Kelp Blades Research Articles

Bimodal spore release heights in the water column enhance local retention and population connectivity of bull kelp, Nereocystis luetkeana.

Dispersal of reproductive propagules determines recruitment patterns and connectivity among populations and can influence how populations respond to major disturbance events. Dispersal distributions can depend on propagule release strategies. For instance, the bull kelp, Nereocystis luetkeana, can release propagules (spores) from two heights in the water column ("bimodal release"): at the water surface, directly from the reproductive tissues (sori) on the kelp's blades, and near the seafloor after the sori abscise and sink through the water column. N. luetkeana is a foundation species that occurs from central California to Alaska and is experiencing unprecedented levels of population declines near its southern range limit. We know little of the kelp's dispersal distributions, which could influence population recovery and restoration. Here, we quantify how bimodal spore release heights affect dispersal outcomes based on a numerical model specifically designed for N. luetkeana. The model incorporates oceanographic conditions typical of the species' coastal range and kelp biological traits. With bimodal release heights, 34% of spores are predicted to settle within 10 m of the parental alga and 60% are predicted to disperse beyond 100 m. As an annual species, bimodal release heights can facilitate the local regeneration of adults within a source kelp forest while also supporting connectivity among multiple forests within broader bull kelp metapopulations. To leverage this pattern of bimodal spore dispersal in bull kelp restoration management, directing resources toward strategically located focal populations that can seed other ones could amplify the scale of recovery.

Kelps may compensate for low nitrate availability by using regenerated forms of nitrogen, including urea and ammonium.

Nitrate, the form of nitrogen often associated with kelp growth, is typically low in summer during periods of high macroalgal growth. More ephemeral, regenerated forms of nitrogen, such as ammonium and urea, are much less studied as sources of nitrogen for kelps, despite the relatively high concentrations of regenerated nitrogen found in the Southern California Bight, where kelps are common. To assess how nitrogen uptake by kelps varies by species and nitrogen form in southern California, USA, we measured uptake rates of nitrate, ammonium, and urea by Macrocystis pyrifera and Eisenia arborea individuals from four regions characterized by differences in nitrogen availability-Orange County, San Pedro, eastern Santa Catalina Island, and western Santa Catalina Island-during the summers of 2021 and 2022. Seawater samples collected at each location showed that overall nitrogen availability was low, but ammonium and urea were often more abundant than nitrate. We also quantified the internal %nitrogen of each kelp blade collected, which was positively associated with ambient environmental nitrogen concentrations at the time of collection. We observed that both kelp species readily took up nitrate, ammonium, and urea, with M. pyrifera taking up nitrate and ammonium more efficiently than E. arborea. Urea uptake efficiency for both species increased as internal percent nitrogen decreased. Our results indicate that lesser-studied, more ephemeral forms of nitrogen can readily be taken up by these kelps, with possible upregulation of urea uptake as nitrogen availability declines.

Interactive effects of temperature and nitrogen on the physiology of kelps (Nereocystis luetkeana and Saccharina latissima)

Kelp forest declines have been linked to warming ocean temperatures worldwide. Ocean warming rarely occurs in isolation, so multiple stressor studies are necessary to understand the physiological responses of kelp to climate change. The canopy-forming bull kelp, Nereocystis luetkeana, is going locally extinct in areas of the Salish Sea that are seasonally warm and nutrient poor, while the understory kelp, Saccharina latissima, persists at those sites. Further, nitrogen availability can alter physiological responses of kelps to temperature stress, including alleviating warming stress. We compared the physiological responses of kelp sporophytes to high temperature stress and nitrogen limitation between two populations of N. luetkeana with different environmental histories (warm and nutrient poor vs. cold and nutrient rich) and between two species, N. luetkeana and S. latissima. Using laboratory mesocosms, we tested the interactive effects of short term (8-9 day) exposure of kelp blades to different temperatures: low (9, 13°C), moderate (15, 16°C), and warm (21°C) at two different nitrogen concentrations: low (1-3 μM) vs. high (>10 μM). We examined a wide array of physiological responses: blade growth, photosynthesis, respiration, photosynthetic yield, nutrient uptake, and tissue C:N. Both kelp species responded negatively to elevated temperatures, but not to low nitrogen levels. Blades of both species showed signs of metabolic stress and reduced growth in the warmest temperature treatment (21°C), at both high and low nitrogen levels, suggesting that N. luetkeana and S. latissima are susceptible to thermal stress over short time periods. Populations of N. luetkeana from warm, nutrient poor and cool, nutrient rich areas were equally susceptible to the effects of ocean warming. Our results suggest that nutrient additions may actually reduce kelp performance at supra-optimal temperatures, and a thorough understanding of kelp responses to coastal temperature and nutrient dynamics is needed to guide conservation and restoration actions.

