A quantitative evaluation of the effects ofAscophyllum harvesting on the littoral ecosystem

138

152

The dominant components of the mid-shore community of 4 sheltered, rocky shores in the south of the Isle of Man, UK, were surveyed. A 2-way factorial experiment to investigate the community structuring roles of Ascophyllum nodosum (hereafter AscophyUum) canopies and Patella vulgata grazing was undertaken at 1 site and monitored over a period of 6 yr. Removal of the canopy had a marked impact on the understorey community, with both direct and indirect effects. In contrast, the effect of limpet removal was limited, owing to the restriction of this grazer to small patches of bare substrate within a turf of red algae. The Ascophyllum canopy directly facilitated the presence of the red algal turf in the mid-shore; canopy removal resulted in bleachmg and death of turf species with a consequent loss of entrapped silt. This degradation of the turf resulted in an increase in the area grazed by limpets and a subsequent increase in limpet recruitment, proportional to the increased area of bare substratum. Thus, the AscophyUwn canopy indirectly limits the population of P. vulgata by facilitating the growth of a red algal turf. Eighteen months after Ascophyllum removal, a mixed canopy of Fucus vesiculosus and Fucus serratus developed. This acted in a similar manner to the original canopy, providlng shade for turf species, which resulted in restoration of the balance between algal turf and limpet grazing. Despite this, the red algal turf had not fully recovered 5 yr after it was originally bleached, illustrating the long-term effects of Ascophyllum canopy loss on this community. Removal of the canopy also resulted in high levels of Ascophyllum recruitment, but the slow growth rate of these juvenile plants meant that nearly 6 yr after canopy removal, an Ascophyllum canopy had still not developed. Nonetheless, we predict that Ascophyllum will eventually outcompete the established Fucus spp. canopy. Detailed exammation of the distribution of Ascophyllum juveniles suggests that the low density of juveniles in the natural population is due to a lack of substratum free from both limpets and space-occupying turf, rather than a direct effect of the canopy.

Section: Patch Sizessupporting

confidence: 80%

Direct and indirect effects of a macroalgal canopy and limpet grazing in structuring a sheltered inter-tidal community

Jenkins¹,

Hawkins²,

Norton³

1999

138

152

“…Menge, 1975;Lub-manipulative experiments. It recolonizes its own zone (Burrows, 1947;Hawkins, 1979;Boaden and Dring, 1980) and sporelings were never found in various Fucus serratus removals (not all detailed here, but see Hawkins, 1979). The factors setting the lower Iimit of Ascophyllum remain unclear.…”

Section: Resultsmentioning

confidence: 99%

Factors determining the upper limits of intertidal canopy-forming algae

Hawkins¹,

Hartnoll²

1985

125

The causes of zonation of canopy-forming algae have been investigated by canopy removal experiments, combined with observations at the upper limits of various species on certain British shores during the exceptionally hot weather of summer 1983. Fucus vesiculosus, F. serratusand Laminaria digitata all extended their range upshore when the species zoned above were experimentally removed, indicating that competition can be directly responsible for setting upper limits of lowand mid-shore canopy algae. Removal of species zoned immediately below permitted down-shore extension of Pelvetia canaliculata, F. spiralis, F. vesiculosus, F. serratus and Himanthalia elongata, confirming the setting of lower limits of these species by competition. Amongst canopy-forming species, P. canaliculata, F. spiralis, L. saccharina, L. digitata and the 'button' stage of H elongata showed signs of damage during summer 1983. There were no signs of damage to F. vesiculosus, F. serratus and Ascophyllum nodosum. Some species of low-shore, turf-forming understorey and encmsting red algae seemed more vulnerable to hot weather than the canopy-forming species. Corallina officinalis and 'lithotharnnia' suffered extensive damage during summer 1983.

“…Improper collecting techniques eliminated some populations in France and recovery was estimated to take up to 15 yr (Keser et al 1981). Removal of rockweed can also have strong negative effects on the abundance and diversity of intertidal animals (Boaden & Dring 1980). In an experiment in Maine comparing regrowth of rockweed under different collection methods (cutting at the holdfast, 15 or 25 cm from the holdfast), Keser et al (1981) found that regrowth was least in areas where plants were cut at the holdfast, especially when grazers (Littorina littorea) were present.…”

Section: Recruitmentmentioning

confidence: 99%

Population consequences of biomass loss due to commercial collection of the wild seaweed Postelsia palmaeformis

Thompson¹,

H²,

Blanchette³

et al. 2010

Commercial take of Postelsia palmaeformis (hereafter Postelsia), an annual kelp found only on wave-exposed, rocky shores of the Northeast Pacific, is increasing rapidly in California where regulation of the edible seaweed 'fishery' is minimal. Many commercial collectors use a frond trimming method they claim is sustainable and allows for multiple collections per year. Unlike cutting at the stipe, which is lethal and can drive populations to extinction, frond trimming preserves the meristem, allowing fronds to regrow. To evaluate the ecological consequences of biomass loss and sustainability of this commercial take method we conducted 2 field experiments. We trimmed fronds at different frequencies and times and then measured: (1) frond regrowth and reproductive output and (2) population recruitment. We explored the potential for geographic variation by replicating the first experiment near the center and southern limit of Postelsia's biogeographic range. Fronds trimmed in April-June were able to regrow and eventually produce viable spores, albeit at somewhat reduced rates. However, spore production was sharply reduced when fronds were trimmed after the onset of sporogenesis (end of July), whether trimmed once or twice. These effects were similar across the geographic range examined but varied in magnitude. Recruitment was 38% greater in populations not subjected to trimming and population sizes were reduced by 40 to 50% when trimmed. A precautionary approach to management should: (1) mandate the frond trimming method, (2) limit collection to once a year and (3) close the commercial season before the onset of reproduction.