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.
This paper uses annual estimates of biomass accumulation and modelled net primary production to show that turbidity substantially limits the carbon storage, fixation and loss in southern hemisphere kelp forests. 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 unaccounted for carbon.
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.
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.
Underwater light is essential for fuelling coastal productivity. However, elevated turbidity, resulting from land-based activities and climate change, is often overlooked as a threat to coastal ...ecosystems. Understanding how low light, and specifically the temporal delivery of light, affects the productivity of forest-forming species is necessary to predict how ecosystems and species will respond to future increases in turbidity. Outdoor mesocosm experiments were used to compare the low-light tolerance of 2 forest-forming macrophytes that vary in their distribution in relation to turbidity, and investigate how the temporal delivery of light, i.e. press vs. pulse low-light disturbance, affects net primary productivity (NPP). We showed that the kelp Ecklonia radiata, which dominates reefs with low turbidity, ismore productive per unit biomass under high-light conditions than the fucoid Carpophyllum flexuosum, which typifies more turbid waters. Under low light, E. radiata suffered greater tissue loss and had lower NPP than C. flexuosum. Under both press and pulse treatments, E. radiata showed significant losses of lamina biomass and reduced NPP, while C. flexuosum showed net growth under press disturbance, and only lost tissue and had reduced NPP under pulse disturbance. The greater tolerance of C. flexuosum to de creased light, and differential responses of E. radiata and C. flexuosum to press and pulse low-light conditions, provide mechanistic support for C. flexuosum being better suited to turbid low-light environments than E. radiata. These results suggest future increases in turbidity may facilitate a shift from kelp-dominated forests to alternate states, resulting in reduced primary productivity.
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.
Kelp and macroalgal forests provide the ecological foundations of many temperate rocky reef ecosystems, but have regionally declined, often due to sea urchin overgrazing and the formation of urchin ...barrens. Sea urchin removal has long been used to investigate kelp-sea urchin dynamics and is increasingly being promoted for kelp forest restoration. In this review, we assess the methods and outcomes of sea urchin removal experiments to evaluate their potential use and feasibility as a tool for restoring macroalgal forests. Seventy-nine sea urchin removal projects were reviewed from temperate subtidal rocky reef systems between 1975 and 2020. Removal methods were often not reported (35%), but included manual culling, including crushing (25%) and chemical application (quicklime, 9%), or relocating sea urchins (13%). Only a small percentage of removals were large in scale (16% > 10 ha) and 92% of these utilized culling. Culling is often the most practical method of urchin removal, but all methods can be effective and we encourage development of new approaches that harvest and utilize low-quality urchins. Urchin removal led to an increase in macroalgae in 70% of studies, and a further 21% showed partial increases (e.g., at one or more sites or set of conditions). Restoration effectiveness is increased by removing essentially all sea urchins from discrete areas of urchin barrens. Sea urchin removal provides a simple, relatively cheap, and effective method that promotes kelp recovery within urchin barrens. However, sea urchin removal does not address the underlying cause of elevated sea urchin populations and is unlikely to provide a long-term solution to restore kelp forests and full ecosystem function on its own. We therefore suggest that if sea urchin removal is considered as a tool for kelp forest restoration, it should be incorporated with other management measures that aim to increase kelp forest resilience and biodiversity (e.g., marine protected areas, predator protection or enhancement). This will ensure that kelp restoration efforts have the greatest ecological, socio-economic and cultural outcomes in the long-term.
