Kelp & Ocean Warming
Climate refugia for kelp within an ocean warming hotspot revealed by stacked species distribution modelling
Marine Environmental Research
Volume 166
April 2021 105267
T.R.Davis, C.Champion, M.A.Coleman
(Fisheries research, DPI, and Southern Cross University)
Highlights
Modelling identified three distinct refuge areas for kelp against climate change
The presence of kelp at two refuges was confirmed using independent data
Identification of refuges will facilitate conservation actions to protect kelp
Abstract
Canopy forming macroalgae are declining globally due to climate change and the identification of refuges for these habitats is crucial for their conservation. This is particularly pertinent in ocean warming hotspots where significant range contractions of kelp have occurred and are projected to continue.
We developed a stacked urchin-kelp species distribution model (SDM) to predict climate refugia for kelp (Ecklonia radiata) in an ocean warming hotspot, south-eastern Australia (from Brisbane to Eden). The optimal stacked-SDM incorporated biotic (uchins and kelp) and abiotic (temperature, depth, radiation) explanatory covariates and was validated using an independent dataset.
Density of the urchin Centrostephanus rodgersii, summer bottom temperature and photosynthetically available radiation at the seabed were significant predictors of kelp cover, highlighting the physiological and ecological influence of these variables on the distribution of kelp.
Our optimal stacked-SDM predicted three spatially distinct refuge areas (at Morton Is, 30 m depth; Seal Rocks, mid depth, Sydney shallow depth), where kelp occurs in deeper waters than surrounding seascapes (and water clarity allows enough light to penetrate). The presence of kelp at two of these refuge areas (not Seal Rocks) was confirmed using independent data. The identification of these refuge areas is crucial for conservation, as they are likely to facilitate the persistence of ecologically and economically important kelp forests as waters warm in shallow areas and kelp retreat to depth under climate change. Furthermore, identification of refugia will enable proactive spatial planning that prioritises new locations for protection to ensure that key kelp habitats can persist in a future of increasing stress.
Further summary:
Data for the models were collected from 23 sites along the coast from Brisbane to Eden. At each site 6-9 transects each 200 m long were used, at 5 – 30 m depth. A Spot-X Squid video was towed along each transect, and a GoPro Hero7 used to take snapshots of the seafloor and six samples that were about 7.6 m2 each were used to estimate % kelp cover and numbers of urchins per m2. The model was validated using observations collected separately in 2019 from the same areas, and showed that kelp was underestimated at some lower latitude sites, and urchins underestimated at some mid and higher latitude sites.
The model explained 61% of the variation in kelp cover, with urchin numbers being the most important predictor, with a linear relationship with sq root of urchin numbers. That means that 4 urchins per m2 caused only double the reduction of kelp as did one urchin. Or that smaller numbers have a relatively larger impact per urchin than higher numbers. However, the model did not extend to more than 4 urchins per sq m and we have a lot high densities in BMP!
Kelp cover increased with bottom summer sea temperatures up to 240 C but then declined, since its optimal temperature range is 22 – 250 C. The model is predicting that, as shallow sea temperatures increase beyond kelp optima, kelp will retreat into deeper cooler waters, but will be restricted to areas with good water clarity so that enough radiation can penetrate the depths.
However, the graph showing kelp cover in the three refugia identified (Moreton Is, Brisbane; Seal rocks and Sydney) show only kelp cover up to 0.6%, unless that is meant to be a proportion, ie 60%. (I would expect that a healthy kelp forest should have 50 – 80% cover?). Kelp cover is highest in mid latitudes (31 – 350S), perhaps due to the urchins?
Graphs show urchin densities and variability increasing with latitude, ie there are more below 360S in southern NSW. Urchin numbers declined with increasing temperatures from 22 – 270C (?) indicating that the larger populations are in cooler waters at higher latitudes, and also with depths not much over 10 m.