Green seaweeds (Ulva spp. and Cladophora spp.) in upper shore rockpools
Image Dale Cartlidge - Green seaweeds (Enteromorpha spp. and Cladophora spp.) in upper shore rockpools Image width ca 1m.
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Ecological and functional relationships
In rockpools high on the shore, the familiar flora and fauna of rockpools is lost, the community becomes greatly depleted consisting of forms that are highly adapted to the rigorous and almost estuarine conditions (Lewis, 1964) as physical factors are the dominant structuring force.
Amongst the fauna, crustaceans predominate. Large populations (720 x 103m²) of the copepod Tigriopus fulvus can occur in upper shore rockpools densely covered by the green alga Ulva intestinalis (Goss-Custard et al., 1979). Tigriopus fulvus is remarkably tolerant of extremes of salinity and temperature (Ranade, 1957). Ranade (1957) stated that Tigriopus fulvus could live normally between salinities of 4-90 psu. In laboratory experiments, Goss-custard et al. (1979) found the species to survive for 15 days in salinities ranging from 42-90 psu, but died after 84 hours in distilled water, and sank to the bottom in salinities greater than 90 psu in a state of apparent death. However, if transferred to seawater (35 psu) after some hours it could recover. In tests with a slowly rising temperature, the death point was 32°C at a salinity of 34 psu, but this rose to 41.8°C at a salinity of 90 psu. Thus high salinities enable Tigriopus fulvus to withstand high temperature, a feature useful for a species living in pools in a zone where insolation and evaporation may be considerable. Despite the instability of the high shore rockpool as a habitat, the copepod benefits from the lower abundance of predators, that are in greater abundance in lower shore rockpools (Dethier, 1980).
Ulva intestinalis provides shelter for the orange harpacticoid copepod, Tigriopus brevicornis, and the chironomid larva, Halocladius fucicola (McAllen, 1999). Ulva intestinalis is often the only seaweed found in supralittoral rockpools, and the copepod and chironomid species utilize the hollow thallus of Ulva intestinalis as a moist refuge from desiccation when the rockpools completely dry out. Several hundred individuals of Tigriopus brevicornis have been observed in a single thallus of Ulva intestinalis (McAllen, 1999).
There are three major sources of food available to the fauna of high shore rockpools: the thalli of Ulva sp. and other macroalgae, the epiphytic micro-organisms attached to the surface of the Ulva and the micro-organisms associated with the substratum (Clark, 1968).
The distribution of grazers, Melarhaphe neritoides and Littorina saxatilis extends into the upper littoral fringe, the former feeding on micro-algae and lichens, the latter grazing on macroalgae and the microalgal film on the rocks. Both winkles favour crevices, especially in dry weather, from which they can forage, but owing to the physical stresses of the upper shore, grazing molluscs are generally lower in abundance than in eulittoral pools allowing green algae to proliferate as a result of reduced grazing pressure.
A band of yellow and grey lichens (£LR.YG£) is usually found immediately above the zone of Verrucaria maura which occurs in this biotope. The fauna of the LR.YG biotope may extend into the LR.G biotope to exploit the lichen. For instance, lichens are fed on by fungivorous Cryptostigmata and other acarid mites and potentially by some lichen dwelling tardigrades (Gerson & Seaward, 1977) and the bristle tail Petrobius maritimus (Joosse, 1976), while rotifers have been reported to consume lichen ascospores (Gerson & Seaward, 1977).
Seasonal and longer term change
Rockpools constitute a distinct environment for which physiological adaptations by the flora and fauna may be required (Lewis, 1964). Physico-chemical parameters within rockpools fluctuate dramatically as a consequence of prolonged separation from the main body of the sea (Huggett & Griffiths, 1986). In general, larger and deep rockpools low on the shore tend to correspond to the sublittoral habitat with a more stable temperature and salinity regime. In contrast, small and shallow pools are especially influenced by insolation, air temperature and rainfall, the effects of which become more significant towards the high shore, where pools may be isolated from the sea for a number of days or weeks (Lewis, 1964).