Sea urchin overgrazing is a major cause of kelp loss and consequently sea urchin removal is frequently promoted as a tool for restoring kelp in urchin barrens. While many sea urchin species support ...high‐end roe fisheries, roe condition is generally lower in barrens than in kelp forests due to reduced food availability. Understanding how roe condition varies between kelp and barren habitats, and in response to kelp restoration, is key to developing effective, culturally and socially acceptable approaches to sea urchin removal and for assessing the potential benefits of kelp restoration. We assessed whether sea urchin roe quality within urchin barrens in northeastern Aotearoa New Zealand can be sufficient for harvest, and whether sea urchin removal can ultimately enhance urchin roe condition (by increasing food quality and quantity). We monitored roe condition (roe weight and gonadosomatic index GSI) in kelp forest and urchin barrens monthly for one year at two sites, and quarterly over two years at four sites following large‐scale (1.6–2 ha) sea urchin removal from urchin barrens. While roe condition was generally lower in barrens, the magnitude of this difference varied seasonally and among sites. Sea urchin roe quality within barrens was sufficient for commercial harvest (GSI ≥ 6%) during spring–summer at three sites. Following large‐scale sea urchin removal, kelp and fucoid density increased over two years (~9–58 plants m−2), but remained low in barren control areas. Although variable, sea urchins from removal areas (restored kelp) typically showed increased GSI, roe weight, and improved roe color within nine months following removal. These results suggest that, at some locations, harvest may provide a viable method for removing urchins from urchin barrens for kelp restoration purposes. However, to ensure sea urchin densities are reduced and maintained below the critical levels needed to achieve restoration outcomes, strategic approaches with critical evaluation of methods are required. Furthermore, we demonstrate that effectively removing urchins and promoting kelp recovery can ultimately improve urchin roe quality.
Abstract
Studies that use CO2 vents as natural laboratories to investigate the impacts of ocean acidification (OA) typically employ control-impact designs or local-scale gradients in pH or pCO2, ...where impacted sites are compared to reference sites. While these strategies can accurately represent well-defined and stable vent systems in relatively homogenous environments, it may not adequately encompass the natural variability of heterogeneous coastal environments where many CO2 vents exist. Here, we assess the influence of spatial heterogeneity on the perceived impacts of OA at a vent system well established in the OA literature. Specifically, we use a multi-scale approach to investigate and map the spatial variability in seawater pH and benthic communities surrounding vents at Whakaari-White Island, New Zealand to better understand the scale and complexity of ecological impacts of an acidified environment. We found a network of vents embedded in complex topography throughout the study area, and spatially variable pH and pCO2 levels. The distribution of habitats (i.e. macroalgal forests and turfing algae) was most strongly related to substratum type and sea urchin densities, rather than pH. Epifaunal communities within turfing algae differed with sampling distance from vents, but this pattern was driven by higher abundances of a number of taxa immediately adjacent to vents, where pH and temperature gradients are steep and white bacterial mats are prevalent. Our results contrast with previous studies at White Island that have used a control-impact design and suggested significant impacts of elevated CO2 on benthic communities. Instead, we demonstrate a highly heterogeneous reef where it is difficult to separate effects of reduced pH from spatial variation in reef communities. We urge that future research carefully considers and quantifies the biological and physical complexity of venting environments, and suggest that in dynamic systems, such as White Island, the use of control-impact designs can oversimplify and potentially overestimate the future impacts of OA.
Sea urchin overgrazing is a leading cause of kelp forest loss and in such cases their removal is increasingly advocated for kelp forest restoration. However, refining removal approaches is needed to ...improve the efficiency and success of restoration, as most previous removal studies have been small scale and require ongoing removals to maintain low densities and allow kelp recovery. We investigated the effectiveness of one‐off urchin removal from large, semi‐discrete areas of urchin barrens as a tool to promote kelp recovery. We removed sea urchins (Evechinus chloroticus) from four areas (1.6–2 ha) of urchin barrens in northeastern New Zealand. Exposed urchins were reduced to approximately 7% of initial densities and remained low, yet cryptic urchin densities increased after 2 years. Kelp (Ecklonia radiata) and fucoid (Sargassum sinclairii, Carpophyllum spp.) densities increased rapidly in removal areas, but remained constant or declined in adjacent control urchin barren areas. Macroalgal canopy recovery varied among and within removal areas, but increased on average from approximately 5 to 43% in 2 years. Densities of large sea urchin predators did not increase with kelp recovery, likely due to ongoing fishing within removal areas. Our results demonstrate that a single urchin removal from large, semi‐discrete areas of urchin barrens can effectively and efficiently promote rapid multi‐species macroalgal recovery without additional actions (e.g. repeated urchin removals or macroalgae enhancement) for at least 2 years. However, this approach does not restore whole ecosystems and consequently restoration benefits through kelp recovery will be temporary without longer‐term urchin management and/or rebuilding of predator populations.
PDF
