Post-glacial redistribution and shifts in productivity of giant kelp forests

Quaternary glacial–interglacial cycles create lasting biogeographic, demographic and genetic effects on ecosystems, yet the ecological effects of ice ages on benthic marine communities are unknown. We analysed long-term datasets to develop a niche-based model of southern Californian giant kelp (Macrocystis pyrifera) forest distribution as a function of oceanography and geomorphology, and synthesized palaeo-oceanographic records to show that late Quaternary climate change probably drove high millennial variability in the distribution and productivity of this foundation species. Our predictions suggest that kelp forest biomass increased up to threefold from the glacial maximum to the mid-Holocene, then rapidly declined by 40–70 per cent to present levels. The peak in kelp forest productivity would have coincided with the earliest coastal archaeological sites in the New World. Similar late Quaternary changes in kelp forest distribution and productivity probably occurred in coastal upwelling systems along active continental margins worldwide, which would have resulted in complex shifts in the relative productivity of terrestrial and marine components of coastal ecosystems.     

AN EXPERIMENTAL ASSESSMENT OF THE EFFECTS OF NUTRIENT ENHANCEMENT ON THE INTERTIDAL KELP HEDOPHYLLUM SESSILE (LAMINARIALES,PHAEOPHYCEAE)1

We tested whether experimentally enhancing nutrients around the kelp Hedophyllum sessile would increase growth, tissue nitrogen, or allocation to phenolic compounds. Packets of time‐released fertilizer were anchored adjacent to fronds in the field, and algae were monitored for several months. Although fertilizer packets increased the concentration of ammonium, nitrate, and phosphorus adjacent to treatment algae by an order of magnitude, there was little evidence that this increased frond growth or size. Hedophyllum individuals showed no tendency to alter allocation patterns in response to nutrient addition. Tissue carbon and nitrogen was unchanged by the nutrient manipulation; most H. sessile had tissue nitrogen concentrations in excess of 2.0% of dry mass. Additionally, the concentration of phloroglucinol equivalents was also unaffected by the presence of increased water column nutrients. Although nutrient concentrations in the water column surrounding the study site show relatively high mean values for ammonium, nitrate, and phosphorus, they are characterized by high spatial and temporal variation. Nonetheless, these data suggest that this intertidal kelp is not limited by nitrogen or phosphorus in wave‐exposed areas in the northeast Pacific Ocean.     

VARIATION IN NITROGEN PHYSIOLOGY AND GROWTH AMONG GEOGRAPHICALLY ISOLATED POPULATIONS OF THE GIANT KELP,MACROCYSTIS PYRIFERA (PHAEOPHYTA)1

Three geographically isolated populations of the giant kelp, Macrocystis pyrifera (L.) C. Ag., were examined for responses to nitrate availability in batch culture experiments using juvenile sporophytes reared from spores in the laboratory. Although maximum rates of nitrate-saturated growth were similar among groups, there were significant quantitative differences in the response to nitrate limitation that can be related to natural patterns of nutrient availability at these sites. Plants from Santa Catalina Island (most oligotrophic) achieved maximum growth rates at ambient nitrate concentrations that were lower than those for plants from Monterey Bay, California (most eutrophic), or Refugio State Beach (near Santa Barbara, California). Tissue nitrogen and amino acid concentrations were highest in plants cultured from Santa Catalina Island populations at all external nitrate concentrations, suggesting that differences in nitrate requirements for growth may reflect the efficiency of nitrate uptake and assimilation at subsaturating nitrate concentrations. Given the different physical environments from which these plants came, the data suggest that geographically isolated populations of M. pyrifera have undergone genetic divergence that can be explained by ecotypic adaptation to unique habitat conditions at these sites.     

