Kelp Forests Research Articles

High-latitude kelps and future oceans: A review of multiple stressor impacts in a changing world.

Kelp forests worldwide are threatened by both climate change and localized anthropogenic impacts. Species with cold-temperate, subpolar, or polar distributions are projected to experience range contractions over the coming decades, which may be exacerbated by climatic events such as marine heatwaves and increased freshwater and sediment input from rapidly contracting glaciers. The northeast Pacific has an extensive history of harvesting and cultivating kelps for subsistence, commercial, and other uses, and, therefore, declines in kelp abundance and distributional shifts will have significant impacts on this region. Gaps in our understanding of how cold-temperate kelp species respond to climate stressors have limited our ability to forecast the status of kelp forests in future oceans, which hampers conservation and management efforts. Here, we conducted a structured literature review to provide a synthesis of the impacts of multiple climate-related stressors on kelp forests in the northeast Pacific, assess existing knowledge gaps, and suggest potential research priorities. We chose to focus on temperature, salinity, sediment load, and light as the stressors most likely to vary and impact kelps as climate change progresses. Our results revealed biases in the existing literature toward studies investigating the impacts of temperature, or temperature in combination with light. Other stressors, particularly salinity and sediment load, have received much less focus despite rapidly changing conditions in high-latitude regions. Furthermore, multiple stressor studies appear to focus on kelp sporophytes, and it is necessary that we improve our understanding of how kelp microstages will be affected by stressor combinations. Finally, studies that investigate the potential of experimental transplantation or selective cultivation of genotypes resilient to environmental changes are lacking and would be useful for the conservation of wild populations and the seaweed aquaculture industry.

What the genome of the leafy seadragon teaches us about rapid evolution of camouflage and adaptations to life in the kelp forest.

What the genome of the leafy seadragon teaches us about rapid evolution of camouflage and adaptations to life in the kelp forest.

Parasite diversity at isolated, disturbed hydrothermal vents.

Habitat isolation and disturbance are important regulators of biodiversity, yet it remains unclear how these environmental features drive differences in parasite diversity between ecosystems. We test whether the biological communities in an isolated, frequently disturbed marine ecosystem (deep-sea hydrothermal vents) have reduced parasite richness and relatively fewer parasite species with indirect life cycles (ILCs) compared to ecosystems that are less isolated and less disturbed. We surveyed the parasite fauna of the biological community at the 9°50'N hydrothermal vent field on the East Pacific Rise and compared it to similar datasets from a well-connected and moderately disturbed ecosystem (kelp forest) and an isolated and undisturbed ecosystem (atoll sandflat). Parasite richness within host species did not differ significantly between ecosystems, yet total parasite richness in the vent community was much lower due to the low number of predatory fish species. Contrary to expectation, the proportion of ILC parasite species was not lower at vents due to a high richness of trematodes, while other ILC parasite taxa were scarce (nematodes) or absent (cestodes). These results demonstrate the success of diverse parasite taxa in an extreme environment and reinforce the importance of host diversity and food web complexity in governing parasite diversity.

Levelling-up rhodolith-bed science to address global-scale conservation challenges

Global marine conservation remains fractured by an imbalance in research efforts and policy actions, limiting progression towards sustainability. Rhodolith beds represent a prime example, as they have ecological importance on a global scale, provide a wealth of ecosystem functions and services, including biodiversity provision and potential climate change mitigation, but remain disproportionately understudied, compared to other coastal ecosystems (tropical coral reefs, kelp forests, mangroves, seagrasses). Although rhodolith beds have gained some recognition, as important and sensitive habitats at national/regional levels during the last decade, there is still a notable lack of information and, consequently, specific conservation efforts. We argue that the lack of information about these habitats, and the significant ecosystem services they provide, is hindering the development of effective conservation measures and limiting wider marine conservation success. This is becoming a pressing issue, considering the multiple severe pressures and threats these habitats are exposed to (e.g., pollution, fishing activities, climate change), which may lead to an erosion of their ecological function and ecosystem services. By synthesizing the current knowledge, we provide arguments to highlight the importance and urgency of levelling-up research efforts focused on rhodolith beds, combating rhodolith bed degradation and avoiding the loss of associated biodiversity, thus ensuring the sustainability of future conservation programs.