Evidence of elevated heavy metals concentrations in wild and farmed sugar kelp (Saccharina latissima) in New England

Seaweed farming in the United States is gaining significant financial and political support due to prospects to sustainably expand domestic economies with environmentally friendly products. Several networks are seeking appropriate synthesis of available science to both inform policy and substantiate the sector’s sustainability claims. Significant knowledge gaps remain regarding seaweed-specific food hazards and their mitigation; a resource-intensive challenge that can inhibit sustainable policies. This is particularly concerning for rapidly expanding Saccharina latissima (sugar kelp) crops, a brown seaweed that is known to accumulate heavy metals linked to food hazards. Here, we present baseline information about concentrations of arsenic, cadmium, lead, and mercury, in both wild and farmed sugar kelp from the New England region. We interpret our findings based on proximity to potential sources of contamination, location on blade, and available heavy metals standards. Contrary to our expectations, high concentrations were widespread in both wild and farmed populations, regardless of proximity to contamination. We find, like others, that cadmium and arsenic consistently reach levels of regulatory concern, and that dried seaweeds could harbor higher concentrations compared to raw products. We also share unique findings that suggest some toxins concentrate at the base of kelp blades. Our results are one step towards aggregating vital data for the region to expand its seaweed farming footprint.

Integrated multi-trophic aquaculture with sugar kelp and oysters in a shallow coastal salt pond and open estuary site.

Sustainable aquaculture includes the aquaculture of non-fed crops that provide ecosystem services including nutrient extraction and water quality improvement. While shellfish are the most farmed sustainable aquaculture crops in the USA, shellfish farmers in the northeastern US have an interest in diversifying their crops and incorporating seaweeds into their farms. In this study, we worked with oyster farmers to investigate the potential for farming sugar kelp, Saccharina latissima, across different environmental regimes in coastal Rhode Island USA. Kelp seed spools were outplanted at two time points in the fall/winter of 2017 and 2018 at four sites and cultivated until harvest the following spring. Kelp performance (length, width, yield), tissue content, and nutrient extraction were determined for each line in each year; oyster growth was also measured monthly for one year at each site. We found that kelp could successfully grow in both shallow coastal lagoons and estuarine sites, although the timing of planting and placement of sites was important. Lines that were planted earlier (as soon as water temperatures<15°C) grew longer and yielded more biomass at harvest; overall, kelp blade yield ranged from 0.36 ± 0.01 to 11.26 ± 2.18 kg/m long line. We report little variation in the tissue quality (C:N) of kelp among sites, but differences in biomass production led to differences in nutrient extraction, which ranged from 0.28 ± 0.04 to 16.35 ± 4.26 g nitrogen/m long line and 8.93 ± 0.35 to 286.30 ± 74.66 g carbon/m long line. We found extensive variability in kelp growth within and between lines and between years, suggesting that crop consistency is a challenge for kelp farmers in the region. Our results suggest that, as there is a lower barrier in terms of permitting (versus starting a new aquaculture farm), it may be a worthwhile investment to add sugar kelp to existing oyster farms, provided they have suitable conditions. At current market rates of US$0.88-$3.30 per kg, farmers in southern New England have the potential to earn US$2,229 per 60 m longline. While seaweed aquaculture is growing, considerable barriers still exist that prevent wide-scale kelp aquaculture adoption by existing aquafarmers.

