Ascophyllum nodosum with epiphytic sponges and ascidians on variable salinity infralittoral rock
Ecological and functional relationships
The biotope is found in very shallow submerged rocky habitats in lagoons, subject to variable or permanently reduced salinity conditions. These particular habitat conditions lead to a variety of seaweed-dominated communities which include fucoids and green filamentous species. The fucoids, more typical of intertidal habitats, penetrate into the subtidal under the reduced salinity conditions which are not tolerated by kelps.
The biotope is dominated by dense stands of Ascophyllum nodosum. The species, and the other macroalgae in the biotope, increase the amount of space available for attachment, they provide shelter from wave action, desiccation and heat, and they are an important food source. High abundances of the characterizing algae may contribute to the oxygen budget of lagoons. In the North Atlantic for example, Ascophyllum nodosum is of great ecological importance because of its high abundance on most sheltered rocky shores, where it must be a major contributor to the oxygen budget of shallow waters to a wide range of intertidal animals (Stengel & Dring, 1997).
Ascophyllum nodosum plants provide a substratum for a variety of attached animal species including the sponge Halichondria panicea, the sea squirts Ciona intestinalis and Botryllus schlosseri and some erect bryozoans.
Growth of epiphytic sponges and ascidians may be slower than in tide-swept habitats because the biotope has weak tidal streams and wave exposure and so will have a limited supply of suspended particles necessary for suspension feeding. However, low water flow environments will favour active rather than passive suspension feeders.
Seasonal and longer term change
has a very long life span where individual fronds can survive for 10-15 years and the holdfast for several decades. The longevity of A. nodosum
contributes to the stability of the biotope. Other fucoid plants found in the biotope, such as Fucus serratus
, have life spans in the order of 3-5 years. However, growth rates of macroalgae do show seasonal changes. For example, in Strangford Lough in Northern Ireland, Stengel & Dring (1997) observed the growth of Ascophyllum nodosum
to be highly seasonal with low growth rates during November and December, and highest growth rates in late spring and early summer. A decline in growth in mid-summer was observed at all shore levels. Faunal groups in the biotope are also likely to show seasonal variation in growth rates and recruitment.
Habitat structure and complexity
Fucoid biotopes provide a variety of habitats and refugia for other species. The dense beds of Ascophyllum nodosum
and the other fucoids in the biotope increases the structural complexity of the habitat providing a variety of resources that are not available on bare rock. Fronds provide space for attachment of encrusting or sessile epifauna and epiphytic algae and provide shelter from wave action, desiccation and heat for invertebrates. For example, the immediate effects of the removal of Ascophyllum
plants are to: destroy the epifauna and flora; increase desiccation; increase predation; increase erosion and aid settlement of other species (Boaden & Dring, 1980). Crevices in the bedrock and overhangs on fucoid rocky shores also increase habitat complexity by providing refugia for a variety of species.
On rocky shores, only about 10% of the primary production is directly cropped by herbivores (Raffaelli & Hawkins, 1996) and this is likely to be similar for lagoon-like habitats. Macroalgae, such as Ascophyllum nodosum
and other fucoids, exude considerable amounts of dissolved organic carbon which are 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 and can make a contribution to the food of many marine species through the production of planktonic larvae and propagules which contribute to pelagic food chains. However, in lagoon-like habitats such as the SIR.AscSAs biotope, where tidal flows and wave exposure are weak larvae and propagules probably enter the food chain of local ecosystems rather than inshore subtidal or offshore ecosystems.
Many rocky shore plants and animals, possess a planktonic stage: gamete, spore or larvae which float in the plankton before settling and metamorphosing into adult form. This strategy allows species to rapidly colonize new areas that become available such as gaps created by storms. For these organisms it has long been evident that recruitment from the pelagic phase is important in governing the density of populations on the shore (Little & Kitching, 1996). Both the demographic structure of populations and the composition of assemblages may be profoundly affected by variation in recruitment rates.
- Fucoid plants are recruited from pelagic sporelings that settle on the substratum. Recruitment of Ascophyllum nodosum is generally poor and in the intertidal few germlings are found on the shore. However, in the sheltered conditions of the lagoon-like SIR.AscSAs biotope, recruitment from local plant stands may be more effective.
- The sponges and ascidians characterizing the biotope all have planktonic larvae and are fairly short-lived. There is therefore, high recruitment and high turnover.
Time for community to reach maturity
The time for an Ascophyllum nodosum
community to reach maturity is likely to be many years because the main characterizing species has very poor recruitment and is very slow growing. Ascophyllum nodosum
does not reach sexual maturity until about 5 years of age and, in the intertidal, individual fronds can live to be up to 15 years old and whole plants for several decades. In their work on fucoid recolonization of cleared areas at Port Erin, Knight and Parke (1950) observed that even eight years after the original clearance there was still no sign of the establishment of an Ascophyllum nodosum
population. There is a long-recognised shortage of sporelings (David, 1943) and the failure of the species to recolonize denuded areas for decades. However, the species is extremely fertile every year and Printz (1956) suggests it must be assumed that some special combination of climatic or environmental conditions is needed for an effective recolonization. If plants are not removed completely Ascophyllum nodosum
plants cut within 10-15cm of the base recover fully in 4-5 years (Printz, 1956). The epiphytic species are likely to colonize algae very rapidly. Most epiphytic species are likely to have planktonic larvae and rapid growth so that colonization of the algae will be rapid. For example, settlement of new colonies of Halichondria panicea
within one year is likely and the species increases in size by about 5% per week (Barthel, 1988). Recovery of the sea squirt Ciona intestinalis
, may take a little longer if adult populations have been lost because the species probably has limited dispersal because the larval stage is very short (hours or days) and larvae are often retained near the adults by mucus threads. However, in Plymouth reproduction is recorded as occurring all year round so recovery from loss within a few years should be possible. Even if some other epiphytic species take longer to return the recovery of the biotope is likely to limited by the recovery of the key species Ascophyllum nodosum
Information on the biotope has been based greatly on the general biology and ecology of Ascophyllum nodosum in the more common intertidal full salinity habitat. It is possible that in reduced and variable salinity lagoonal habitats life history characteristics of the species, such as growth rates, longevity and reproduction may be different.
This review can be cited as follows:
Ascophyllum nodosum with epiphytic sponges and ascidians on variable salinity infralittoral 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=328&code=1997>