Shortfalls in the protection of persistent bull kelp forests in the USA

Kelp forests are one of the most productive ecosystems on earth, providing critical ecosystem services. Despite their global importance, their persistence in the face of human pressure and climate change is uncertain. We present a 38-year quarterly time series of satellite imagery that maps the distribution and persistence of surface canopy-forming kelp (dominated by the bull kelp, Nereocystis leutkeana) forests along eleven degrees of latitude in the western Pacific of the USA. We estimate kelp persistence as the fraction of years occupied by kelp canopy in the time series and evaluate the representation of kelp in marine protected areas (MPAs). While 3.6 % of kelp habitat is fully protected and 10.1 % is partially protected, only 0.7 % of the highly persistent kelp which may be indicative of climate refugia are fully protected. Regionally, the amount of kelp fully protected inside MPAs decreases from Central Northern California (8.0 %) to Oregon (5.9 %), Northern California (1.7 %), and Washington (0 %). Five years after the 2014–2016 marine heatwaves, kelp forests did not recover in California (∼90 % loss for both regions), while Oregon and Washington remained near pre-heatwave values. The low amount of protection in Northern California is concerning and likely exacerbates the vulnerability of kelp and associated species to marine heatwaves. Meeting a target of protecting 10 % of existing kelp habitat will require a 2.5-fold increase in kelp representation in MPAs. Moreover, we propose protecting highly persistent kelp is a cost-effective approach to increase representation and efficacy of MPAs to support climate resilience of kelp forest ecosystems.

Assessing the carbon storage value of kelp forest restoration in the Hauraki Gulf Marine Park, New Zealand: Lessons from no-take Marine Protected Areas

Anthropogenic CO2 emissions are one of the principal driving forces of climate change. Climate change leads to systematic changes of ecosystems that cause significant impacts on species interactions, ecosystem processes, and result in profound socio-economic consequences. While carbon offsetting of terrestrial forests and their monetary value are well accepted, the carbon offsetting potential and value of coastal and marine ecosystems (Blue Carbon, BC) remains excluded from the international carbon accounting framework. In this study, we estimated the carbon offsetting value of kelp forests (Ecklonia radiata) against urchin barrens (dominated by crustose coralline algae) using the standing stock biomass as a metric. We propose that standing stock biomass is a crucial next step in introducing BC to the general carbon offsetting framework. Our approach is informed by long-term ecological research conducted in a marine reserve, which highlights the potential of utilizing trophic cascades to limit the dominance of urchin barrens in shallow coastal habitats. We found that the value of carbon standing stock in current kelp forests with reforestation in the Hauraki Gulf Marine Park (HGMP) is worth up to $NZ 7.9 million, based on 2021 carbon market pricing in New Zealand. This simple step towards realizing the value of marine conservation and its multiple benefits to society highlights the missed opportunities when policy and plans do not align with ecology and economics. Including the ocean's contributions to carbon storage is a critical step in using monetization techniques to overcome climate change challenges. Monetizing the carbon offsetting value of kelp forests contributes to establishing funding mechanisms to attract diverse investments in marine ecosystem restoration while providing ecological benefits in ocean governance.

Dynamics of juvenile salmon and forage fishes in nearshore kelp forests

Abstract North‐east Pacific juvenile salmon and forage fishes, including the endangered salmon species Chinook (Oncorhynchus tshawytscha), coho (Oncorhynchus kisutch), chum (Oncorhynchus keta), and pink (Oncorhynchus gorbuscha), and the forage fishes species of Pacific herring (Clupea pallasi), surf smelt (Hypomesus pretiosus), and sand lance (Ammodytes hexapterus), utilize kelp environments. Many details of the kelp forest ecosystem function for these fishes are lacking. Kelp forests, salmon, and forage fishes are declining precipitously as the global climate shifts and developments along coastal shorelines expand. It is therefore essential to understand kelp forest function for these species. Analysis of 7 years of snorkelling survey videos indicates that both forage fishes and salmon use kelp forests throughout the outmigration season, and that their interactions occur in small and large groups, primarily along the outer edge of kelp beds. Over the course of outmigration, juvenile Chinook and coho salmon encounter sand lance first, followed by smelt and herring. The majority of interactions are intermingling, in which a subset proceed to predation, primarily on herring. It is important to develop and implement specific fishery and habitat conservation measures to preserve and restore these functions. Long‐term research has shown that intact, conserved nearshore habitats function better ecologically than restored habitats, and individual kelp forests can function differently for forage fishes and salmon. Therefore, conservation plans should be developed to conserve wild kelp forests which are documented to provide ecosystem function for salmon and forage fishes, by protecting them from various development impacts, including dredging, filling, and water‐quality decline. Coastal restoration is often only successful when ecosystem‐limiting factors that result in a loss of habitat are resolved. Kelp forest restoration, therefore, should be prioritized for regions of known high historical kelp forest importance, with restoration actions focused on correcting the limiting factors that caused loss and/or degradation.

