Persistence of seaweed forests in the anthropocene will depend on warming and marine heatwave profiles

Marine heatwave duration and intensity interact to reduce physiological tipping points of kelp species with contrasting thermal affinities

BACKGROUND AND AIMS

Marine heatwaves (MHWs) are widely recognized as pervasive drivers of ecosystem change, yet our understanding of how different MHW properties mediate ecological responses remains largely unexplored. Understanding MHW impacts on foundation species is particularly important, given their structural role in communities and ecosystems.

METHODS

We simulated a series of realistic MHWs with different levels of intensity (Control: 14 °C, Moderate: 18 °C, Extreme: 22 °C) and duration (14 or 28 d) and examined responses of two habitat-forming kelp species in the southwest UK. Here, Laminaria digitata reaches its trailing edge and is undergoing a range contraction, whereas Laminaria ochroleuca reaches its leading edge and is undergoing a range expansion.

KEY RESULTS

For both species, sub-lethal stress responses induced by moderate-intensity MHWs were exacerbated by longer duration. Extreme-intensity MHWs caused dramatic declines in growth and photosynthetic performance, and elevated bleaching, which were again exacerbated by longer MHW duration. Stress responses were most pronounced in L. ochroleuca, where almost complete tissue necrosis was observed by the end of the long-duration MHW. This was unexpected given the greater thermal safety margins assumed with leading edge populations. It is likely that prolonged exposure to sub-lethal thermal stress exceeded a physiological tipping point for L. ochroleuca, presumably due to depletion of internal reserves.

CONCLUSIONS

Overall, our study showed that exposure to MHW profiles projected to occur in the region in the coming decades can have significant deleterious effects on foundation kelp species, regardless of their thermal affinities and location within respective latitudinal ranges, which would probably have consequences for entire communities and ecosystems.

The impacts of artificial light at night on the ecology of temperate and tropical reefs

Despite 22% of the world's coastal regions experiencing some degree of light pollution, and biologically important artificial light at night (ALAN) reaching large portions of the seafloor (greater than 75%) near coastal developments, the impacts of ALAN on temperate and tropical reefs are still relatively unknown. Because many reef species have evolved in response to low-light nocturnal environments, consistent daily, lunar, and seasonal light cycles, and distinct light spectra, these impacts are likely to be profound. Recent studies have found ALAN can decrease reproductive success of fishes, alter predation rates of invertebrates and fishes, and impact the physiology and biochemistry of reef-building corals. In this paper, we integrate knowledge of the role of natural light in temperate and tropical reefs with a synthesis of the current literature on the impacts of ALAN on reef organisms to explore potential changes at the system level in reef communities exposed to ALAN. Specifically, we identify the direct impacts of ALAN on individual organisms and flow on effects for reef communities, and present potential scenarios where ALAN could significantly alter system-level dynamics, possibly even creating novel ecosystems. Lastly, we highlight large knowledge gaps in our understanding of the overall impact of ALAN on reef systems. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Marine heatwaves and decreased light availability interact to erode the ecophysiological performance of habitat-forming kelp species

Coastal marine ecosystems are threatened by a range of anthropogenic stressors, operating at global, local, and temporal scales. We investigated the impact of marine heatwaves (MHWs) combined with decreased light availability over two seasons on the ecophysiological responses of three kelp species (Laminaria digitata, L. hyperborea, and L. ochroleuca). These species function as important habitat-forming foundation organisms in the northeast Atlantic and have distinct but overlapping latitudinal distributions and thermal niches. Under low-light conditions, summertime MHWs induced significant declines in biomass, blade surface area, and Fv/Fm values (a measure of photosynthetic efficiency) in the cool-water kelps L. digitata and L. hyperborea, albeit to varying degrees. Under high-light conditions, all species were largely resistant to simulated MHW activity. In springtime, MHWs had minimal impacts and in some cases promoted kelp performance, while reduced light availability resulted in lower growth rates. While some species were negatively affected by summer MHWs under low-light conditions (particularly L. digitata), they were generally resilient to MHWs under high-light conditions. As such, maintaining good environmental quality and water clarity may increase resilience of populations to summertime MHWs. Our study informs predictions of how habitat-forming foundation kelp species will be affected by interacting, concurrent stressors, typical of compound events that are intensifying under anthropogenic climate change.

Biological Impacts of Marine Heatwaves

Climatic extremes are becoming increasingly common against a background trend of global warming. In the oceans, marine heatwaves (MHWs)-discrete periods of anomalously warm water-have intensified and become more frequent over the past century, impacting the integrity of marine ecosystems globally. We review and synthesize current understanding of MHW impacts at the individual, population, and community levels. We then examine how these impacts affect broader ecosystem services and discuss the current state of research on biological impacts of MHWs. Finally, we explore current and emergent approaches to predicting the occurrence andimpacts of future events, along with adaptation and management approaches. With further increases in intensity and frequency projected for coming decades, MHWs are emerging as pervasive stressors to marine ecosystems globally. A deeper mechanistic understanding of their biological impacts is needed to better predict and adapt to increased MHW activity in the Anthropocene.

Intergrading reef communities across discrete seaweed habitats in a temperate-tropical transition zone: Lessons for species reshuffling in a warming ocean

Temperate reefs are increasingly affected by the direct and indirect effects of climate change. At many of their warm range edges, cool-water kelps are decreasing, while seaweeds with warm-water affinities are increasing. These habitat-forming species provide different ecological functions, and shifts to warm-affinity seaweeds are expected to modify the structure of associated communities. Predicting the nature of such shifts at the ecosystem level is, however, challenging, as they often occur gradually over large geographical areas. Here, we take advantage of a climatic transition zone, where cool-affinity (kelp) and warm-affinity (Sargassum) seaweed forests occur adjacently under similar environmental conditions, to test whether these seaweed habitats support different associated seaweed, invertebrate, coral, and fish assemblages. We found clear differences in associated seaweed assemblages between habitats characterized by kelp and Sargassum abundance, with kelp having higher biomass and seaweed diversity and more cool-affinity species than Sargassum habitats. The multivariate invertebrate and fish assemblages were not different between habitats, despite a higher diversity of fish species in the Sargassum habitat. No pattern in temperature affinity of the invertebrate or fish assemblages in each habitat was found, and few fish species were exclusive to one habitat or the other. These findings suggest that, as ocean warming continues to replace kelps with Sargassum, the abundance and diversity of associated seaweeds could decrease, whereas fish could increase. Nevertheless, the more tropicalized seaweed habitats may provide a degree of functional redundancy to associated fauna in temperate seaweed habitats.