The effects of the experimental removal of green sea urchins and of ice-scour on sublittoral benthic macro-algal communities in eastern Newfoundland

Keats, D. W. (1986) The effects of the experimental removal of green sea urchins and of ice-scour on sublittoral benthic macro-algal communities in eastern Newfoundland. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The results of an urchin removal experiment which was conducted over a four year period at a site in Conception Bay, Newfoundland are reported. The experiment was carried out to determine how the benthic algal community at a heavily grazed sublittoral Newfoundland site, subjected to periodic ice scour, would respond to a reduction in grazing pressure. Three transects were selected: one from which urchins were removed manually; one from which urchins were removed using quicklime, and one as a control. Sampling was conducted within each of three depth ranges: 0-2m, the shallow macro-algal zone; 2-3m, the upper portion of the urchin dominated zone; and 6-9m, the middle portion of the urchin dominated zone. The occurrence of ice-scour during two out of four years of the study allowed an assessment of its effects on biomass, diversity, and species composition of the algal community. -- In the shallow zone (0-2m) urchins had no detectable effect on macroalgal biomass. Biomass, species composition and diversity were affected, however, by the occurrence and timing of ice-scour. In the absence of ice-scour, Alaria esculenta formed a canopy which shaded the substratum; few species coexisted, and diversity was low. Winter and spring ice scour, which was temporally and spatially patchy, removed the Alaria canopy and made available well lighted rocky substratum for colonization. Scoured patches were· colonized by annual algae, resulting in an increase in diversity. Biomass returned to pre-scour levels approximately 2 mo after scouring, but diversity remained high due to the persistence of several species of annual algae in patches among the unscoured Alaria. -- In the urchin dominated zone macro-algal biomass increased following urchin removal by both manual and quicklime techniques. The upper portion of the resulting communities became dominated by Alaria esculenta, with very few co-existing species. This dominance by Alaria was also typical of the shallow zone in years during which no ice-scour occurred. Below the zone of Alaria domination (2-3m) the community became dominate~ by Desmarestia aculeata, with many other co-existent species (including Agarum cribrosum, Alaria esculenta, and Laminaria digitata). -- Algal biomass in the control area was generally low, except during the summer of 1982, when an increase in both biomass and species richness occurred. The species composition, however, differed from that on the removal transects in that the control area was dominated by annual algae such as Acrothrix novaeangliae, Chordaria flagelliformis, Dictyosiphon foeniculaceus, Ectocarpus siliculosus, and Eudesme virescens. Alaria and Desmarestia aculeata were unimportant in this association because the algae recruited in the late spring and early summer, after the period of Alaria and Desmarestia recruitment. -- Urchin biomass in the control area was greatest at 2-3m. Density was greatest in the middle of the urchin dominated zone (6-9m), where numerous juveniles occurred in crevices, and among branched and undercut encrusting coralline algae. -- In addition to urchins, the herbivore guild in the study area consisted of six species of molluscs (Acmaea testudinalis, Lacuna vincta, Margarites helicinus, Ischnochiton alba, Tonicella marmorea, and T. rubra). Densities of Acmaea and the three chitons were greatest at 6-9m. Recruitment and density of the latter three species showed no detectable relationship with urchin removal. Recruitment of the annual Lacuna and Margarites occurred during the early autumn, in shallow water, particularly in the presence of macro-algae. Survival into the following spring was greatest on the macro-algae in the urchin removal areas.

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