Quantitative changes

Qualitative changes

There is a very marked seasonality within European maerl communities. Both Jacquotte (1962) and J. Cabioch (1969) investigated the seasonal changes on the maerl and reported that of the "constant association" of characteristic epifloral species about half were found throughout the year, while most of the others were collected only in summer. Jacquotte found Halopitys incurvus to be more frequent in winter, and Cabioch reported that a few crustose species were more abundant during the winter months. Jacquotte attributed the seasonal changes in the epiflora of the maerl beds in the Mediterranean to seasonal changes in illumination, as the temperature at depth remained more or less constant throughout the year.

A study in Galway Bay focussed on seasonality of two subtidal maerl beds during 1980 and 1981 (Maggs, 1983a, 1983b). The two maerl beds chosen differ considerably: the Carraroe site at 5 m depth is exposed to wave action and strong currents, and the principal maerl species is Phymatolithon calcareum, while the Finavarra bed at 10 m is more sheltered, and is composed largely of Lithothamnium corallioides. The cover of macroalgae and the number of species were counted monthly. The algal diversity increased in the summer, probably due to the greater stability of the beds as a result of the calmer weather. Although nutrient levels in seawater are very low in summer, macroalgae can generally utilise stored nitrogen, e.g. by metabolising pigments (so that red algae become yellow), to continue growth and reproduction.

All cover and presence data were analysed by cluster analysis (CLUSTAN) and DECORANA. Seasonal changes in both total algal abundance and diversity were apparent on both maerl beds, but were more marked at 5 m which was dominated in summer by almost 100% cover of Dictyota dichotoma, while at 10 m the maximum cover was 60%. DECORANA (Figure 4) showed clearly that the composition of samples followed an annual cycle, which was more marked at Carraroe than at Finavarra. Likely causes of this were the greater seasonal changes in the shallow depths at Carraroe in environmental variables such as temperature, photon irradiance, amount of blue light, and wave perturbation. The maerl community had several features in common with other communities on mobile substrata, including a high proportion of ephemeral species. The majority of the perennial species are crustose.

Preliminary studies of faunal seasonality on maerl in the BIOMAERL programme have also found dramatic seasonal variations, such as massive juvenile recruitments and mortality, and the switching of feeding mode depending on food availability (J. Hall-Spencer, pers. comm.). An example is the seasonal pattern of population densities of the infaunal holothurian Neopentadactyla mixta at Bute, where this species was recorded only in March and April, and was apparently absent during the rest of the year (BIOMAERL, unpublished data).

One important aspect of seasonal changes in biodiversity is that of heteromorphic life histories of algae, in which an erect phase (usually the haploid gametophyte) alternates with a cryptic crustose, boring or filamentous phase (typically the diploid sporophyte). The maerl epiflora can be divided into three main groups of species:

1. Present as mature thalli all year, probably mostly quite long-lived.
2. Present as the erect form for only part of the year, then as cryptic crustose or perennating fragments for the rest of the year.
3. Absent from the community for part of the year and depending on input from reproducing populations for their presence.

Examples of maerl epiphytes with heteromorphic phases are given in the table below. The different phases in the heteromorphic life histories of these species which are of survival value on mobile substrata must be related to the environmental conditions which obtain during each season. In general, the erect phases of the life history are found only during the summer months. Each species must respond appropriately to the main environmental conditions of temperature, light, and daylength. A combination of field studies and laboratory culture experiments can provide some understanding of the nature of the complex interactions of the environmental stimuli governing the life histories of some of the maerl epiphytes, and thus may be used to explain - and possibly predict - some of the seasonally related changes in the maerl epiflora.

Over the course of a medium-term investigation, species can both appear and disappear in a non-seasonal pattern. Maggs (1983a) reported that during a 2-year-long sampling programme 9 conspicuous species disappeared from the maerl beds under investigation while a further 3 species appeared in the biotope. The highlights the problems that may be encountered in trying to interpret data from monitoring the epifloral component of maerl beds.

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