Although concern here is primarily with mussel beds as "Biogenic
Reefs" of intrinsic conservation interest in their own right, it is recognised that
in many places the mussels are of as much importance directly as food for birds and
invertebrate predators, or as key functional features of the ecosystems of the large
shallow embayments. Responses to natural events here are reviewed primarily in the context
of the intrinsic interest of the reef features, but these other contexts also need to be
borne in mind.
The presence and scale of the mussel bed mounds is governed by the
complex interplay of features that on the one hand cause them to build up and on the other
break them down. Stock density is influenced by recruitment, predation and density
dependent mortality, together with factors that affect feeding; the production of faeces
and growth all build up the mounds. Waves, currents, predation and sometimes ice scour or
sand burial, limit, erode or carry away the mounds.
The dynamics and sensitivity of mussel beds forming biogenic mounds
under various conditions needs to be considered at the various different levels of scale.
In the long-term, persistent mound forming beds tend to remain in the same place, though
patches and individual mounds within them are very much more dynamic. Above the scale of
the individual, consideration has also to be given to dynamics at the scale of clumps.
Differences in recruitment, growth and mortality are likely to occur at all scales and
similarly in the associated fauna and flora.
Far less is known of the dynamics and sensitivities of the gravel
deposit type of bed than the commercially exploited estuary channel beds.
In general, Mytilus is considered to have a strong ability to
recover from disturbance (Seed & Suchanek, 1992).
Many mussel reefs are vulnerable to total destruction by storms and
tidal surges in the Wash, Morecambe Bay and the Wadden Sea, and on occasion this may
involve hundreds of hectares. The number of mussel beds in the Schleswig-Holstein part of
the Wadden Sea mapped by aerial survey decreased from 94 in 1989 to 49 in 1991 as a result
of severe storms in early 1990 (Nehls & Thiel, 1993). Storms were a major factor in
limiting persistent mussel beds to the shelter of the islands while in more exposed areas
the beds were highly dynamic. Young (1985) demonstrated experimentally that mechanical
agitation stimulated byssus production in mussels. Erratic storms are likely therefore to
be more damaging than where equivalent wave forces are the norm on exposed rocky shores.
Ice flows can sweep away beds in the Wash and the Wadden Sea in the
most severe winters. Sand burial of Mytilus reefs occurs occasionally in Morecambe
Bay (Dare, pers. comm.). Large scale sand movements are also common in other places, such
as parts of the Cumbrian Coast and Solway Firth (e.g. Perkins, 1967; 1968; 1970; 1971;
Perkins et al., 1980), and can be expected to bury Mytilus beds and reefs on
Long-term climate change
Beukema (1992) used the observed macrobenthos changes in the western
part of the Wadden Sea over mild winters in the 1969-1991 period to forecast responses to
warming in the long term. Mild winters resulted in greater weight loss by individuals over
the winter followed by poor recruitment in the following summer and subsequent problems
both for the fishery and dependent predators such as eiders. In part, recruitment failure
after mild winters is thought to be due to greater numbers of predatory small crabs on the
flats at the crucial time of settlement. On the other hand, severe ice cover over hard
winters can also greatly reduce mussel densities, e.g. in the Wadden See (Nehls &
Thiel, 1993) and the Wash (Dare, pers. comm.). Increased mussel predation by birds was
reported for several areas during the harsh winter of 1964 (see chapter IV).
Predation is an important influence on all mussel populations and has
been discussed in detail in chapter IV. See also the effects of mild winters on crab
predation in the paragraph above. However, Nehls & Thiel (1993) considered that bird
predation was less important in causing losses of entire adult mussel populations than
factors such as storm loss.
Dense phytoplankton blooms can, on occasion, be detrimental to Mytilus
edulis, although serious effects at the population level have only occasionally been
reported. Dense blooms of a non-flagellated chrysophycean alga in North America caused
heavy mortalities in M. edulis (Tracey, 1988). Blooms of the toxic dinoflagellate Gyrodinium
aureolum have been reported to cause some mortalities of M. edulis in Norway
(Tangen, 1977), and sublethal toxic including acute effects on clearance rate and marked
cellular damage to the gut in Mytilus in the UK (Widdows et al., 1979), although,
in general, reports of G. aureolum blooms seem to suggest that kills of fish,
lugworms and other invertebrates are more frequent and widespread than are kills of
mussels (Boalch, 1979; Helm et al., 1974; Tangen, 1977). A bloom of the flagellate Phaeocystis
poucheti, which produces copious amounts of glutinous material, caused reproductive
failure in Mytilus in the Dutch Wadden Sea as a result of the inability of the
mussels to feed (Pieters et al., 1980).