Genetic linkage map construction and QTL mapping of blade length and width in Saccharina japonica using SSR and SNP markers

MAS (marker assisted selection) is useful for cultivar development in the lower plant Saccharina japonica. In this study, 167 screened SSR markers and 125 individuals were used to construct a S.japonica genetic linkage map with 27 linkage groups and a total Haldane length of 2069 cM. Constructed linkage groups were aligned with assembled pseudo-chromosomes of S. japonica genome sequences (NCBI: MEHQ00000000) and anchored to 23 of 31 pseudo-chromosomes. A genetic linkage map including 33 linkage groups was constructed with combined 3397 SSR and SNP markers. In the integrated genetic map used for QTL mapping, SSR marker M1895 was closely associated with the major QTL for blade length and width with LOD threshold of &amp;gt;5.0. Annotation indicated that SjPT, coded as a high affinity phosphate transporter, was mapped to this major QTL with cDNA sequence of 1422 bp, which coded 473 aa for 8 transmembrane helics. qRT-PCR and the transcriptomic detection confirmed that SjPT transcription was consistent with increases in kelp blade length and width. Marker M1895 appears to be particularly useful for MAS (marker assisted selection) in S. japonica breeding.

Optimisation of at-sea culture and harvest conditions for cultivated Macrocystis pyrifera: yield, biofouling and biochemical composition of cultured biomass

Seaweed cultivation is gaining interest world-wide for both food and non-food applications. Global seaweed aquaculture production currently exceeds 32 Mt WW per annum but is dominated (86% of total) by Asian countries. To meet future demand for seaweed products, regions beyond Asia with aquaculture production potential are being explored. The goal of this study was to assess the suitability of the native kelp Macrocystis pyrifera (Phaeophyceae, Laminariales), for aquaculture in Tasmania, south-eastern Australia. M. pyrifera was cultivated on seeded twine on loops (1 – 5 m depth) along 100-m longlines at two sites (Okehampton Bay and Great Taylor Bay) from April-November 2020. Temporal and spatial variability in (1) yield (kg m-1, WW), (2) biofouling (% coverage), and (3) biochemical composition (including proximate composition, fatty acids, dietary minerals, heavy metal profiling, C, N, H, S concentrations and C:N ratio, antioxidants (phenolic compounds), and pigments (Chl-a, Chl-c, fucoxanthin)) was compared amongst the two cultivation sites, at two depths (1 and 5 m) from harvests between July – November 2020. Yield (kg m-1, WW) did not significantly change across harvest times, but was greater at a depth of 1 m compared to 5 m. Biofouling on the kelp blades increased significantly in early spring (September). The biochemical composition of the cultured biomass varied over time, between sites and with depth for most of the compounds analysed. Higher lipid, protein and ash content was reported for cultures cultivated at Okehampton Bay compared to Great Taylor Bay and at 5 m compared to 1 m depth, and levels of these macronutrients decreased during the harvest period. The iodine content was slightly above the tolerable content for dried seaweed products in Australia and New Zealand. The combined results of yield, biofouling, and biochemical composition suggest that, for an April deployment at the sites investigated, M. pyrifera should be harvested in July-August (mid to late winter) to optimise yield and quality of the cultured kelp biomass. These findings provide a better understanding of the variation in growth and quality of cultivated M. pyrifera biomass in the region, and inform future management and development of kelp aquaculture in south-eastern Australia and in a global context.

The effects of cultivation deployment- and harvest-timing, location and depth on growth and composition of Saccharina latissima at the Swedish west coast

A study has been conducted to shed light on the effect of cultivation parameters on growth and chemical composition of Saccharina latissima. Longline cultivation took place at two separate locations in the Koster archipelago on the Swedish west coast, centred around three experiments that explored duration of pre-deployment hatchery processes (Exp 1), deployment and harvest time-frames (Exp 2), and cultivation at 2 m vs. 4 m depth (Exp 3). For all experiments the effects of these parameters were evaluated in terms of size/weight of specimens, and for experiments Exp 2 and Exp 3, concentrations of moisture, ash, carbohydrates, proteins, fatty acids, phlorotannins and common heavy metals were determined.The specific parameters used in this study are likely to vary from site to site, nevertheless trends were observed that resonate in the literature, and these lead to some general recommendations. The weight of harvested kelp blades was higher for the later harvest, earlier deployment and when grown at a shallower depth. Carbohydrates concentration increased with later harvests and at shallower depth. Later harvests also increased the concentration of phlorotannin, while ash and total fatty acids decreased. Protein and fucoidan content was higher for the deeper lines. Growth and chemical composition of the seaweeds showed only minor differences between the two study sites, mostly relating to heavy metal content.

