An Automated Method for Mapping Giant Kelp Canopy Dynamics from UAV

Satellite and aerial imagery have been used extensively for mapping the abundance and distribution of giant kelp (Macrocystis pyrifera) in southern California. There is now great potential for using unoccupied aerial vehicles (UAVs) to map kelp canopy at very high resolutions. However, tides and currents have been shown to affect the amount of floating kelp canopy on the water surface, and the impacts of these processes on remotely sensed kelp estimates in this region have not been fully quantified. UAVs were used to map fine-scale changes in canopy area due to tidal height and current speed at kelp forests off the coast of Palos Verdes, CA and Santa Barbara, CA. An automated method for detecting kelp canopy was developed that was 67% accurate using red-green-blue (RGB) UAV imagery and 93% accurate using multispectral UAV imagery across a range of weather, ocean, and illumination conditions. Increases in tidal height of 1 m reduced the amount of floating kelp canopy by 15% in Santa Barbara and by over 30% in Palos Verdes. The effect of current speed on visible kelp canopy was inconclusive, but there was a trend towards lower canopy area with increased current speed. Therefore, while tidal height and current speed can introduce significant variability to estimates of kelp abundance, the magnitude of this variability is site specific. Still, UAVs are a valuable tool for mapping of kelp canopy and can provide greater spatial resolution and temporal coverage than is possible from many satellite sensors. This data can provide insight into the patterns and drivers of high frequency fluctuations in kelp abundance.

A nonnative habitat‐former mitigates native habitat loss for endemic reef fishes

Animals that select the best available habitats are most likely to succeed in degraded environments, but ecological change can create evolutionarily unfamiliar habitats that may be under‐ or over‐utilized by native fauna. In temperate coastal waters, eutrophication and grazing have driven a global decline in native seaweeds and facilitated the establishment of non‐native seaweeds that provide novel macrophyte habitat. We tested whether a non‐native kelp canopy (wakame Undaria pinnatifida) functions as a viable habitat or ecological trap for several endemic reef fishes on urchin‐grazed reefs in southern Australia. We assessed the willingness of fish to utilize native vs. wakame kelp canopy via a laboratory habitat choice experiment and by recording natural recruitment to specially constructed boulder reefs with manipulated kelp canopy. We also compared fish communities on natural reefs using a before‐after‐control‐impact survey of wakame patches, and to assess the quality of wakame habitat for resident fish, compared fitness metrics for fish collected from habitats with native vs. wakame kelp canopy. Endemic fishes did not distinguish between the native or wakame canopy but preferred both to barren reef habitats. On urchin‐grazed natural reefs, fish occurred in higher abundance and diversity where seasonal wakame canopy was present. Fitness metrics in fish collected from wakame patches were comparable to those in fish from adjacent native kelp patches. These findings indicate that the non‐native canopy provides a viable habitat for endemic fish and may play a role in sustaining native fauna populations in this degraded ecosystem. More broadly, we recommend that managers consider the role of non‐native habitats within the context of environmental change, as endemic fauna may benefit from non‐native habitat‐formers in areas where their native counterparts cannot persist.

Destructive grazing, epiphytism, and disease: the dynamics of sea urchin - kelp interactions in Nova Scotia

We measured the rate of advance of urchin (Strongylocentrotus droebachiensis) feeding aggregations (fronts) as they destructively grazed kelp beds (Laminaria longicruris) at both a wave-exposed site and a sheltered site in Nova Scotia over 3.5 years. The grazing fronts were composed of high densities of large adults (up to 98 and 70 per 0.25 m2 at the exposed and sheltered sites, respectively). Urchins in the recently formed barrens, or in adjacent kelp beds, occurred at much lower densities and consisted mainly of juveniles. The fronts moved onshore into shallower water at each site, but their rate of advance varied markedly between sites and over time at each site, ranging from 0 to 4 m·month-1. The rate of advance of a front was related to the biomass of urchins; fronts did not advance below a threshold biomass of ~2 kg·m-2. Infestations of kelp by an epiphytic bryozoan (Membranipora membranacea) caused marked reductions in kelp canopy cover and biomass during winter, but the canopy regenerated through recruitment of juvenile sporophytes in spring. A localized outbreak of disease decimated S. droebachiensis at the exposed site in 1993, which enabled kelp to recolonize the barrens. Surviving urchins gradually reaggregated and resumed destructive grazing after ~1.5 years. A recurrence of disease in 1995 eliminated urchins at both sites and terminated the transition from kelp beds to barrens on a coastal scale. Our findings have important implications for the management of the urchin fishery, which targets grazing fronts for harvesting.