Differences in rocky reef habitats related to human disturbances across a latitudinal gradient

Decadal dynamics of subtidal barrens habitat

Rocky reef barrens that are devoid of macroalgae can be created by various mechanisms, and are often maintained in the long-term by grazing urchins. The persistence of barrens varies greatly among locations, although few studies have investigated the stability of this habitat over multiple decades, particularly at large spatial scales. We used aerial images to test for differences in temporal trajectories of shallow (2-10 m) barrens at 21 sites (average size 12 ha) over 30 years across 500 km of coastline in New South Wales, Australia. Longer-term (40-68 yr) trajectories of barrens cover were documented for five of the sites and these generally reflected the 30-year patterns. Averaged across all sites, barrens area increased at a rate of 19.9 ± 8.4 m² per year per hectare of reef from 1980s-2010s. Importantly, however, 55% of sites had stable or fluctuating (±10% cover) barrens over this period, rather than displaying continual increases. Although the extent of shallow barrens increases with latitude, the temporal dynamics of barrens did not differ among three latitudinal regions where barrens are the most extensive. Associations between variability in barrens cover and environmental variables indicated that reef topography might pay a role in influencing barrens. Examples of such long-term persistence of extensive barrens are relatively rare and potential reasons for this and possible future changes are discussed.

Morphological variation of a rapidly spreading native macroalga across a range of spatial scales and its tolerance to sedimentation

Understanding how species' traits can shape winners and losers of environmental change can help resolve drivers of current community composition patterns and predict future drivers. Sedimentation is one of the main environmental stressors shaping coastal marine communities and tolerance of high sedimentation rates (e.g. via morphological variation) may allow for competitive dominance. In New South Wales, Australia, the abundance and range of the native green macroalga Caulerpa filiformis have increased over recent decades, apparently associated with sediment disturbance. We used field measurements to test hypotheses about morphological variability in C. filiformis in relation to local- and large-scale environmental variation in water depth, sediment cover and latitude. Using a lab experiment, we tested hypotheses about survival and morphological change under different sedimentation regimes. In the field, C. filiformis fronds were more elongated and less branched when a sediment veneer is present and when water depth increased (i.e. reduced light). At larger spatial scales, frond length and width decreased with increased latitude, but latitude was less important in explaining the variation C. filiformis' length than were depth or sedimentation. Our lab experiment showed a high tolerance to sedimentation, aided by increased investment in vertical growth. This study shows that rapid morphological plasticity is a likely key attribute of the spreading native macroalga C. filiformis. We argue that having a broad environmental tolerance is key to define a species success under environmental change.

Living on the edge: Early life history phases as determinants of distribution in Pyura praeputialis (Heller, 1878), a rocky shore ecosystem engineer

The distribution of intertidal organisms can depend on processes operating early in their life history. The ascidian Pyura praeputialis, a mid- to low-intertidal habitat-forming ecosystem engineer, was strongly associated with specific types of habitat (biogenic vs. bare rock). We examined field patterns and performed laboratory and field experiments to assess the nature of this association. Recruits were frequently found on the tunics of conspecifics and clumps of turfing coralline algae. Larvae preferred these same habitats in a series of laboratory settlement assays. Laboratory-reared juveniles (20- & 50-days-old) survived poorly on bare rock in the laboratory, while those on rugose surfaces - the tunic of adults and turfing corallines - showed high survivorship. Field-collected juveniles (<2 cm) affixed to these rugose habitats also exhibited high survivorship in the field. We conclude that both pre and post-settlement processes determine spatial pattern in this important habitat-forming taxon. The acute sensitivity of juveniles to desiccating conditions was unexpected in an intertidal organism.