Spatial organization of the kelp microbiome at micron scales

BackgroundElucidating the spatial structure of host-associated microbial communities is essential for understanding taxon-taxon interactions within the microbiota and between microbiota and host. Macroalgae are colonized by complex microbial communities, suggesting intimate symbioses that likely play key roles in both macroalgal and bacterial biology, yet little is known about the spatial organization of microbes associated with macroalgae. Canopy-forming kelp are ecologically significant, fixing teragrams of carbon per year in coastal kelp forest ecosystems. We characterized the micron-scale spatial organization of bacterial communities on blades of the kelp Nereocystis luetkeana using fluorescence in situ hybridization and spectral imaging with a probe set combining phylum-, class-, and genus-level probes to localize and identify > 90% of the microbial community.ResultsWe show that kelp blades host a dense microbial biofilm composed of disparate microbial taxa in close contact with one another. The biofilm is spatially differentiated, with clustered cells of the dominant symbiont Granulosicoccus sp. (Gammaproteobacteria) close to the kelp surface and filamentous Bacteroidetes and Alphaproteobacteria relatively more abundant near the biofilm-seawater interface. A community rich in Bacteroidetes colonized the interior of kelp tissues. Microbial cell density increased markedly along the length of the kelp blade, from sparse microbial colonization of newly produced tissues at the meristematic base of the blade to an abundant microbial biofilm on older tissues at the blade tip. Kelp from a declining population hosted fewer microbial cells compared to kelp from a stable population.ConclusionsImaging revealed close association, at micrometer scales, of different microbial taxa with one another and with the host. This spatial organization creates the conditions necessary for metabolic exchange among microbes and between host and microbiota, such as provisioning of organic carbon to the microbiota and impacts of microbial nitrogen metabolisms on host kelp. The biofilm coating the surface of the kelp blade is well-positioned to mediate interactions between the host and surrounding organisms and to modulate the chemistry of the surrounding water column. The high density of microbial cells on kelp blades (105–107 cells/cm2), combined with the immense surface area of kelp forests, indicates that biogeochemical functions of the kelp microbiome may play an important role in coastal ecosystems.8EDo4SKhFZv5JhLfh-35e4Video abstract

A deep dive into the epibiotic communities on aquacultured sugar kelp Saccharina latissima in Southern New England

Sugar kelp cultivation at the southern end of its range on the east coast of North America is being pursued commercially for human consumption, which demands a high-quality product. Blade quality can be compromised by attached organisms – epibionts. Biweekly examination of epibionts on sugar kelp was conducted April–May 2018, on a kelp farm in eastern Long Island Sound, CT, USA. Culturable Vibrio spp. were not present on kelp blades until May and were limited to only old sections. No Vibrio colonies were human pathogens V. parahaemolyticus or V. vulnificus, based upon ToxR-specific multiplex PCR assays. Neither epibenthic cyanobacteria Lyngbya spp. nor the dinoflagellate Prorocentrum lima, microbes of concern because of toxigenicity, were detected on kelp by microscopy or metabarcoding of partial rRNA genes. The lacy bryozoan was the only epibiotic animal observed that could cause damage to kelp, but its abundance was low.Summarizing most-common sequence reads, Gammaproteobacteria was the most abundant bacterial group on kelp blades (49%) and Alphaproteobacteria were the most abundant in seawater (39%). Bacillariophyta were the most abundant eukaryotes on kelp blades (36%) and Dinoflagellata were the most abundant eukaryotes in seawater (43%). Molecular operational taxonomic unit matrices were used for non-metric multidimensional scaling; the most prominent structure for both prokaryotic and eukaryotic communities was the separation between blade and seawater samples. This spatial separation explained 81% and 76% of the variation among prokaryotic and eukaryotic samples, respectively. Indicator Species Analysis identified Gammaproteobacteria (55%) and Bacillariophyta (56%) to be the most important blade indicator prokaryotes and eukaryotes, respectively. A closer examination of indicator species temporal patterns and their ecophysiology suggested that Aquimarina, Parcubacteria, and Peronosporomycetes are potential pathogens to sugar kelp. Ciliates may be the most important grazers that keep epiphytes (Bacillariophyta, Rhodophyta, and Phaeophyta) and Peronosporomycetes on kelp in check.