- Weather conditions exert a considerable influence on temperature and salinity. Water temperature in pools follows the temperature of the air more closely than that of the sea. In summer, shallow pools or the surface waters of deeper pools are warmer by day, but may be colder at night, and in winter may be much colder than the sea (Pyefinch, 1943). In deeper pools, the vertical temperature gradation usually present in summer, reverses during winter owing to density stratification, so that ice may form (Naylor & Slinn, 1958).
- High air temperatures cause surface evaporation of water from pools, so that salinity steadily increases, especially in pools not flooded by the tide for several days. Alternatively, high rainfall will reduce pool salinity or create a surface layer of brackish/nearly fresh water for a period. However, the extent of temperature and salinity change is affected by the frequency and time of day at which tidal inundation occurs. If high tide occurs in early morning and evening the diurnal temperature follows that of the air, whilst high water at midday suddenly returns the temperature to that of the sea (Pyefinch, 1943). Heavy rainfall, followed by tidal inundation can cause dramatic fluctuations in salinity, and values ranging from 5-30 psu have been recorded in a period of 24 hours (Ranade, 1957). Rockpools in the supralittoral, littoral fringe and upper eulittoral are liable to gradually changing salinities followed by days of fully marine or fluctuating salinity at times of spring tide (Lewis, 1964).
- Other physico-chemical parameters in rockpools demonstrate temporal change. The biological community directly affect oxygen concentration, carbon dioxide concentration and pH, and are themselves affected by changes in the chemical parameters. Throughout the day, algae photosynthesize and produce oxygen, the concentration of which may rise to three times its saturation value, so that bubbles are released. During photosynthesis algae absorb carbon dioxide and as concentrations fall, the pH rises. pH values >9 were recorded in rockpools on the Isle of Cumbrae (Morris & Taylor, 1983). At night changes occur in the opposite direction. Respiration utilizes much of the available oxygen and pH decreases.
Fluctuations especially in the abundance of green seaweeds is likely owing to the marked changes in salinity and temperature during the year. For instance, surface layers of Ulva
may be bleached in the summer.
Habitat structure and complexity
Rockpools vary greatly in their physical features. Pools in bedrock may be shallow and well-lit or deep and shaded with overhanging sides and vertical surfaces. Algae growing within provide additional surface for colonization and for shelter. There is also a tendency for loose substrata (sand, stones, rocks) to accumulate in pools, the instability of which may cause abrasion and affect species diversity. Amongst rockpools, deep crevices may be found, around the entrance of which small mussels may cluster. Crevices also support their own specialized fauna with many air-breathing arthropods such as centipedes, millipedes, beetles, pseudoscorpions and primitive onchidellid pulmonates (see Lewis, 1964).
Macroalgae and the microbial film of bacteria, blue-greens, diatoms, fungi and protozoans are the primary producers in this biotope. Accumulations of algal debris are also likely in high shore rockpools and such detrital material contributes to overall productivity. Information specific to the community was not found, but Workman (1983) gave an estimate of primary production by microalgal films on the high shore in the British Isles to be in the region of 60 g C/m²/yr, much of which will be utilized directly by grazers.
Rockpools in the supralittoral, littoral fringe or upper eulittoral which are subject to variable salinity and widely fluctuating temperatures are characterized by the ephemeral green alga Ulva
spp. or the filamentous green alga Cladophora
Species of the genus Ulva
are rapidly growing opportunists, favoured by the frequency and speed of their reproduction. The short lived plants reach maturity at a certain stage of development rather than relying on an environmental trigger. Ulva intestinalis
can be found in reproductive condition at all times of the year, but maximum development and reproduction occur during the summer months especially towards the northern end of the distribution of the species (Burrows, 1991). The life history consists of an isomorphic (indistinguishable except for the type of reproductive bodies produced) alternation between haploid gametophytic and diploid sporophytic generations, but can be modified by environmental conditions (Burrows, 1959; Moss & Marsland, 1976; Reed & Russell, 1978).
