Slow Community Development Enhances Abiotic Limitation of Benthic Community Structure in a High Arctic Kelp Bed

We examined the patterns of propagule recruitment to assess the timescale and trajectory of succession and the possible roles of physical factors in controlling benthic community structure in a shallow High Arctic kelp bed in the Beaufort Sea, Alaska. Spatial differences in established epilithic assemblages were evaluated against static habitat attributes (depth, distance from river inputs) and environmental factors (temperature, salinity, current speed, underwater light) collected continuously over 2–6 years. Our measurements revealed that bottom waters remained below freezing (mean winter temperatures ∼−1.8°C) and saline (33–36) with negligible light levels for 8–9 months. In contrast, the summer open water period was characterized by variable salinities (22–36), higher temperatures (up to 8–9°C) and measurable irradiance (1–8 mol photons m–2 day–1). An inshore, near-river site experienced strong, acute, springtime drops in salinity to nearly 0 in some years. The epilithic community was dominated by foliose red algae (47–79%), prostrate kelps (2–19%), and crustose coralline algae (0–19%). Strong spatial distinctions among sites included a positive correlation between cover by crustose coralline algae and distance to river inputs, but we found no significant relationships between multi-year means of physical factors and functional groups. Low rates of colonization and the very slow growth rates of recruits are the main factors that contribute to prolonged community development, which augments the influence of low-frequency physical events over local community structure. Mortality during early succession largely determines crustose coralline algal and invertebrate prevalence in the established community, while kelp seem to be recruitment-limited. On scales > 1 m, community structure varies with bathymetry and exposure to freshwater intrusion, which regulate frequency of primary and physiological disturbance. Colonization rates (means of 3.3–69.9 ind. 100 cm–1 year–1 site–1) were much lower than studies in other Arctic kelp habitats, and likely reflect the nature of a truly High Arctic environment. Our results suggest that community development in the nearshore Beaufort Sea occurs over decades, and is affected by combinations of recruitment limitation, primary disturbance, and abiotic stressors. While seasonality exerts strong influence on Arctic systems, static habitat characteristics largely determine benthic ecosystem structure by integrating seasonal and interannual variability over timescales longer than most ecological studies.

3. Life in the coastal ocean

The coastal regions of the Global Ocean comprise a narrow strip of ocean extending from the shoreline to the edge of the continental shelf. This coastal ocean environment accounts for only about 7 per cent of the area of the Global Ocean, but it is of huge importance to human society. Roughly 44 per cent of the human population lives within 150 kilometres of a coast, meaning that the coastal ocean is heavily impacted by human activities. ‘Life in the coastal ocean’ describes the kelp bed habitats, the seagrass meadows, and soft-bottom communities, before considering the impact of harmful phytoplankton blooms, biological invasions, and the increase in plastic debris in our oceans.

Effects of elevated temperature and sedimentation on grazing rates of the green sea urchin: implications for kelp forests exposed to increased sedimentation with climate change

Sea urchin grazing rates can strongly impact kelp bed persistence. Elevated water temperature associated with climate change may increase grazing rates; however, these effects may interact with local stressors such as sedimentation, which may inhibit grazing. In Alaska, glacial melt is increasing with climate change, resulting in higher sedimentation rates, which are often associated with lower grazer abundance and shifts in macroalgal species composition. The short-term effects of elevated temperature and sediment on grazing were investigated for the green sea urchin, Strongylocentrotus droebachiensis (O.F. Müller, 1776), in Kachemak Bay, Alaska (59° 37′ 45.00″ N, 151° 36′ 38.40″ W) in early May 2017. Feeding assays were conducted at ambient temperature (6.9–9.8 °C) and at 13.8–14.6 °C with no sediment and under a high sediment load. Grazing rates significantly decreased in the presence of sediment, but were not significantly affected by temperature. Along with sediment impacts on settlement and post-settlement survival, grazing inhibition may contribute to the commonly observed pattern of decreased macroinvertebrate grazer abundance in areas of high sedimentation and increased sedimentation in the future may alter sea urchin grazing in kelp forests.