Wave attenuation by flexible vegetation (and suspended kelp) with blade motion: Analytical solutions

Vegetation is one of the most important components of nature-based coastal protection due to its ability to dissipate wave energy. To quantify the wave attenuation by vegetation, traditional analytical models assume rigid vegetation and use bulk drag coefficient (CD) or effective blade length (le) techniques to consider the effects of blade motion, where CD and le are conventionally fitted as a function of KC (the Keulegan–Carpenter number) and CaL (with Ca the Cauchy number and L the ratio of the blade length to wave excursion), respectively. These parameters do not include the full blade dynamics and so the empirical formulas of CD and le are different for varying vegetation with blade dynamics. To obtain analytical solutions for CD and le, an analytical wave attenuation model for flexible vegetation (and kelp) was developed in this study by simplifying and linearizing the blade motion. Compared with a wide range of experiments for both submerged vegetation and suspended kelp canopies, the simplified analytical model underestimated the wave decay coefficient (kD) by 27%, but with a small NRMSE (normalized root mean square error by the range of the measured data) of 0.054. In comparison, the numerical model with full nonlinearity underestimated the wave decay coefficient by 11.7% with NRMSE=0.063. To reduce the underestimation of the analytical model due to the simplification and linearization, a modification factor defined as the ratio of the numerically calculated kD and the analytically calculated kD was fitted. With the modification factor, the underestimation of the analytical model was reduced to 10.1%. Based on the analytical model, analytical solutions for CD and le were derived, which showed a similar precision with the experimentally fitted CD and le based on KC and CaL, respectively. Thus, the analytical solutions for CD and le could be a reliable alternative when the experimentally calibrated CD and le are not available. Using the analytical wave attenuation model, a case study showed the wave attenuation by cultivated Saccharina latissima changes seasonally with the kelp growth. When the kelp blade reaches 2.4 m long after 7 months of growth, the kelp farms with 50 longlines (over a distance of 200 m in the direction of wave propagation) in 8 m-deep water may attenuate wave energy by 29% for 1 m-high waves with the period of 6 s. The wave attenuation can be enhanced to 43% when the farms are located in 5 m-deep water. To provide considerable wave attenuation of kelp with adequate long blades around the year, biennial and multiple partial harvesting techniques are recommended.

The effect of distal‐end trimming onSaccharina latissimamorphology, composition, and productivity

AbstractAs kelp cultivation increases around the world, so does the need for farm management strategies that produce specific crop characteristics, optimize yield, widen harvesting windows, and prevent biomass loss. Distal‐end trimming of macroalgae has been recommended as a farm management method addressing these needs. In this study, we trimmed cultivatedSaccharina latissimasporophytes grown in the Western Gulf of Maine (WGoM) to 60 cm above the stipe–blade interface. We characterized the effect of trimming on the morphology, tissue nutrient content, stable isotope ratio, and nitrate reductase activity of the kelp. We also evaluated the economic trade‐offs of trimming using a simple production model. The results suggest that trimming the blade to 60 cm may have minimal biological consequences. Additionally, the trimming appears to benefit “short” kelp blades in proximity to the trimmed blades. Daily yield (% increase in weight day−1) after trimming was initially lower than the control, but late‐season daily yields and crop‐retention following storms were markedly improved. Ultimately, we conclude that growers could use trimming to acquire kelp biomass earlier in the season, retain late‐season biomass, and potentially increase the total revenue gained from kelp farming if price premiums can be exacted for this biomass.