Patterns of spatial and temporal dynamics of mixed Mytilus edulis and M. trossulus populations in a small subarctic inlet (Tyuva Inlet, Barents Sea)

Subarctic populations of “cryptic” blue mussel species Mytilus edulis (ME) and M. trossulus (MT) are less studied than Arctic and boreal populations. Ecological features of ME and MT in sympatry are poorly known everywhere. We studied the habitat segregation of ME and MT and the interannual dynamics of their mixed settlements at the Murman coast of the Barents Sea, the northeastern boundary of the Atlantic littoral mussel communities. Previous data on mussels from this area are 50-100 years old. The 3-km-long Tyuva Inlet (Kola Bay) was used as the study site. Mussels were found in the littoral and the sublittoral down to a depth of 4 m. Their characteristic habitats were sandbanks, littoral rocks, sublittoral kelp forests and “the habitat of the mussel bed” in the freshened top of the inlet. The main spatial gradients explaining the variability of demographics of the settlements (abundance, age structure, size) were associated with the depth and the distance from the inlet top. ME and MT were partially segregated by depth: ME dominated in the sublittoral and MT, in the littoral. In addition, ME dominated throughout the mussel bed. The ratio of species in the mixed settlements varied over time: between 2004 and 2010 the proportions of MT decreased everywhere, by 22% on average. The habitat distribution of mussels apparently changed with time: we found that mussels were abundant in kelp forests, where they had rarely been observed in the 20th century. We suggest that the spatial and temporal dynamics of subarctic mussels can be partly explained by the competition between ME and MT and their differing sensitivity to environmental factors.

Optimising Kelp Cultivation to Scale up Habitat Restoration Efforts: Effect of Light Intensity on “Green Gravel” Production

Kelp forests are disappearing worldwide due to climate change and human stressors, highlighting the need for active interventions. A new restoration approach, “green gravel”, has been shown to be a potentially effective tool to restore endangered kelp forests. However, green gravel is still a novel technique that needs better experimentation and optimisation at all process stages. Contamination by other algal species is one of the critical factors in early-stage green gravel production because their overgrowth can lead to the loss of the seeded material. In this study, we assessed the effect of light intensity on kelp growth and on the coverage of contaminating algae on green gravel. Our results show that under high lights, kelps displayed faster growth (recruits on average more than three times the size and covering a six-times-larger area in high light intensity than in low light), but there was also a higher percentage of contaminating algae. In contrast, the green gravel cultivated under low lights showed almost no signs of algal contamination, but the area occupied by kelps and the length of the lamina were dramatically lower. Due to the cultivation conditions, opportunistic species can grow fast. This advantage is expected to disappear once the green gravel is deployed. To obtain cleaner cultures and to avoid the risk of losing the cultivated material, we would advise starting rearing under lower light intensity to reduce the risk of contamination but ensure kelp growth and then increasing the light intensity to boost it. Clear and appropriate protocols are absolutely necessary to minimise production costs and times and for the scaling-up of future attempts at marine forest restoration.

Impact of Super Typhoon ‘Hinnamnor’ on Density of Kelp Forest and Associated Benthic Communities in Jeju Island, Republic of Korea

This study was carried out to determine the levels of resistance and resilience of kelp forests to large-scale physical disturbances. Our study site, Seongsan, Jeju Island, was impacted by super typhoon ‘Hinnamnor’. Before the typhoon, Seongsan had shown high ecosystem stability. Our results indicated that the ecological stability of a kelp forest facing a severe typhoon is strongly linked to the prevailing environmental conditions. Although typhoon impact resulted in a significant loss of brown macroalgae canopy, Ecklonia cava remained dominant within the kelp forest community. Resistance and resilience levels strongly depended on water temperature and movement and presence of turf-forming algae. Hence, hydrodynamic and biological factors strongly influence the overall stability of a kelp forest. We also report the first occurrences of a scleractinian coral species (i.e., Montipora millepora) at Seongsan, which became visible after canopy loss following the typhoon. Our findings provide valuable ecological information about the benthic community of kelp-dominated ecosystems and are essential to mitigate the impacts of expected climate change-driven rises in seawater temperature and the frequency of super typhoons.

