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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.

Identiferoai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45479
Date January 1985
CreatorsFujita, Rodney Masanori
PublisherBoston University
Source SetsBoston University
Languageen_US
Detected LanguageEnglish
TypeThesis/Dissertation
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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