Drag force and reconfiguration of cultivated Saccharina latissima in current

The design of aquaculture systems requires an understanding of the drag forces on cultivated kelp. This study measured the drag on line segments of cultivated Saccharina latissima in a towing tank. The drag on segments of farm line with full kelp bundles and with stipes alone (fronds removed) was measured at tow speeds of 0.10 to 0.50 m/s. The drag on individual fronds cut from the line was also measured. Video images were collected to evaluate the plant reconfiguration. Both kelp blades and stipes contributed to the total drag force on the line bundle. Within the velocity range of our experiments, the kelp blades were essentially horizontal. However, the pronation of kelp stipes increased as flow velocity increased. The reconfiguration of kelp stipes was observed to decrease the vertical extent of the kelp bundle. Due to this reconfiguration, the measured force, F, increased with velocity, U, at a rate slower than quadratic, and was consistent with scaling laws derived for reconfiguration. Specifically, F∼Uα with α=1.35±0.17.

The stress and strain of life – how differences in the mechanical properties and cellular composition enable the kelp Laminaria digitata to thrive in different hydrodynamic environments

Sessile organisms such as macroalgae located in the intertidal and shallow subtidal zones are subject to a hydrodynamically diverse environment, controlling the variation of intraspecific morphology and distribution. Kelp forests experience both waves and/or currents, yet, how kelp blade material mechanically differs between these various hydrodynamic environments and what drives the variation in strength and extensibility are not fully understood. Here, the mechanical properties, cellular composition and blade tissue thickness of the meristematic region and distal tips of the kelp Laminaria digitata blades were quantified and compared between seasons and among three hydrodynamic environments: wave dominated, current dominated and a benign hydrodynamic environment. Kelps associated with energetic environments, generally tended to be stronger yet more extensible than those growing in the benign hydrodynamic environment. Higher extensibility was located at the meristematic region whereas tissue was stronger in the distal tip of the blade. Linking both cellular composition and mechanical properties, results suggest enhancement of medulla cells in the meristematic region increases extensibility, potentially protecting the thallus during increased storm activity while growing in a wave/current exposed habitat. Investment in cortex cells towards the tip of the blade suggests an increase in strength of the region, which is susceptible to breakage. However, the lack of variation in the proportion of medulla and cortex cellular layers between distinct hydrodynamic environments revealed that the potential overall strategy for avoiding breakage in energetic hydrodynamic environments is that of investing energy into the increased thickness of blade tissue.

Photosynthetic pigments of co-occurring Northeast Atlantic Laminaria spp. are unaffected by decomposition

Along Northeast Atlantic coasts, the genus Laminaria dominates kelp forests. Two species, L. digitata and L. hyperborea, are cold temperate whilst the third, L. ochroleuca, is warm temperate. In northern France and the southern British Isles, all 3 species now co-occur, with a gradual rise in predominance of L. ochroleuca evident over recent decades due to climate-driven range shifts. Despite recent focus on the export of photosynthetically viable macroalgal detritus, the effect of decomposition on detrital pigment concentration remains unknown for northern hemisphere kelps. Furthermore, notwithstanding their importance in determining habitat suitability, the photosynthetic pigments of all 3 species have not yet been assessed comparatively within the same forest. Here, we present results of a field experiment aimed to test potential changes in the pigments of decomposing kelp blade fragments. Using spectrophotometry and Gaussian peak spectral deconvolution to quantify pigments, we found that shallow benthic decomposition over 1 mo did not affect major photosynthetic pigment concentrations in any of the examined species. Moreover, the 2 boreal species were similar in their sporophyte pigment stoichiometry and had more chlorophyll a, chlorophyll c, fucoxanthin and minor carotenoids (β,β-carotene and zeaxanthin) than L. ochroleuca. This resulted in total pigment content that was 82 and 74% higher in L. digitata and L. hyperborea than in L. ochroleuca. These differences correspond approximately to each species’ latitudinal distribution and photosynthetic performance. Our results suggest photosynthetic viability of Northeast Atlantic kelp in the initial detrital phase and a potential reduction of forest-scale photosynthetic capacity under continued ocean warming.

