Two catastrophic-scale storm disturbances of a giant kelp forest community were followed by very different oceanographic conditions, the warm. nutrient-stressed period of the 1982-1984 El Nino and the cold, nutrient-rich La Nina of 1988-1989. Here w e compare the fates of the 2 postdisturbance algal communities, or '2 cohorts', under condltions determined by large-scale, lowfrequency oceanographic events. Succession and population dynamics of the competitive d o m~n a n t kelp. Macrocystis pyrifera, and understory kelps, Pterygophora californlca and Laminaria farlowu, were followed a t 5 permanent sites in the Point Loma kelp forest near San Diego. California, USA. where kelps have been mapped quarterly since 1983. There was intense kelp recruitment after both disturbances. The different oceanographic cond~tions, however, strongly affected the population dynamlcs of M. pynfera and ~t s competitive interactions with the lower standing species. Poor M pyrifera growth, canopy formation, and survival during the El Nino apparently allowed the persistence of understory populations. Extraordinary conditions for M. pyrifera growth during the La Niria were associated with the near extinction of understory populations. The number of stipes per plant and stipe dens~ty are indices of IM, pyr~fera growth and carrying capacity, respect~vely, which appear to be very senslt~ve to environmental condltions. The 2 cohorts exh~blted very different stipe patterns. In both cases, the anomalous oceanographic conditions lasted for about 2 yr after the disturbances, but the effects on kelp community structure persisted for the lives of the M. pyrifera cohorts, despite average or relatively poor conditions later In summary, these data suggest that large-scale, low-frequency oceanographic phenomena are important to kelp forest successional processes, population dynamics, and competitive interactions among kelp guilds

Field experiments were conducted on 2 subtidal sandflats to identify the short-term impacts of commercial scallop dredging on macrobenthic communities. The 2 sites (1400 m2) were situated 14 km apart, both at about 24 m depth, with similar exposure aspects and were characterised by infaunal communities dominated by small and short-lived species. Prior to dredging, preliminary sampling failed to reveal significant differences in the density of common macrofauna within each site, although community composition was distinctly different between sites The experiment was initiated by using a commercial scallop dredge to dredge half of each study site. Macrofauna samples were collected in both the dredged and adjacent reference plot at each site irnmed~ately after dredging and again 3 mo later. The density of common macrofaunal populations at each site decreased as a result of dredging, with some populations still significantly different from the adjacent reference plot after 3 mo. Significant compositional differences in the assemblage structure between dredged and reference plots were also recorded at each site over the course of the experiment. The f~n d~n g s of this experiment are considered a conservative assessment of bottom disturbance by fishing because of the area of seabed used, the types of community present and the intensity of disturbance used in the experiment. The findings of this and similar short-term experiments are discussed in light of the need to predict and assess possible large-scale changes to benthic communities as a result of habitat disturbance by fishing.

Settlement plates of coral rock were moored on the Central Great Barrier Reef in order to evaluate the distribution of competent bivalve larvae and post-settlement growth and survivorship of oysters in different regions. Oysters were chosen because they dominate plates. The regions and sites were outer reef (Myrmidon), central reef (John Brewer), inner reef (Pandora) and an inshore non-reef habitat (Bowling Green Bay). Cross-shelf differences were studied by transferring plates from John Brewer Reef to the other sites with the return of some plates to John Brewer to control transfer effects. Cross-reef and fish predation effects were studied by transferring caged and uncaged plates to the fore reef, reef flat and back reef at John Brewer. Transfer to the back reef also evaluated the effect of Acropora colonies as relative refugia from fish predation and sand flats as an area relatively free of the invertebrate predators associated with coral rubble. There were clear regional settlement differences. Myrmidon Reef had lowest densities (6 2 ind 500cn1-'), high species richness ( l ? of 24 species identified in the project), and low equitabllity dominated by Ostrea foliurn. John Brewer ( l 6 species) had high densities (> 100 ind. 5 0 0 c m -~) and hlgh specles equitability. Pandora Reef had the most species (20) and the highest densities because of a massive settlement of Pinctada maxima, most of which fell off the plates after recovery. Bowling Green Bay had low density (-16 ind. 500cn1-~) and only 13 species, Ostrea angasi being a strong dominant. Average bivalve sizes increased from the outer to the inner reef. The cross-shelf transplant experiments showed strong trends in the growth rate with Myrmidon having the slowest and John Brewer and Pandora having the fastest growth. Growth rates of Plnctada and Ostrea spp. and Crassostrea commercialis differed significantly across the shelf. Ostrea spp. were largest at John Brewer, while Pinctada spp. and most Crassostrea spp. were largest at Pandora. At Bowling Green Bay oysters were smaller than at Pandora. Survivorship of Crassostrea spp. was about the same in all areas, but Pinctada and Ostrea spp. had significantly higher mortality at Bowling Green Bay and Pandora. In the cross-reef experiment immediate fish predation was highest on plates set in sand and lowest on plates under an Acropora thicket and there was much less fish predation at the reef flat and back reef sites. Oysters suffered very large mortalities from predation by invertebrates, especially gastropods, at all sites, but this was reduced for plates over sand substrata.