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The role of nitrogen supply variability in regulating nitrogen uptake by macroalgae and in structuring a macroalgal community

Temporal variability in nutrient supply has not generally been considered to be a factor controlling macroalgal community structure. Frequent sampling in environments with multiple nitrogen (N) inputs reveals that N supply varies greatly on scales of hours, days and weeks. This thesis describes studies of the adaptations of macroalgae to variability in N supply and the effects of this variability on the species composition of a macroalgal community. A study was made of the efficacy of using N flux (water flow rate X N concentration) to predict the growth rate of Gracilaria tikvahiae (Rhodophyta). The relationship between growth rate and flow rate was affected by concentration and the relationship between growth rate and N concentration was affected by flow rate such that growth was a linear function of N flux. Thus, N flux proved to be a better predictor of growth rate than water flow rate or N concentration taken alone. Increased N flux during a period of high N concentration increased both the maximum photosynthetic rate and photosynthetic rates at low irradiances of G. tikvahiae and Enteromorpha clathrata. Storage of N taken up during N pulses (relatively short episodes of high N availability) as photosynthetic pigment protein complexes would thus allow macroalgae to fix carbon rapidly to supply carbohydrates necessary for growth during N-limitation and for rapid N uptake during the next N pulse. The effects of nutritional history on NH⁺₄ uptake by Ulva lactuca, Gracilaria tikvahiae and Enteromorpha spp. were investigated. Algae grown at low N fluxes or in N-free medium took up NH⁺₄ much more rapidly than algae grown at high N fluxes. Enteromorpha spp. took up NH⁺₄ more rapidly than the other species. Rapid uptake by algae immediately after collection from the field, after growth at low N fluxes or after growth in N-free medium suggests that N-limited or starved algae in the field can take up pulses of N rapidly (in excess of immediate requirements). G. tikvahiae, Enteromorpha spp., and filva lactuca grew in N-free medium at rates similar to those observed at the high and low N fluxes for 14, 10 and 9 days (respectively), indicating that N taken up during periods of high N availability can support growth in the absence of N. Experimental manipulation of the frequency with which N pulses occurred in the field showed that the frequency with which N pulses occur in nature may be important in regulating the species composition and productivity of macroalgal communities.
Date January 1985
CreatorsFujita, Rodney Masanori
PublisherBoston University
Source SetsBoston University
Detected LanguageEnglish
RightsThis work is being made available in OpenBU by permission of its author, and is available for research purposes only. All rights are reserved to the author.

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