Oxygen metabolism shapes microbial settlement on photosynthetic kelp blades compared to artificial kelp substrates.

We examined factors shaping community assembly of the bull kelp (Nereocystis luetkeana) microbiome by comparing microbial biofilm formation on photosynthetic kelp blade tissues and artificial kelp substrates ('agar substrates') deployed into a kelp forest. New kelp blade tissues were colonized by markedly distinct microbial taxa relative to agar substrates during the same time interval, even when agar substrates were infused with N. luetkeana blades, suggesting that microbial settlement onto kelp surfaces is more than just attraction to a polysaccharide-rich surface. Further, common seawater taxa such as Colwellia sp. and Psychromonas sp. became abundant on agar substrates but avoided new kelp blade tissues, indicating that host-specific factors may deter certain surface-associated marine microbial taxa. Over two-thirds of the bacterial taxa in the kelp microbiome were associated with strictly aerobic metabolisms; thus, photosynthetic production of O2 may favour aerobic microbial metabolisms. While living kelp blades primarily recruited aerobic microbes, including the obligate aerobe Granulosicoccus sp., microbes that colonized agar substrates were predominantly facultative anaerobes. We also found that infusion of kelp tissues into agar substrates altered microbial community composition and lowered taxonomic diversity relative to control agar substrates, suggesting that non-living components of the kelp blade also impact microbial community assembly.

Hydrodynamic characteristics of a full-scale kelp model for aquaculture applications

Hydrodynamic characteristics are needed to optimize the design of kelp aquaculture systems. To support this need, the objective of this study was to resolve both the normal and tangential drag forces acting on a dense aggregate of kelp blades using full-scale physical model tests. The physical model was designed to match the exposed length, individual blade flexural rigidity, the number of blades per unit width, the mass/length of biomass, and the aggregate mass density of kelp cultured at the University of New England experimental aquaculture lease site in Saco Bay, Maine USA. Tow tests were conducted at the United States Naval Academy in a tank with the dimensions of 116m × 7.9m × 4.9 m. The large tank size enabled the use of the full-scale physical model, minimizing dynamic similarity issues. In a series of tests, the model was towed in orientations both aligned and perpendicular to the tow direction. Horizontal and vertical reaction forces were measured for five tow speeds, along with the deflection of the dense kelp aggregate. With these datasets and the wet weight biomass per length of the model, normal and tangential drag forces were calculated. Drag components were processed into corresponding normal and tangential drag-area values. The drag-area representation was chosen since reference areas were ambiguous for both the actual kelp and model. At the higher speeds, the total horizontal drag in the aligned configuration were slightly lower than for the perpendicular orientation. Normal drag-areas as a function of tow speed ranged from 2.36 m2/m to 1.39 m2/m for the aligned case and from 2.49 to 1.88 m2/m for the perpendicular case. Tangential drag-areas as a function of tow speed ranged from 0.264 m2/m to 0.0325 m2/m for the aligned case and from 0.213 to 0.0415 m2/m for the perpendicular case. A transition from a bluff body to a streamlined body occurred as the tow speeds increased. To investigate this transition, horizontal components of the normal and tangential drag forces were reconstructed with the results of the tow tests. The reconstructed forces were obtained using a force balance system of equations with drag-area values for tow speeds less than 0.25 m/s extrapolated from the experimental datasets. For both aligned and perpendicular orientations, the model-aggregate reconfigured at a threshold of 0.25 m/s. We defined the threshold for reconfiguration as the tow speed at which the horizontal component of tangential drag equaled or exceeded the horizontal component of the normal drag. The drag-area results from this study can be incorporated into a dynamic fluid-structure interaction model representing kelp aggregates as a finite element beam prescribed with in-situ values of length, volume, mass density and flexural-rigidity of kelp material.

Carbon export is facilitated by sea urchins transforming kelp detritus.