The haploid gametophytes of Ulva
produce enormous numbers of biflagellate motile gametes which cluster and fuse to produce a sporophyte (diploid zygote). The sporophyte matures and produces by meiosis large numbers of quadriflagellate zoospores that mature as gametophytes, and the cycle is repeated. Both gametes and spores may be released in such quantities into rock pools or slack water that the water mass is coloured green (Little & Kitching, 1996). Together spores and gametes are termed 'swarmers'. Swarmers are often released in relation to tidal cycles, with the release being triggered by the incoming tide as it wets the thallus. However, the degree of release is usually related to the stage of the spring/neap tidal cycle, so allowing regular periodicity and synchronization of reproduction (Little & Kitching, 1996). Christie & Evans (1962) found that swarmer release of Ulva intestinalis
from the Menai Straits, Wales, peaked just before the highest tides of each neap-spring cycle. Mobility of swarmers belonging to Ulva intestinalis
can be maintained for as long as 8 days (Jones & Babb, 1968). Algae such as Ulva intestinalis
tend to have large dispersal shadows, with propagules being found far from the nearest adult plants, e.g. 35 km (Amsler & Searles, 1980).
Information on the ecology of reproduction and propagation of the genus Cladophora
is limited. Reproduction is asexual, and achieved by the release of quadriflagellate zoospores and biflagellate isogametes formed in the terminal cells of fronds. The life history consists of an isomorphic (indistinguishable except for the type of reproductive bodies produced) alternation of gametophyte and sporophyte generations, the plants are dioecious (Burrows, 1991). Both zoospores and gametes can be found at most times of the year. Archer (1963) was unable to find any correlation between the time of reproduction, the state of tide or environmental conditions. Most species of Cladophora
attach to the substratum by multicellular, branching rhizoids (van den Hoek, 1982); these basal holdfasts may serve as resistant structures from which new growths can arise.
Fraser (1936) describes the ecology and life-history of the copepod Tigriopus fulvus
. The species mates throughout the year. Females of the species release a sex pheromone promoting sexual recognition and attraction in males (Lazzaretto et al.
, 1994). Females brood the fertilized eggs which may be released between 5-15 days after the appearance of the female's egg sac, the time being shorter in summer and longer in winter. A single female may produce numerous juveniles from several egg sacs without further mating. From the time of hatching a juvenile attains an adult form and the ability to reproduce in about two months (Fraser, 1936).
Internal fertilization occurs in all species of winkle (littorinids). Melarhaphe neritoides
releases its eggs into the plankton, whilst the female Littorina saxatilis
broods its eggs which hatch as live young. Although animals with planktonic larvae have a greater dispersive ability than those with direct development, the production of crawling, live young from egg capsules or brood pouches reduces reproductive losses and permits exploitation of locally favourable conditions. It can also lead to inbreeding and genetic isolation of populations. For instance, owing to dispersion in the plankton the population of Melarhaphe neritoides
is genetically homogenous, which is reflected in their uniform colour. Littorina saxatilis
, which bears live young and are variable in size and colour (Hawkins & Jones, 1992).
Time for community to reach maturity
To recruit, grow and reproduce in the unpredictable environment of high shore rockpools, the flora and fauna within need to be capable of rapid recruitment, early maturation and rapid growth in order to exploit the habitat, thus it is likely that the community would be considered mature in terms of species present and capable of reproduction within a few months.
For example, with the exception of Cladophora rupestris
whose turfs may persist for many years, the macroalgal species, e.g. Ulva
and Prasiola stipitata
which are characteristic of this biotope are seasonal and short lived (ephemeral) algae, which recruit rapidly to available substrata. For instance, the thalli of Ulva intestinalis
, which arise from spores and zygotes, grow within a few weeks into thalli that reproduce again, and the majority of the cell contents are converted into reproductive cells. The species is also capable of dispersal over a considerable distance. For instance, Amsler & Searles (1980) showed that swarmers of a coastal population of Ulva
reached exposed artificial substrata on a submarine plateau 35 km away. Ulva
is amongst the first multicellular algae to appear on substrata that have been cleared following a disturbance, e.g. following the Torrey Canyon
oil spill in March 1967, species of the genus Ulva
rapidly recruited to areas where oil had killed the herbivores that usually grazed on them, so that a rapid greening of the rocks (owing to a thick coating of macroalgae) was apparent by mid-May (Smith, 1968).
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This review can be cited as follows:
Green seaweeds (Ulva spp. and Cladophora spp.) in upper shore rockpools.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=246&code=1997>