Correlation between Intertidal Seaweed Community Composition and Sea Water Temperature Patterns on a Geographical Scale

Quantitative seaweed community data were collected from the lower intertidal region of 21 semi-exposed shores along ca 600 km of coastline around the southern tip of Africa. There is a gradient of ambient sea water temperature regimes at these sites, with annual means from 12.9 to 16.7 °C, and a difference between maximum and minimum monthly means of from 1.8 to 5.8 °C. An analysis by Reciprocal Averaging has revealed a community gradient which follows closely the gradient of temperature change around the coastline. Values for the first axis of the analysis correlate significantly with annual mean, maximum monthly mean and minimum monthly mean sea temperatures. Community samples from the warmest region of False Bay, where temperatures are raised, are similar to those from geographically distant sites with comparable temperature regimes. Apart from one site with a very different community caused by sand-scouring effects, there was no effect of substratum on community composition on the different rock types sampled. It is concluded that not only the absolute ranges of seaweed species distributions, but also the structure and composition of seaweed communities on a biogeographical scale can be correlated with seawater temperature regime. Introduction . , ~ Lj* -u *· r potheses. Geographic distributions of congeneric speFactors affecting the distribution of intertidal seaweed cies seem to be unaffected by competition (Pielou communities differ in relative importance with the 1977). scale of the investigation. In small scale investigations, The evidence for the control of seaweed distributions the study of a bay or short stretch of coast, such by seawater temperature is in two general categories: factors as wave exposure, substrate, sea water tern^ Λ . , ,. perature, exposure to air, salinity, turbidity and a *: Correlations of floral discontinues with disconvariety of biological factors (especially interference f' m "*' "TM? TMTM***TM ^/'/S* . Λ , Ν t u ,. . u and Donze 1967, Hoek 1982, Murray and Littler competition a;nd herbivory) have been shown to be ΛΛΛΛ „ , „ ' ^ t , ' 0 , «TM„ ^ , ·*· ι · A-ff + Α ι κ· ι,· 1981, Searles 1984, Bolton 1986, South 1987, Bolton critical in different areas. On a larger biogeographic ' ,ΛΜν i t and Steeenea 1987) scale, however, sea water temperature has been con6 & ' tinuously emphasised in the literature as an overriding 2. Detailed laboratory studies in culture linking physfactor controlling the distribution of seaweed floras, iological temperature tolerance limits to geographic Although it has been suggested that biological factors distribution limits (e.g. Hoek 1982, Cambridge et al. may influence large scale distribution (e.g. Gaines 1984, 1987, L ning 1984, Yarish et al. 1984, 1986, and Lubchenko 1982) there is to date no case with 1987, Bolton and Anderson 1987, Anderson and Bolwell documented evidence supporting such hyton 1989). Botanica Marina / Vol. 33 / 1990 / Fasc. 5 Copyright © 1990 Walter de Gruyter · Berlin · New York 448 Bolton and Anderson: Correlation between community composition and sea water temperature In a review of the latter approach (Breeman 1988) detailed distribution and temperature tolerance data was analysed for ca 60 species in the North Atlantic, and it was concluded that "in all but a few species, thermal responses accounted for the location of boundaries. " Thus the evidence that aspects of sea water temperature regimes control distributions of individual seaweed species by limitation of specific life history stages is overwhelming. This dominant role of a single physical factor in controlling distributions is unusual in ecology. One fact that has thus far largely been overlooked is that the vast majority of studies investigating causes of seaweed distribution are based on presence/absence data; that is they show that there is a good correlation between the total ranges of individual species and ambient temperature regimes. This may be construed as a benefit when considering the evolutionary biogeography of seaweed floras [for example Lawson (1988) states that it is an 'advantage . that every species is taken into account, so an infrequent microscopic filament may be considered (to be) as characteristic of a flora as a common laminarian']. However, investigating the effects of sea water temperature on seaweed community patterns along an environmental gradient is primarily an ecological rather than an evolutionary topic (see Discussion). It could be hypothesised that a significant number of species may be relatively abundant for a certain portion of their range, although their relative" importance in the community at certain points along the total range may be limited by factors other than sea water temperature. In order to test this hypothesis we must investigate correlations between seaweed community data, incorporating abundance estimates, and prevailing environmental factors. This has been discussed by Murray and Littler (1981) who "emphasize the lack of definitive work demonstrating direct numerical correlations of species distributions with temperature data". In the present study seaweed communities were sampled around the southern tip of Africa, along a coastline with a definite gradient of seawater temperature regimes. An attempt is made to correlate community composition with the prevailing environmental conditions using computer assisted ordination techniques.