With the increasing imperative for societies to act to curb climate change by increasing carbon stores and sinks, it has become critical to understand how organic carbon is produced, released, transformed, transported, and sequestered within and across ecosystems. In freshwater and open-ocean systems, shredders play a significant and well-known role in transforming and mobilizing carbon, but their role in the carbon cycle of coastal ecosystems is largely unknown. Marine plants such as kelps produce vast amounts of detritus, which can be captured and consumed by shedders as it traverses the seafloor. We measured capture and consumption rates of kelp detritus by sea urchins across four sampling periods and over a range of kelp detritus production rates and sea urchin densities, in northern Norway. When sea urchin densities exceeded 4m-2, the sea urchins captured and consumed a high percentage (ca. 80%) of kelp detritus on shallow reefs. We calculated that between 1.3 and 10.8kg of kelp m-2 are shredded annually from these reefs. We used a hydrodynamic dispersal model to show that transformation of kelp blades to sea urchin feces increased its export distance fourfold. Our findings show that sea urchins can accelerate and extend the export of carbon to neighboring areas. This collector-shredder pathway could represent a significant flow of small particulate carbon from kelp forests to deep-sea areas, where it can subsidize benthic communities or contribute to the global carbon sink.

Living on the Edge: Variation in the Abundance and Demography of a Kelp Forest Epibiont

The ecology of an epibiont may depend not only on the dynamics of its biogenic habitat but also on microclimate variation generated within aggregations of its host, a process called physical ecosystem engineering. This study explored variation in the abundance and demography of Membranipora, a suspension-feeding bryozoan, within forests of giant kelp (Macrocystis pyrifera) off the coast of Santa Barbara, California, USA. First, we assessed differences in Membranipora abundance between the edge and interior of kelp forests. The occurrence of Membranipora on kelp blades and its percent cover on occupied blades were higher along forest edges than interiors. Second, we conducted observational studies and field experiments to understand spatial variation in substrate longevity, colony mortality, larval recruitment, and colony growth rates. A higher density of recruits and colonies occurred along forest edges than interiors, suggesting kelp acts like a sieve, whereby larvae settle to edge blades first. Moreover, growth rates along the edge were up to 45% higher than forest interiors. Reduced current speeds, combined with feeding by exterior colonies, may have lowered the uptake of suspended food particles by interior colonies. These results suggest that variation in Membranipora abundance is due in part to differences in colony growth between forest edges and interiors, and not solely the result of recruitment limitation. Our results highlight the importance of ecosystem engineers in influencing the ecological dynamics of epiphytic flora and fauna in marine systems.

Variation of isotope composition in kelps from Kongsfjorden (Svalbard)

Kelp-derived detritus is a potential food item at the base level of marine food webs. A good knowledge of the factors that influence stable isotope composition in kelps is essential for reliable food web models. The variation of stable isotope signatures was analysed in three species of kelp in Kongsfjorden, Svalbard. Between 15 and 2.5 m depth, towards higher photon fluence rate, Alaria esculenta was gradually enriched in 13C while 15N did not change. The carbon/nitrogen ratio decreased with depth. 2- and 3-year-old kelp individuals had significantly higher δ15N values in their blades than 5- and 6-year-old ones. C/N ratio in blades remained statistically the same over the range of 2–6 years old plants. A two-way ANOVA did not show any interactive effects between depth and age group. A. esculenta as well as Saccharina latissima and Laminaria digitata exhibited enrichment in heavy carbon isotopes in the blades compared to the stipes, while within each species, δ15N values remained the same between blades and stipes. A more detailed analysis of kelp blades showed that independent of location in the fjord, year or season, and species, young tissue was more enriched in 13C than meristem and that decaying apical tissue was depleted relative to young tissue. The degree of this 13C depletion varied with kelp species. Meristem tissue appeared most uniform in the blades compared to young and decaying tissues and its δ13C signature was characteristic for each species with no significant difference in δ15N values. Decaying tissue showed no difference in δ13C between species. Carbon and nitrogen contents along the blades were species-specific and varied with growing location and year or season. Some advice is given on the temporal, spatial, and tissue-related choice of samples for different research objectives.