Evaluation of remote sensing techniques for monitoring giant kelp populations

Photographs and maps of the floating canopy of the giant kelp, Macrocystis pyrifera, provide an important data source to monitor nearshore water quality in southern California. Declines in water quality related to turbidity from coastal development, ocean discharges, and non-point source runoff have caused reductions in the areal extent of these kelp beds. Historically the kelp beds have been monitored by a variety of methods including small format infrared and color photography. New digital remote sensing instruments combined with geographical information system (GIS) databases offer an efficient method for collecting and analyzing data on changes in kelp bed size and location. SPOT satellite imagery has been found to provide adequate resolution for mapping the larger beds of giant kelp along the California coast. Beds smaller than 10 ha are not resolved well with SPOT imagery and need to be mapped with a resolution greater than the 20 m pixel size provided by the SPOT multispectral imagery. Imagery from a prototype of the Positive Systems ADAR system, an airplane mounted multispectral video sensor, provided a spatial resolution of 2.3 m in 4 spectral bands. ADAR imagery taken on 2 October 1991 of the San Onofre Kelp Bed in northern San Diego County showed 39% more kelp than small format color infrared photography made during the same time period.     

Commercial potential of seaweeds from St Lawrence Island,Alaska

This study assessed the possibility of using drift and subtidal seaweeds from St Lawrence Island, Alaska (lat. 63°N) for sale by the native population after simple processing. Over 125 km of coastline were surveyed for distribution of both drift and subtidal seaweeds. Drift seaweed wet weight densities ranged from 0.2 to over 9 kg m⁻², with an average of over 4 t km⁻¹ in the areas sampled. Attached, benthic seaweed densities ranged from 0.15 to 0.32 kg m⁻². Thirty and 35% of the biomass was composed of Agarum cribrosum and species of Laminaria, respectively, both as drift and as benthic seaweed. Data from tagged Laminaria indicated that growth rates were relatively slow for most of the year. The drift seaweed resource on the coasts south and west of the city of Gambell appeared to have good potential for a small-scale commercial harvest. (*author for correspondence)     

Laminaria culture for reduction of dissolved inorganic nitrogen in salmon farm effluent

Finfish culture is a growing industry, and it causes a nutrient loading problem. To investigate the feasibility of an integrated culture of kelp and salmon, 15-cm long kelp (Laminaria saccharina) was grown in salmon culture effluent. The objectives were to test the effects of flow rate and kelp density on dissolved inorganic nitrogen removal (DIN), and DIN uptake and growth by the kelp. NH ₄ ⁺ , NO ₃ ^– and DIN (NH ₄ ⁺ + NO ₃ ^– ) loadings were in the ranges 6.2–25.4, 12.9–40.0, 19.7–52.7 mol 1^–1, respectively, over the experimental period.Surplus uptake of nitrogen was not evident, because the C:N ratio (10–11) was constant in all experiments. During light periods, the kelp removed from 170–339 mol 1^–1 h^–1, and approximately 26–40% of the incoming DIN. The DIN uptake rate, based on daylight sampling periods, ranged between 6.1–22.5 mol g^–1 dry mass h^–1. The highest-flow rate, lowest-density tank had the highest DIN uptake rate. Debris from the fish effluent settling on the kelp thalli in the low-flow rate tanks affected uptake. Mean DIN uptake rate based on 3 days of growth for all flow-density combinations ranged between 5.4–8.3 mol g^–1 dry mass h^–1. The kelp utilized NH ₄ ⁺ and NO ₃ ^– equally.The growth ranged between 6.5–9% d^–1. The biomass production ranged from 1–2 g per sampling period. The highest growth rate and biomass production were achieved by kelp in the highest-flow rate, lowest-density tank. Lower DIN concentrations due to higher DIN removal rates in the other tanks and light limitation due to self-shading in the high-density tanks were probably responsible for the reduced growth rate in these tanks.Author for correspondence     

Seaweed biogeochemistry: Global assessment of C:N and C:P ratios and implications for ocean afforestation

