Kelp carbon sink potential decreases with warming due to accelerating decomposition.

Cycling of organic carbon in the ocean has the potential to mitigate or exacerbate global climate change, but major questions remain about the environmental controls on organic carbon flux in the coastal zone. Here, we used a field experiment distributed across 28° of latitude, and the entire range of 2 dominant kelp species in the northern hemisphere, to measure decomposition rates of kelp detritus on the seafloor in relation to local environmental factors. Detritus decomposition in both species were strongly related to ocean temperature and initial carbon content, with higher rates of biomass loss at lower latitudes with warmer temperatures. Our experiment showed slow overall decomposition and turnover of kelp detritus and modeling of coastal residence times at our study sites revealed that a significant portion of this production can remain intact long enough to reach deep marine sinks. The results suggest that decomposition of these kelp species could accelerate with ocean warming and that low-latitude kelp forests could experience the greatest increase in remineralization with a 9% to 42% reduced potential for transport to long-term ocean sinks under short-term (RCP4.5) and long-term (RCP8.5) warming scenarios. However, slow decomposition at high latitudes, where kelp abundance is predicted to expand, indicates potential for increasing kelp-carbon sinks in cooler (northern) regions. Our findings reveal an important latitudinal gradient in coastal ecosystem function that provides an improved capacity to predict the implications of ocean warming on carbon cycling. Broad-scale patterns in organic carbon decomposition revealed here can be used to identify hotspots of carbon sequestration potential and resolve relationships between carbon cycling processes and ocean climate at a global scale.

Leveraging the blue economy to transform marine forest restoration.

The UN Decade of Ecosystem Restoration is a response to the urgent need to substantially accelerate and upscale ecological restoration to secure Earth's sustainable future. Globally, restoration commitments have focused overwhelmingly on terrestrial forests. In contrast, despite a strong value proposition, efforts to restore seaweed forests lag far behind other major ecosystems and continue to be dominated by small-scale, short-term academic experiments. However, seaweed forest restoration can match the scale of damage and threat if moved from academia into the hands of community groups, industry, and restoration practitioners. Connecting two rapidly growing sectors in the Blue Economy-seaweed cultivation and the restoration industry-can transform marine forest restoration into a commercial-scale enterprise that can make a significant contribution to global restoration efforts.

Homogenization and miniaturization of habitat structure in temperate marine forests.

Humans are rapidly transforming the structural configuration of the planet's ecosystems, but these changes and their ecological consequences remain poorly quantified in underwater habitats. Here, we show that the loss of forest-forming seaweeds and the rise of ground-covering 'turfs' across four continents consistently resulted in the miniaturization of underwater habitat structure, with seascapes converging towards flattened habitats with smaller habitable spaces. Globally, turf seascapes occupied a smaller architectural trait space and were structurally more similar across regions than marine forests, evidencing habitat homogenization. Surprisingly, such habitat convergence occurred despite turf seascapes consisting of vastly different species richness and with different taxa providing habitat architecture, as well as across disparate drivers of marine forest decline. Turf seascapes contained high sediment loads, with the miniaturization of habitat across 100s of km in mid-Western Australia resulting in reefs retaining an additional ~242 million tons of sediment (four orders of magnitude more than the sediments delivered fluvially annually). Together, this work demonstrates that the replacement of marine forests by turfs is a generalizable phenomenon that has profound consequences for the ecology of temperate reefs.

Ecological feedbacks stabilize a turf-dominated ecosystem at the southern extent of kelp forests in the Northwest Atlantic.

Temperate marine ecosystems globally are undergoing regime shifts from dominance by habitat-forming kelps to dominance by opportunistic algal turfs. While the environmental drivers of shifts to turf are generally well-documented, the feedback mechanisms that stabilize novel turf-dominated ecosystems remain poorly resolved. Here, we document a decline of kelp Saccharina latissima between 1980 and 2018 at sites at the southernmost extent of kelp forests in the Northwest Atlantic and their replacement by algal turf. We examined the drivers of a shift to turf and feedback mechanisms that stabilize turf reefs. Kelp replacement by turf was linked to a significant multi-decadal increase in sea temperature above an upper thermal threshold for kelp survival. In the turf-dominated ecosystem, 45% of S. latissima were attached to algal turf rather than rocky substrate due to preemption of space. Turf-attached kelp required significantly (2 to 4 times) less force to detach from the substrate, with an attendant pattern of lower survival following 2 major wave events as compared to rock-attached kelp. Turf-attached kelp allocated a significantly greater percentage of their biomass to the anchoring structure (holdfast), with a consequent energetic trade-off of slower growth. The results indicate a shift in community dominance from kelp to turf driven by thermal stress and stabilized by ecological feedbacks of lower survival and slower growth of kelp recruited to turf.