Assessing Future Projections of Warm‐Season Marine Heatwave Characteristics With Three CMIP6 Models

AbstractMarine heatwaves in the summertime when temperatures may exceed organisms' thermal thresholds (“warm‐season marine heatwaves (MHWs)”) have huge impacts on the health and function of ecosystems like kelp forests and coral reefs. While previous studies showed that MHWs are likely to become more frequent and severe under future climate change, there has been less analysis of the thermal properties of warm‐season MHWs or on the effects of climate model biases on these projections. In this study, we examine CMIP6 model ability to simulate five key thermal properties of warm‐season MHWs, and evaluate the global pattern of future projections for coral reef and kelp systems. The results show that the duration, accumulated heat stress and peak intensity are projected to increase by >60 days, 160°C·day and 1°C, respectively, across most of the ocean by the end of the 21st century. In contrast, the duration of “priming” (a period of sub‐lethal heat stress prior to substantial MHW heat stress) is projected to decrease by >30 days in the tropics, potentially reducing organisms' ability to acclimate to heat stress. The projected increases in MHW duration and accumulated heat stress in some coral reef and kelp forest locations, however, are likely overestimated due to model limitations in simulating surface winds, deep convection and some other processes that influence MHW evolution. The findings point to some possible processes to target in model development and regional biases to be considered when projecting the impacts of MHWs on marine ecosystems.

Examining the Potential of Sandy Marine Sediments Surrounding Giant Kelp Forests to Provide Recycled Nutrients for Growth

Examining the Potential of Sandy Marine Sediments Surrounding Giant Kelp Forests to Provide Recycled Nutrients for Growth

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.

Why Earth’s giant kelp forests are worth $500 billion a year

Why Earth’s giant kelp forests are worth $500 billion a year

Shading by giant kelp canopy can restrict the invasiveness of Undaria pinnatifida (Laminariales, Phaeophyceae).

The spread of non-indigenous and invasive seaweeds has increased worldwide, and their potential effects on native seaweeds have raised concern. Undaria pinnatifida is considered among the most prolific non-indigenous species. This species has expanded rapidly in the Northeast Pacific, overlapping with native communities such as the iconic giant kelp forests (Macrocystis pyrifera). Canopy shading by giant kelp has been argued to be a limiting factor for the presence of U. pinnatifida in the understory, thus its invasiveness capacity. However, its physiological plasticity under light limitation remains unclear. In this work, we compared the physiology and growth of juvenile U. pinnatifida and M. pyrifera sporophytes transplanted to the understory of a giant kelp forest, to juveniles growing outside of the forest. Extreme low light availability compared to that outside (~0.2 and ~4.4 mol photon ⋅ m-2 ⋅ d-1 , respectively) likely caused a "metabolic energy crisis" in U. pinnatifida, thus restricting its photoacclimation plasticity and nitrogen acquisition, ultimately reducing its growth. Despite M. pyrifera juveniles showing photoacclimatory responses (e.g., increases in photosynthetic efficiency and lower compensation irradiance, Ec ), their physiological/vegetative status deteriorated similarly to U. pinnatifida, which explains the low recruitment inside the forest. Generally, our results revealed the ecophysiological basis behind the limited growth and survival of juvenile U. pinnatifida sporophytes in the understory.

How far to build it before they come? Analyzing the use of the Field of Dreams hypothesis in bull kelp restoration.

In restoration ecology, the Field of Dreams hypothesis posits that restoration efforts that create a suitable environment could lead to the eventual recovery of the remaining aspects of the ecosystem through natural processes. Natural processes following partial restoration has led to ecosystem recovery in both terrestrial and aquatic systems. However, understanding the efficacy of a "Field of Dreams" approach requires a comparison of different approaches to partial restoration in terms of spatial, temporal, and ecological scale with what would happen given more comprehensive restoration efforts. We explore the relative effect of partial restoration and ongoing recovery on restoration efficacy with a dynamical model based on temperate rocky reefs in Northern California. We analyze our model for both the ability and rate of bull kelp forest recovery under different restoration strategies. We compare the efficacy of a partial restoration approach with a more comprehensive restoration effort by exploring how kelp recovery likelihood and rate change with varying intensities of urchin removal and kelp outplanting over different time periods and spatial scales. We find that, in the case of bull kelp forests, setting more favorable initial conditions for kelp recovery by implementing both urchin harvesting and kelp outplanting at the start of the restoration project has a bigger impact on the kelp recovery rate than applying restoration efforts through a longer period of time. Therefore, partial restoration efforts, in terms of spatial and temporal scale, can be significantly more effective when applied across multiple ecological scales in terms of both the capacity and rate for achieving the target outcomes.