Algal carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P) ratios are fundamental for understanding many oceanic biogeochemical processes, such as nutrient flux and climate regulation. We synthesized literature data (444 species, >400 locations) and collected original samples from Tasmania, Australia (51 species, 10 locations) to update the global ratios of seaweed carbon-to-nitrogen (C:N) and carbon-to-phosphorus (C:P). The updated global mean molar ratio for seaweed C:N is 20 (ranging from 6 to 123) and for C:P is 801 (ranging from 76 to 4102). The C:N and C:P ratios were significantly influenced by seawater inorganic nutrient concentrations and seasonality. Additionally, C:N ratios varied by phyla. Brown seaweeds (Ochrophyta, Phaeophyceae) had the highest mean C:N of 27.5 (range: 7.6–122.5), followed by green seaweeds (Chlorophyta) of 17.8 (6.2–54.3) and red seaweeds (Rhodophyta) of 14.8 (5.6–77.6). We used the updated C:N and C:P values to compare seaweed tissue stoichiometry with the most recently reported values for plankton community stoichiometry. Our results show that seaweeds have on average 2.8 and 4.0 times higher C:N and C:P than phytoplankton, indicating seaweeds can assimilate more carbon in their biomass for a given amount of nutrient resource. The stoichiometric comparison presented herein is central to the discourse on ocean afforestation (the deliberate replacement of phytoplankton with seaweeds to enhance the ocean biological carbon sink) by contributing to the understanding of the impact of nutrient reallocation from phytoplankton to seaweeds under large-scale seaweed cultivation.     

NEUTRAL LIPIDS AS MAJOR STORAGE PRODUCTS IN ZOOSPORES OF THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Neutral lipids, consisting primarily of triacylglycerols, were found to be a major form of carbon reserve in zoospores of the giant kelp Macrocystis pyrifera (L.) C. Ag. The fluorescent stain Nile Red revealed large lipid droplets in the posterior end of the cell, which comprised 20–41% of cellular carbon in newly released spores. Flow cytometric analyses of newly released spores stained with Nile Red revealed considerable variation in the neutral lipid content among spores that was independent of spore size. Lipid droplets were consumed during germination in spores maintained either under constant light or in continual darkness. The availability of light appeared to delay, but did not preclude, lipid use. The rate of lipid use during germination varied considerably among germlings with some cells consuming all of their lipid reserves within 5 h after release. In addition to zoospores, lipid droplets were observed in both male and female gametes. Numerous droplets were observed in eggs, while single lipid droplets were observed in sperm. Neutral lipid droplets were not observed in gametophytes or sporophytes except in developing gametes and spores. Large lipid reserves thus seem to be confined to the microscopic life history stages that presumably have relatively high energy demands. By serving as a supplemental fuel reserve, neutral lipids may be important in extending the effective range of zoospore dispersal.     

AUXIN,AN ENDOGENOUS REGULATOR OF GROWTH IN ALGAE?1,2

The effects of 5 pairs of synthetic auxins and chemically related, but biologically inactive, derivatives (e.g., 2,4- and 3,5-dichlorophenoxyacetic acid, respectively) on blade growth of the brown alga, Alaria esculenta (L.) Grev.(Laminariales) were studied. The significant result was the inability to divide the growth responses into auxin and non-auxin groups. At high concentrations (10^?3 M) 9 test substances inhibited blade growth; lower concentrations (<10^?3 M) were either slightly inhibitory or showed no effect. The data suggest that a higher plant auxin-specific recognition mechanism is lacking in this alga. As this mechanism is central to higher plant concepts of growth control by auxin, the role of auxin as an endogenous regulator of growth in Alaria, and perhaps other algae as well, seems unlikely. This interpretation is consistent with recent negative biochemical data from auxin binding studies using subcellular preparations from algae.     

Isolated, distal blade discs of the brown alga Laminaria digitata form sorus, but not discs, near to the meristematic transition zone

Blade discs of vegetative thalli of Laminaria digitata (Huds.) Lamour. from Helgoland (North Sea) cut at 5–15 cm distance from the blade/stipe transition, formed sorus in the laboratory after 7–12weeks, 5 months earlier than whole fronds in the field. Sorus formation occurred in a broad range of daylength regimes or temperatures, at 8–16 h light pe rday and 6–12 ^°C. No sorus was developed during three months by meristematic blade discs cut from the lowermost 3 cm portion of the blade, nor from whole thalli cultured in parallel to isolated blade discs. These findings point to the possible existence of sporulation inhibitors produced by the laminarian meristem. This revised version was published online in August 2006 with corrections to the Cover Date.