Synergistic negative effects of thermal stress and altered food resources on echinoid larvae.

Multiple changes to the marine environment under climate change can have additive or interactive (antagonistic or synergistic) effects on marine organisms. Prompted by observations of anomalously warm sea temperatures and low chlorophyll concentrations during the 2013-2016 warm "Blob" event in the Northeast Pacific Ocean, we examined the combined effects of thermal stress and a shift in food resources on the development of a larval echinoid (Strongylocentrotus droebachiensis) in the laboratory. A high concentration of phytoplankton yielded faster echinus rudiment development at warm versus historical temperature, indicating a mitigating effect of abundant food on thermal stress; however, low phytoplankton concentration or a shift in diet to suspended kelp detritus, yielded slow development and high mortality at warm temperature. The results indicate a synergistic negative effect of thermal stress and altered food resources on larvae of a keystone marine species.

Fertilization limitation of Diadema antillarum on coral reefs in the Florida Keys.

Mass mortality of the sea urchin Diadema antillarum due to disease outbreaks in 1983 and 1991 decimated populations in the Florida Keys, and they have yet to recover. Here, we use a coupled advection-diffusion and fertilization-kinetics model to test the hypothesis that these populations are fertilization limited. We found that fertilization success was ≥ 96% prior to the first disease outbreak, decreased substantially following recurrent disease to 3%, and has since remained low. By investigating the combined effects of physical factors (population spatial extent and current velocity) and sea urchin behavior (aggregation) on density-dependent fertilization success, we show that fertilization success at a given density increases with increasing population spatial extent and decreasing current velocity, and is greater under simulated aggregation behavior of D. antillarum. However, at present population densities, the increase in fertilization success due to aggregation is < 1%, even under the most favorable physical conditions. These results indicate that populations are severely fertilization limited, and that Allee effects at low population density will continue to limit recovery. Our results can serve as a practical guide to managers in the development of coral reef restoration strategies, including the design of a D. antillarum restocking program to obtain reproductively viable populations.

Validating the identity of Paramoeba invadens, the causative agent of recurrent mass mortality of sea urchins in Nova Scotia, Canada.

Green sea urchins Strongylocentrotus droebachiensis along the coast of Nova Scotia, Canada, suffer mass mortalities from infection by the pathogenic amoeba Paramoeba invadens Jones, 1985. It has been speculated that P. invadens could be a form of Neoparamoeba pemaquidensis, a species associated with disease in S. droebachiensis and lobsters in the northeast USA. During a disease outbreak in fall 2011, we isolated amoebae from moribund urchins collected from 4 locations along ~200 km of coastline. In laboratory infection trials, we found that timing and rate of morbidity corresponded to that of similar experiments conducted in the early 1980s, when P. invadens was first identified. All isolates had a similar size and morphology to the original description, including an absence of microscales. Sequences of nuclear SSU rDNA show that disease was caused by one 'species' of amoeba across the range sampled. Phylogenetic analyses prove that P. invadens is not conspecific with N. pemaquidensis, but is a distinct species most closely related to N. branchiphila, a suspected pathogen of sea urchins Diadema aff. antillarum in the Canary Islands, Spain. Morphology and closest phylogenetic affinities suggest that P. invadens would be assignable to the genus Neoparamoeba; however, nuclear SSU rDNA trees show that Neoparamoeba and Paramoeba are phylogenetically inseparable. Therefore, we treat Neoparamoeba as a junior synonym of Paramoeba, with P. invadens retaining that name, and N. pemaquidensis and N. aestuarina reverting to their original names (P. pemaquidensis and P. aestuarina), and with new combinations for N. branchiphila Dykova et al., 2005, and N. perurans Young et al., 2007, namely P. branchiphila comb. nov. and P. perurans comb. nov.