At all stations, a large interannual variability of the benthic communities was combined with a variation on a roughly decadal scale and a couple of unusual years presumably following disturbances. A general description of the temporal development will be followed by a more detailed discussion of the effects of influential factors in subsequent chapters.
The multivariate community analysis identified several groups of years with a similar community composition at all stations and a couple of exceptional years.
The groups represent roughly the 70ies, 80ies and 90ies, but they are not distinctly separated by strong changes. Exceptional community compositions were recorded in most cases following extremely cold winters (see e.g. Fig.
4.3.4a).
The transitions between the periods are rather gradual changes in species composition and dominance structure, and each period is itself subject to large interannual community fluctuations.
The periods correspond roughly to the observed shifts in various compartments of the biological system of the North Sea in the late 70ies and the late 80ies 70ies, which are suspected to be at least partly due to methodological changes, the above-mentioned periods do not differ significantly in total density of organisms, species number or community structure. Major changes of these parameters are reflections of short-term fluctuations often related to disturbances rather than persistent changes over longer periods. The largest changes in the benthic communities follow severe winters, which strongly influence macrofaunal communities in coastal areas (Ziegelmeier 1964; 1970;
Gerdes 1977; Buhr 1981; Dörje et al. 1986; Arntz & Rumohr 1986; Beukema 1989; Krönck et al. 1998; Armonies et al. 2001).
The clearest development can be Seen at the two deeper stations, the Silty- Sand-station (SSd) and the station east of the White Bank (WB).
The most prominent feature of the community development at SSd are the changes following the severe winters of 1970, '79, '86 and '96 (Fig. 4.4.4).
Following all of these winters, the densities of many species decline markedly and a few opportunistic species, mostly small polychaetes, increase, as already found in earlier years by Ziegelmeier (1 964; 1970). While the community during 1993-95 approached a similar composition as already found during the 70ies, the change initiated in 1996 is followed by a recovery from 1997 to '99 that results in a similar community composition as found during the 80ies. In 2000, the community approaches the situation of the early 90ies again, although
A. fdiformis has not regained its former densities, while several small bivalve species (Nucula nifidosa, Mysella bidentata, Montacuta ferruginosa, Tellina fabula, Abra spp.) reach extremely high densities.
A similar temporal development is visible in the shorter series from WB. Here, most of the 80ies represent a rather persistent community dominated by change in the community occurs in 1992193. Strong decreases of the dominant species, especially A. filiformis, lead to a more even community structure, which persists until 1995. The severe winter of I995196 initiates the largest changes observed at this station, with a strong decrease of most species. This is followed by a massive recruitment mainly of the species that were dominant during the 80ies, and by 1999 the community has reached a similar composition as during the early 90ies.
Although a roughly similar temporal development is discernible at the shallower Silt-station (SIt) and Fine-Sand-station (FSd), large interannual fluctuations render the picture less clear.
The community at SIt is characterised by the lowest interannual similarity of all stations, with very large fluctuations of most species. The community composition following the cold winters is not as clearly different from other years, although most of these years are to some extent separated from the respective periods, with generally low species numbers and abundances. The community development following 1996 is similar to the development after 1970, and the community approaches in the 90ies the situation of the 70ies again. Both periods are characterised by a dominance of Nucula nifidosa, which was less abundant during the 80ies, when Diasfylis rafhkei dominated the community.
Although interannual similarity is higher at FSd, the community is dominated by opportunistic species like Magelona mirabilis and other small polychaetes during most years. Therefore, the years following most severe winters are not as clearly separated from the remaining years. However, marked changes are visible following 1996, and a situation similar to the early 90ies is only reached again by the year 2000.
The separation of roughly decadal periods should not be interpreted as periods of relative stability separated by major transition.
Each of these periods is characterised by strong fluctuations and often a development within the period. Periods without major changes are the exce~tion, and sianificant inter-annual chanaes are the rule.
A.5.5).
Large inter-annual changes and interruptions by "exceptional" years make a statistical detection of temporal trends difficult. An application of regression analysis or smoothing methods or filters to extract long-term development against interannual ("random?") variability would also ignore the occurrences of distinct disturbances, which are a characteristic feature of these shallow-water systems. Therefore, the simple visual inspection and description of the development of the communities and nonparametric correlation analyses with environmental factors are probably the best way to interpret the present data.
As each station represents a clearly distinct type of community, the nature of the community changes differs. Fluctuations in total abundance or species density are not in parallel between the stations, except for a few common features. Lower total abundance and species number found at most stations during the early 70ies may in part by related to low penetration depth of the grab especially at FSd and SSd. As the increasing species number in the late 70ies was also observed at SIt, where it is not correlated to methodological changes, it should be based on real increases in species density. From then until 1995, no major trends in the species number are observed. The development during the 9Oies is remarkably similar between the stations, with a relatively high species number until '95 (with the exception of a low species number in '93). At all stations, species number drops after the severe winter of 1995196 with a recovery after two to three years.
Beyond these similar development of the species density, the temporal development of the benthic communities at the different stations does not run very much in parallel and a couple of species do even exhibit opposing trends at different stations. While large inter-annual fluctuation make a detection of long-term trends difficult, a number of significant correlations to environmental variables was detected at all stations.