Filamentous green seaweeds on low salinity infralittoral mixed sediment or rock
Ecological and functional relationships
Many of the species in this community are reliant on or affected by one-and-other. Some examples of interactions are given below.
Dense shading by the seasonal growths of green filamentous algae prevents other macrophytes from growing but may offer shelter to invertebrates.
The aplisiomorph mollusc Akera bullata feeds on filamentous algae and is especially found in this sort of very sheltered variable or low salinity biotope (see, for instance, Thompson & Seaward, 1989).
Absence of significant grazing species because of low salinity conditions most likely is essential to the development of the dense algal mats.
Mysid shrimps are dependant on a supply of plankton on which to feed.
By feeding on zooplankton, mysids may enhance eutrophication in nutrient-rich brackish lakes by reducing grazing pressure on phytoplankton. (Aaser et al., 1995).
The three-spined stickleback Gasterosteus aculeatus feeds on mysids (Rademacher & Klis, 1996).
The ascidian Ciona intestinalis and the mussel Mytilus edulis, where they occurs in the biotope, may have a significant effect on phytoplankton biomass (Riisgård et al., 1993 for Ciona intestinalis).
Seasonal and longer term change
The biotope shows significant seasonal change especially in algal growth and includes many ephemeral species. For instance, Cladophora rupestris
attains maximum development in summer, Cladophora sericea
grows in spring and summer and Chaetomorpha linum
is abundant in spring and summer (Burrows, 1991). For animal species, Sarda et al.
(1998) working on saltmarsh communities that included several of the characterizing species in this biotope concluded that macroinfaunal densities peaked in June following spring recruitment. Also, three-spined sticklebacks Gasterosteus aculeatus
may colonize the habitat seasonally. The ecosystem may be unstable. Riisgård et al
. (1993) noted that, in a sheltered lagoonal system, during a long period of calm and warm weather, the filamentous algal mat rose to the surface releasing high amounts of nutrients and causing a phytoplankton bloom. One of the mysid species likely to occur in this biotope, Neomysis integer
, performs a diel vertical migration, rising towards the surface waters during the night and returning to the deeper waters at daylight where it remains throughout the day (Hough & Naylor, 1992).
Habitat structure and complexity
Both hard and soft substrata occur in this biotope with further surfaces for settlement provided by the filamentous algae that dominate the biotope especially in summer. Algae form a smothering blanket that nevertheless might provide a habitat for small crustaceans and snails that gain shelter amongst the filaments and feed on the algae.
Dense mats of filamentous green algae may adversely affect primary production though self-shading. Krause-Jensen et al.
(1996) observed that the self-shading produced by dense mats of Chaetomorpha linum
could cause a switch from being net productive to a status where consumption exceeded production possibly making such shallow macrophyte communities inherently unstable. Pedersen & Borum (1996) noted that fast growing algae were nitrate limited in experimental studies.
Sessile species such as the dominant algae, as well as mussels and ascidians where they occur and the infaunal polychaetes recruit from planktonic propagules. Some others such as the aplisiomorph mollusc Akera bullata
are likely to recruit from planktonic stages but also by migration. Some other species are highly mobile (for instance, fish, mysids, starfish, crabs) and most likely migrate into the biotope, possibly on a seasonal basis.
Time for community to reach maturity
The species in the biotope are predominantly fast growing and are likely to settle readily from planktonic propagules or migrate into the habitat. Development of the community will depend on the time of year that a suitable habitat becomes available, the proximity of similar biotopes with migratory species that can move-in, and the time taken for a balance to be established. Filamentous green algae would be expected to dominate at first with species such as mysid shrimps and fish colonizing rapidly. Settlement and growth of other species will occur especially during spring and summer. Herbivores such as the aplisiomorph mollusc Akera bullata
probably do not have a critical role to play in freeing space for settlement of other hard substratum species but domination by filamentous algae may limit space available for, for instance, solitary ascidians and mussels. With the domination by rapidly-settling algae and the presence of migratory species such as mysids, sticklebacks and green shore crabs, it would be expected that the biotope would be recognised within a few months of the habitat being available in proximity to other examples of the biotope. Full species richness would be likely within a year or slightly more.
The presence of Beggiatoa sp. in the biotope suggests that de-oxygenated pockets occur and that hypoxia may be a feature that component species need to be tolerant of.
This review can be cited as follows:
Filamentous green seaweeds on low salinity infralittoral mixed sediment or 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=157&code=1997>