No abstract

The ability to sustain growth at low availability of nltrogen (N) was examined in 6 species of macroalgae with different growth strategies by compdrlng substrate dependent growth kinetics. The N rcqulred to support optimal growth and the N uptake kinetics of 2 slow-growing algae. Fucus r~esicu-losus and Codium fragilc, and 4 fast-growing specles, Chnetolnorpha Ijn~~rn, Cladophora serica, Cerarn~um rubrum and Ulva lactuca, were experimentally determ~ned In summer when the algae were N limited. The N required to support maximum growth vaned 16-fold among specles, w~t h fdst-gro\v-ing algae having the highest N demands. The high N requirements of ephemeral algae were caused by u p to 13-fold h~g h e r growth ratps and 2-to 3-fold higher N content at maximum growth. Also, the fastgrowing specles took up ammonium and nitrate 4 to 6 times faster per u n~t of biomass than slo\v-growing speclcs at both low and high substrate concentrations, but the ratios of maxlinum Pi uptake to requirements were larger among the slow-growing algae. Thus, the fast-growing specles tended to requlre relatively higher external concrntrations of inorganic N to saturate the11 growth Under N limited conditions, all 6 macroalgae were able to explo~t pulses of high concentrations of ammonium by taking u p ammonium a t transiently enhanced rates (i.e. surge uptake). Uptake was, however, only marginally enhanced at low, and naturally occurring, concentrations of ammonium, suggesting that surge uptake is of minor ecological importance. Our results show that large, slow-growing macroalgae may be better able to meet their N requirements at low N availability than fast-growing species. This is consistent with the common observation that nutrient-poor coastal areas are dominated by slowgrowing macroalgae rather than ephemeral species, although ephemeral species have higher N uptake capacities.

The role of algal size as a controlling factor for nltrogen uptake kinetics is examined by comparing published values of N uptake rate and half-saturation constants in micro-and macroalgae. The uptake kinetics differ substantially among algae very different in size. Microalgae take up nitrogen much faster per unit of biomass than macroalgae at both high and low substrate concentrations, and microalgae have significantly higher affinity for nitrogen than macroalgae. These typical differences in the uptake kinetics among small and large algae are commonly attributed to size-specific differences in the relative surface area (SA:V). Regression analysis demonstrates that size-specific variations in the kinetic parameters can be attributed to changes in relative surface area over an extensive range of algal sizes, covering both micro-and macroalgae. These results agree with previously described relationships between maximum uptake rate of nutrients (nitrogen and phosphorus) and SA:V within narrow size-ranges (either phytoplankton or macroalgae), and emphasize the existence of a general coupling between physiological and morpholog~cal properties in algae. KEY WORDS: Nitrogen uptake Microalgae . Macroalgae . Allometry 0 Inter-Research 1995 Resale of full article not permitted

Attached Sargassum muticum (Yendo) Fensholt was first observed in the westernmost part of Limfjorden (Denmark) in 1984, and it has since colonized Limfjorden from west to east at a rate of 15 to 17 km yr -1. By 1997, S. muticum had become the most dominant macroalga, covering more than 6% of the bottom between 0 and 6 m depth, corresponding to ca 35% cover of the hard substrate. In 1990, the cover of S. muticum was strongly correlated with distance from the original source area in the westernmost part of the estuary, suggesting that colonization was still in progress. By 1997, the cover of S. muticum was less correlated with distance from the original source area and more strongly correlated with the amount of hard substrate, indicating that colonization was reaching its climax. The absence of S. muticum from certain parts of Limfjorden by 1997 is therefore best explained by lack of hard substrate in these areas, and not by insufficient colonization time. The increased abundance of S. muticum between 1990 and 1997 affected species richness and diversity of the macroalgal community only marginally. However, multivariate community analysis revealed significant changes in the macroalgal community structure that were closely related to the increased abundance of S. muticum. Not only did the dominance of S. muticum increase significantly from 1990 to 1997, but the cover of several indigenous species belonging to the genera Laminaria, Fucus, and Codium decreased during the same period, indicating competitive displacement through competition for hard substrate and light.KEY WORDS: Sargassum muticum · Macroalgae · Invasion · Macroalgal community structure Resale or republication not permitted without written consent of the publisher

Changes in the size of intracellular nitrogen pools and the potential feedback by these pools on maximum N uptake (NH4+ and NO3−) rates were determined for Chaetomorpha linum (Müller) Kützing grown sequentially under nutrient‐saturating and nutrient‐limiting conditions. The size of individual pools in N‐sufficient algae could be ranked as residual organic N (RON) comprised mainly of amino acids and amino compounds > protein N > NO3− > NH4+ > chlorophyll N. When the external N supply was removed, growth rates remained high and individual N pools were depleted at exponential rates that reflected both dilution of existing pools by the addition of new biomass from growth and movement between the pools. Calculated fluxes between the tissue N pools showed that the protein pool increased throughout the N depletion period and thus did not serve a storage function. RON was the largest storage reserve; nitrate was the second largest, but more temporary, storage pool that was depleted within 10 days. Upon N resupply, the RON pool increased 3 × faster than either the inorganic or protein pools, suggesting that protein synthesis was the rate‐limiting step in N assimilation and caused a buildup of intermediate storage compounds. Maximum uptake rates for both NH4+ and NO3− varied inversely with macroalgal N status and appeared to be controlled by changes in small intracellular N pools. Uptake of NO3− showed an initial lag phase, but the initial uptake of NH4+ was enhanced and was present only when the intracellular NH4+ pool was depleted in the absence of an external N supply. A strong negative correlation between the RON pool size and maximum assimilation uptake rates for both NH4+ and NO3− suggested a feedback control on assimilation uptake by the buildup and depletion of organic compounds. Enhanced uptake and the accumulation of N as simple organic compounds or nitrate both provide a temporary mechanism to buffer against the asynchrony of N supply and demand in C. linum.