occur naturally. These blooms have tended to
occur each spring in the North Sea water with a smaller peak in the autumn. However, recent studies
(FRS, 2007) indicate that the pattern has changed to a single bloom throughout the summer.
Additionally, Harmful Algal Blooms (HABs) involving nuisance or noxious species can occur. These
blooms can result in changes to the ecosystem causing discolouration, fish and marine organism
mortality, deoxygenation and foam formation. The causes of HAB include rapid reproduction of a
species, reduced grazing pressure or alterations in light, temperature, salinity and nutrients (Johns
and Reid, 2001).
3.6.2. BENTHOS
Bacteria, plants and animals living on or within the seabed sediments are collectively referred to as
the benthos. Species living on top of the sea floor may be sessile (e.g. seaweeds) or freely moving
(e.g. starfish) and are collectively referred to as epibenthic organisms. Animals living within the
sediment are termed infaunal species (e.g. clams, tubeworms and burrowing crabs) while animals
living on the surface are termed epifaunal (e.g. mussels, crabs, starfish and flounder). Semi‐infaunal
animals, including sea pens and some bivalves, lie partially buried in the sea bed. Benthic species may
also be classified in terms of their size. Macrobenthos are organisms greater than 1 mm in size,
microbenthos are smaller than 50 µm and the meiobenthos (50 µm to 1 mm) lie in between. These
classifications, together with examples of representative groups, are shown in Table 3‐10.