Ulva spp. on freshwater-influenced or unstable upper eulittoral rock
Ecological and functional relationships
The community predominantly consists of algae which cover the rock surface and creates a patchy canopy. In doing so, the algae provides an amenable habitat in an otherwise hostile environment, exploitable on a temporary basis by other species. For instance, Ulva intestinalis provides shelter for the orange harpacticoid copepod, Tigriopus brevicornis, and the chironomid larva of Halocladius fucicola (McAllen, 1999). The copepod and chironomid species utilize the hollow thalli of Ulva intestinalis as a moist refuge from desiccation when rockpools completely dry. Several hundred individuals of Tigriopus brevicornis have been observed in a single thallus of Ulva intestinalis (McAllen, 1999). The occasional grazing gastropods that survive in this biotope no doubt graze Ulva.
Seasonal and longer term change
- During the winter, elevated levels of freshwater runoff would be expected owing to seasonal rainfall. Also, winter storm action may disturb the relatively soft substratum of chalk and firm mud, or boulders may be overturned.
- Seasonal fluctuation in the abundance of Ulva spp. Would therefore be expected with the biotope thriving in winter months. Porphyra also tends to be regarded as a winter seaweed, abundant from late autumn to the succeeding spring, owing to the fact that the blade shaped fronds of the gametophyte develop in early autumn, whilst the microscopic filamentous stages of the spring and summer are less apparent (see recruitment process, below).
Habitat structure and complexity
Habitat complexity in this biotope is relatively limited in comparison to other biotopes. The upper shore substrata, consisting of chalk, firm mud, bedrock or boulders, will probably offer a variety of surfaces for colonization, whilst the patchy covering of ephemeral algae provides a refuge for faunal species and an additional substratum for colonization. However, species diversity in this biotope is poor owing to disturbance and changes in the prevailing environmental factors, e.g. desiccation, salinity and temperature. Only species able to tolerate changes/disturbance or those able to seek refuge will thrive.
The biotope is characterized by primary producers. Rocky shore communities are highly productive and are an important source of food and nutrients for neighbouring terrestrial and marine ecosystems (Hill et al., 1998). Macroalgae exude considerable amounts of dissolved organic carbon which is taken up readily by bacteria and may even be taken up directly by some larger invertebrates. Dissolved organic carbon, algal fragments and microbial film organisms are continually removed by the sea. This may enter the food chain of local, subtidal ecosystems, or be exported further offshore. Rocky shores make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains.
The life histories of common algae on the shore are generally complex and varied, but follow a basic pattern, whereby there is an alternation of a haploid, gamete-producing phase (gametophyte-producing eggs and sperm) and a diploid spore-producing (sporophyte) phase. All have dispersive phases which are circulated around in the water column before settling on the rock and growing into a germling (Hawkins & Jones, 1992).
Ulva intestinalis is generally considered to be an opportunistic species, with an 'r-type' strategy for survival. The r-strategists have a high growth rate and high reproductive rate. 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.
The life cycle of Porphyra involves a heteromorphic (of different form) alternation of generations, that are either blade shaped or filamentous. Two kinds of reproductive bodies (male and female (carpogonium)) are found on the blade shaped frond of Porphyra that is abundant during winter. On release these fuse and thereafter, division of the fertilized carpogonium is mitotic, and packets of diploid carpospores are formed. The released carpospores develop into the 'conchocelis' phase (the diploid sporophyte consisting of microscopic filaments), which bore into shells (and probably the chalk rock) and grow vegetatively. The conchocelis filaments reproduce asexually. In the presence of decreasing day length and falling temperatures, terminal cells of the conchocelis phase produce conchospores inside conchosporangia. Meiosis occurs during the germination of the conchospore and produces the macroscopic gametophyte (blade shaped phase) and the cycle is repeated (Cole & Conway, 1980).
Time for community to reach maturity
Disturbance is an important factor structuring the biotope, consequently the biotope is characterized by ephemeral algae able to rapidly exploit newly available substrata and that are tolerant of changes in the prevailing conditions, e.g. temperature, salinity and desiccation. For instance, following the Torrey Canyon tanker oil spill in mid March 1967, which bleached filamentous algae such as Ulva and adhered to the thin fronds of Porphyra, which after a few weeks became brittle and were washed away, regeneration of Porphyra and Ulva was noted by the end of April at Marazion, Cornwall. Similarly, at Sennen Cove where rocks had completely lost their cover of Porphyra and Ulva during April, by mid-May had occasional blade-shaped fronds of Porphyra sp. up to 15 cm long. These had either regenerated from basal parts of the 'Porphyra' phase or from the 'conchocelis' phase on the rocks (see recruitment processes). By mid-August these regenerated specimens were common and well grown but darkly pigmented and reproductively immature. Besides the Porphyra, a very thick coating of Ulva (as Enteromorpha) was recorded in mid-August (Smith 1968). Such evidence suggests that the community would reach maturity relatively rapidly and probably be considered mature in terms of the species present and ability to reproduce well within six months.
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This review can be cited as follows:
Ulva spp. on freshwater-influenced or unstable upper eulittoral rock.
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=104&code=1997>