The challenge of estimating kelp production in a turbid marine environment.

Coastal kelp forests produce substantial marine carbon due to high annual net primary production (NPP) rates, but upscaling of NPP estimates over time and space remains difficult. We investigated the impact of variable underwater photosynthetically active radiation (PAR) and photosynthetic parameters on photosynthetic oxygen production of Laminaria hyperborea, the dominant NE-Atlantic kelp species, throughout summer 2014. Collection depth of kelp had no effect on chlorophyll a content, pointing to a high photoacclimation potential of L. hyperborea towards incident light. However, chlorophyll a and photosynthesis versus irradiance parameters differed significantly along the blade gradient when normalized to fresh mass, potentially introducing large uncertainties in NPP upscaling to whole thalli. Therefore, we recommend a normalization to kelp tissue area, which is stable over the blade gradient. Continuous PAR measurements revealed a highly variable underwater light climate at our study site (Helgoland, North Sea) in summer 2014, reflected by PAR attenuation coefficients (Kd ) between 0.28 and 0.87 m-1 . Our data highlight the importance of continuous underwater light measurements or representative average values using a weighted Kd to account for large PAR variability in NPP calculations. Strong winds in August increased turbidity, resulting in a negative carbon balance at depths >3-4m over several weeks, considerably impacting kelp productivity. Estimated daily summer NPP over all four depths was 1.48 ± 0.97 g C · m-2 seafloor · d-1 for the Helgolandic kelp forest, which is in the range of other kelp forests along European coastlines.

Climate change leaves urchins on a rock in a hard place to live

The kelp forests off the West Coast of North America are often described as underwater rainforests – dense, beautiful and teeming with life. Unfortunately, climate change is altering seawater temperature, acidity and oxygen content in the forests, which could be highly stressful for the sea creatures that inhabit them. One of the most valuable players in the kelp forests are red sea urchins, which function as engineers, controlling how dense the forests are through their consumption of kelp. While urchins may be able to cope with changes in one of these stressors at a time, they might not be able to handle changes in all three simultaneously. Ultimately, urchin populations may collapse if they cannot endure this triple threat, leaving the future of the kelp forests uncertain. Emily Donham at the University of California, Santa Cruz, USA, worked with a team of researchers at Reef Check Foundation, USA, Monterey Bay Aquarium Research Institute, USA, and the University of California, Santa Cruz, to find out how resilient red urchins in California are to the environmental changes they face because of climate change.First, the researchers needed to find out what ocean conditions are like in California's urchin habitat. But California is a gigantic state, so the urchins living off the coast of northern California may be accustomed to different ocean conditions when compared with urchins from the south and, thus, have different vulnerabilities to climate change. So, the team placed sensors that measured seawater temperature, oxygen and water acidity in kelp forests along the length of the state. After determining the seawater conditions of the different populations, the team collected northern and southern urchins and brought them back to the lab. Then they held the urchins in seawater that mimicked the two regions and measured how much the urchins grew, ate and expended energy. They predicted that the urchins would thrive more in ocean conditions that mimicked their home environments.As expected, the sea urchins performed better in the water that mirrored their own natural conditions, such that when the northern urchins were held in the southern-type water, they had lower survival rates and grew less than the southern urchins. The same thing could be said for the southern urchins, which outperformed the northern urchins across most tests, but did so to a much greater extent when held in the waters mimicking their own home southern water conditions.The team also discovered that urchins in the north are used to experiencing simultaneous fluctuations in seawater temperature, acidity and oxygen levels, which makes these changes more predictable and therefore potentially easier for the urchins to react to. In contrast, the southern urchins appear to be accustomed to a more unpredictable environment. As climate change will make seawater conditions more erratic and variable, the team expected that the northern urchins would struggle more than the southern urchins when exposed to a climate change scenario.To test this, Donham and the team conducted a second study in which they exposed urchins from each population to conditions that simulated how climate change will affect their local environments. They found that climate change will harm urchins across California, as both populations grew and ate less in the future climate change conditions. But, surprisingly, the urchins in the north had a greater chance of surviving those troubling times. This is concerning news for urchins and kelp forests in the south but, hopefully, the populations will persist if enough urchins are able to withstand the coming changes. Ultimately, the best thing we can do for urchins is fight to keep climate change predictions from becoming reality.

