Interactions among chronic and acute impacts on coral recruits: the importance of size-escape thresholds

Newly settled recruits typically suffer high mortality from disturbances, but rapid growth reduces their mortality once size-escape thresholds are attained. Ocean acidification (OA) reduces the growth of recruiting benthic invertebrates, yet no direct effects on survivorship have been demonstrated. We tested whether the reduced growth of coral recruits caused by OA would increase their mortality by prolonging their vulnerability to an acute disturbance: fish herbivory on surrounding algal turf. After two months' growth in ambient or elevated CO2 levels, the linear extension and calcification of coral (Acropora millepora) recruits decreased as CO2 partial pressure (pCO2) increased. When recruits were subjected to incidental fish grazing, their mortality was inversely size dependent. However, we also found an additive effect of pCO2 such that recruit mortality was higher under elevated pCO2 irrespective of size. Compared to ambient conditions, coral recruits needed to double their size at the highest pCO2 to escape incidental grazing mortality. This general trend was observed with three groups of predators (blenny, surgeonfish, and parrotfish), although the magnitude of the fish treatment varied among species. Our study demonstrates the importance of size-escape thresholds in early recruit survival and how OA can shift these thresholds, potentially intensifying population bottlenecks in benthic invertebrate recruitment.

Benthic and substrate cover data derived from a time series of photo-transect surveys for the Eastern Banks, Moreton Bay Australia, 2004-2015

This paper describes seagrass species and percentage cover point-based field data sets derived from georeferenced photo transects. Annually or biannually over a ten year period (2004-2015) data sets were collected using 30-50 transects, 500-800 m in length distributed across a 142 km**2 shallow, clear water seagrass habitat, the Eastern Banks, Moreton Bay, Australia. Each of the eight data sets include seagrass property information derived from approximately 3000 georeferenced, downward looking photographs captured at 2-4 m intervals along the transects. Photographs were manually interpreted to estimate seagrass species composition and percentage cover (Coral Point Count excel; CPCe). Understanding seagrass biology, ecology and dynamics for scientific and management purposes requires point-based data on species composition and cover. This data set, and the methods used to derive it are a globally unique example for seagrass ecological applications. It provides the basis for multiple further studies at this site, regional to global comparative studies, and, for the design of similar monitoring programs elsewhere.

Hide, survive and hitch a ride, the dispersal of Antarctic krill into the nursery habitats

Antarctic krill (Euphausia superba), a key species of Southern Ocean food webs plays a central role in ecosystem processes, community dynamics of apex predators and as a commercial fishery target. A decline in krill abundance during the late 20th century in the SW Atlantic sector has been linked to a concomitant decrease in sea ice, based on the hypothesis that sea ice acts as a feeding ground for overwintering larvae. However, evidence supporting this hypothesis has been scarce due to logistical challenges of collecting data in austral winter. Here we report on a winter study that involved diver observations of larval krill in their under-ice environment, ship-based studies of krill, sea ice physical characteristics, and biophysical model analyses of krill-ocean-ice interactions. We present evidence that complex under-ice topography is vital for larval krill in terms of dispersal and advection into high productive nursery habitats, rather than the provision by the ice environment of food. Further, ongoing changes in sea ice will lead to increases in sea-ice regimes favourable for overwintering larval krill but shifting southwards. This will result in ice-free conditions in the SW Atlantic, which will be conducive for enhancing food supplies due to sufficient light and iron availability, thus enhancing larvae development and growth. However, the associated impact on dispersal and advection may lead to a net shift in krill from the SW Atlantic to regions further east by the eastward flowing ACC and the northern branch of the Weddell Gyre, with profound consequences for the Southern Ocean pelagic ecosystem.

(Table 1) Radiocarbon dating of a lake sediment core, southern West Greenland

The dominant processes determining biological structure in lakes at millennial timescales are complex. In this study, we used a multi-proxy approach to determine the relative importance of in-lake versus indirect processes on the Holocene development of an oligotrophic lake in SW Greenland (66.99°N, 50.97°W). A 14C and 210Pb-dated sediment core covering approximately 8500 years BP was analyzed for organic-inorganic carbon content, pigments, diatoms, chironomids, cladocerans, and stable isotopes (d13C, d18O). Relationships among the different proxies and a number of independent controlling variables (Holocene temperature, an isotope-inferred cooling period, and immigration of Betula nana into the catchment) were explored using redundancy analysis (RDA) independent of time. The main ecological trajectories in the lake biota were captured by ordination first axis sample scores (18-32% variance explained). The importance of the arrival of Betula (ca. 6500 years BP) into the catchment was indicated by a series of partial-constrained ordinations, uniquely explaining 12-17% of the variance in chironomids and up to 9% in pigments. Climate influences on lake biota were strongest during a short-lived cooling period (identified by altered stable isotopes) early in the development of the lake when all proxies changed rapidly, although only chironomids had a unique component (8% in a partial-RDA) explained by the cooling event. Holocene climate explained less variance than either catchment changes or biotic relationships. The sediment record at this site indicates the importance of catchment factors for lake development, the complexity of community trends even in relatively simple systems (invertebrates are the top predators in the lake) and the challenges of deriving palaeoclimate inferences from sediment records in low-Arctic freshwater lakes.