Long-term marine protection enhances kelp forest ecosystem stability.

Trophic downgrading destabilizes ecosystems and can drive large-scale shifts in ecosystem state. While restoring predatory interactions in marine reserves can reverse anthropogenic-driven shifts, empirical evidence of increased ecosystem stability and persistence in the presence of predators is scant. We compared temporal variation in rocky reef ecosystem state in New Zealand's oldest marine reserve to nearby fished reefs to examine whether protection of predators led to more persistent and stable reef ecosystem states in the marine reserve. Contrasting ecosystem states were found between reserve and fished sites, and this persisted over the 22-year study period. Fished sites were predominantly urchin barrens but occasionally fluctuated to short-lived turfs and mixed algal forests, while reserve sites displayed unidirectional successional trajectories toward stable kelp forests (Ecklonia radiata) taking up to three decades following protection. This provides empirical evidence that long-term protection of predators facilitates kelp forest recovery, resists shifts to denuded alternate states, and enhances kelp forest stability.

The value of ecosystem services in global marine kelp forests.

While marine kelp forests have provided valuable ecosystem services for millennia, the global ecological and economic value of those services is largely unresolved. Kelp forests are diminishing in many regions worldwide, and efforts to manage these ecosystems are hindered without accurate estimates of the value of the services that kelp forests provide to human societies. Here, we present a global estimate of the ecological and economic potential of three key ecosystem services - fisheries production, nutrient cycling, and carbon removal provided by six major forest forming kelp genera (Ecklonia, Laminaria, Lessonia, Macrocystis, Nereocystis, and Saccharina). Each of these genera creates a potential value of between $64,400 and $147,100/hectare each year. Collectively, they generate between $465 and $562 billion/year worldwide, with an average of $500 billion. These values are primarily driven by fisheries production (mean $29,900, 904 Kg/Ha/year) and nitrogen removal ($73,800, 657 Kg N/Ha/year), though kelp forests are also estimated to sequester 4.91 megatons of carbon from the atmosphere/year highlighting their potential as blue carbon systems for climate change mitigation. These findings highlight the ecological and economic value of kelp forests to society and will facilitate better informed marine management and conservation decisions.

Coastal darkening substantially limits the contribution of kelp to coastal carbon cycles.

Macroalgal-dominated habitats are rapidly gaining recognition as important contributors to marine carbon cycles and sequestration. Despite this recognition, relatively little is known about the production and fate of carbon originating from these highly productive ecosystems, or how anthropogenic- and climate-related stressors affect the role of macroalgae in marine carbon cycles. Here, we examine the impact of increasing turbidity on carbon storage, fixation and loss in southern hemisphere kelp forests. We quantified net primary production (NPP) and biomass accumulation (BA), and estimated carbon release via detritus and dissolved organic carbon (DOC) across a large-scale turbidity gradient. We show that increased turbidity, resulting in a 63% reduction in light, can result in a 95% reduction in kelp productivity. When averaged annually, estimates of NPP and BA per plant at high-light sites were nearly six and two times greater than those at low-light sites, respectively. Furthermore, the quantity of carbon fixed annually by kelp forests was up to 4.7 times greater than that stored as average annual standing stock. At low-light sites, the majority of C goes directly into tissue growth and is subsequently eroded. In contrast, excess production at high-light sites accounts for up to 39% of the total carbon fixed and is likely released as DOC. Turbidity is expected to increase in response to climate change and our results suggest this will have significant impacts on the capacity of kelp forests to contribute to carbon sequestration pathways. In addition to demonstrating that turbidity significantly reduces the quantity of carbon fixed by kelp forests, and subsequently released as detritus, our results highlight the negative impacts of turbidity on a large source of previously unaccounted for carbon.

Seasonal and spatial variation in photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient.

Understanding the photoacclimation response of macroalgae across broad spatial and temporal scales is necessary for predicting their vulnerability to environmental changes and quantifying their contribution to coastal primary production. This study investigated how the photosynthesis-irradiance response and photosynthetic pigment content of the kelp Ecklonia radiata varies both spatially and seasonally among seven sites located across a turbidity gradient in the Hauraki Gulf, north-eastern New Zealand. Photosynthesis-irradiance curves were derived under laboratory conditions for whole adult E. radiata using photorespirometry chambers. Lab-derived photosynthesis-irradiance curves in summer were also compared with in situ measurements made on kelp at each of the seven study sites. Photosynthetic parameters and pigments showed clear seasonal patterns across all sites as demonstrated by higher photosynthetic pigment levels and photosynthetic efficiency occurring in autumn and winter, and higher maximum rates of photosynthesis and respiration occurring in summer. Lamina biomass was similar across sites, yet thalli exhibited a clear photokinetic response to increasing turbidity. At turbid sites photosynthetic pigment levels and photosynthetic efficiency was higher, and respiration and saturation and compensation irradiances lower, compared to high-light sites. The results presented here further our understanding of low-light acclimation strategies in kelp and highlight the degree of seasonality in photosynthetic parameters. Though E. radiata demonstrates a clear capacity to photoacclimate to a degrading light environment, further research is needed to investigate the extent to which the observed acclimation can offset the likely negative effects of increasing turbidity on kelp forest primary production.