Organellar genomes of giant kelp from the southern hemisphere

ABSTRACT Macrocystis pyrifera is a foundation species that creates kelp forests and supports essential ecosystem services across coastal environments. Over the past half-century, more than 95% of giant kelp forests have declined around Tasmania, Australia, due to climate change, causing a near-complete loss of the ecosystems and services they support. Compared with northern hemisphere giant kelp ecosystems, Australian populations have received little research attention in genomic and other genetic analyses. We present the complete mitochondrial and chloroplast genomes of Macrocystis pyrifera from Tasmania. Both organellar genomes were similar to published Laminariales genomes in length, GC content, gene composition and synteny. A phylogeny constructed by combining protein-coding genes from both genomes showed Tasmanian specimens clustered with M. pyrifera specimens from the northern hemisphere. Genetic differences in protein genes between the Tasmanian M. pyrifera and the northern hemisphere specimens were overall low, but some ribosomal protein genes presented higher values of nonsynonymous mutations. The most divergent gene, the mitochondrial conserved hypothetical protein ORF377, can provide insights into the evolution of the species. This gene has been proposed as a suitable molecular marker for population genetic research in Fucales and may also be helpful for intraspecific studies of M. pyrifera. The complete mitochondrial and chloroplast genomes of Tasmanian M. pyrifera provide important genetic data and critical information for further evolutionary and population studies and for managing these endangered and disappearing populations.

Persistence of southern California giant kelp beds and alongshore variation in nutrient exposure driven by seasonal upwelling and internal waves

Kelp beds provide significant ecosystem services and socioeconomic benefits globally, and prominently in coastal zones of the California Current. Their distributions and abundance, however, vary greatly over space and time. Here, we describe long-term patterns of Giant Kelp (Macrocystis pyrifera) sea surface canopy area off the coast of San Diego County from 1983 through 2019 along with recent patterns of water column nitrate (NO3-) exposure inferred from in situ temperature data in 2014 and 2015 at sites spanning 30 km of the coastline near San Diego California, USA. Site-specific patterns of kelp persistence and resilience were associated with ocean and climate dynamics, with total sea surface kelp canopy area varying approximately 33-fold over the almost 4 decades (min 0.34 km2 in 1984; max 11.25 km2 in 2008, median 4.79 km2). Site-normalized canopy areas showed that recent kelp persistence since 2014 was greater at Point Loma and La Jolla, the largest kelp beds off California, than at the much smaller kelp bed off Cardiff. NO3- exposure was estimated from an 11-month time series of in situ water column temperature collected in 2014 and 2015 at 4 kelp beds, using a relationship between temperature and NO3- concentration previously established for the region. The vertical position of the 14.5°C isotherm, an indicator of the main thermocline and nutricline, varied across the entire water column at semidiurnal to seasonal frequencies. We use a novel means of quantifying estimated water column NO3- exposure integrated through time (mol-days m-2) adapted from degree days approaches commonly used to characterize thermal exposures. Water column integrated NO3- exposure binned by quarters of the time series showed strong seasonal differences with highest exposure in Mar - May 2015, lowest exposure in Sep - Dec 2014, with consistently highest exposure off Point Loma. The water column integrated NO3- signal was filtered to provide estimates of the contribution to total nitrate exposure from high frequency variability (ƒ >= 1 cycle 30 hr-1) associated predominantly with internal waves, and low frequency variability driven predominantly by seasonal upwelling. While seasonal upwelling accounted for > 90% of NO3- exposure across the full year, during warm periods when seasonal upwelling was reduced or absent and NO3- exposure was low overall, the proportion due to internal waves increased markedly to 84 to 100% of the site-specific total exposure. The high frequency variability associated with internal waves may supply critical nutrient availability during anomalously warm periods. Overall, these analyses support a hypothesis that differences in NO3- exposure among sites due to seasonal upwelling and higher frequency internal wave forcing contribute to spatial patterns in Giant Kelp persistence in southern California. The study period includes anomalously warm surface conditions and the marine heatwave associated with the “Pacific Warm Blob” superimposed on the seasonal thermal signal and corresponding to the onset of a multi-year decline in kelp canopy area and marked differences in kelp persistence among sites. Our analysis suggests that, particularly during periods of warm surface conditions, variation in NO3- exposure associated with processes occurring at higher frequencies, including internal waves can be a significant source of NO3- exposure to kelp beds in this region. The patterns described here also offer a view of the potential roles of seasonal and higher frequency nutrient dynamics for Giant Kelp persistence in southern California under continuing ocean surface warming and increasing frequency and intensity of marine heatwaves.