HR: 09:00h
AN: B51D-05    [Abstracts]
TI: Effects of Deepwater Algae on C-N-P Cycling in Permeable Sediments
AU: * Sansone, F J
EM: sansone@hawaii.edu
AF: Department of Oceanography, 1000 Pope Rd, University of Hawaii, Honolulu, HI 96822,
AU: Spalding, H L
EM: hspaldin@hawaii.edu
AF: Department of Botany, 3190 Maile Way, University of Hawaii, Honolulu, HI 96822,
AU: Smith, C M
EM: celia@hawaii.edu
AF: Department of Botany, 3190 Maile Way, University of Hawaii, Honolulu, HI 96822,
AB: Extensive deepwater (50-100 m depth) meadows of native and introduced macroalgae occur offshore of the main Hawaiian Islands. Sediment porewater (to 25 cm sediment depth) and overlying seawater were collected during September and December 2004, and November and December 2006 from 13 vegetated and non-vegetated deepwater sites on sand and muddy-sand seafloors offshore from Oahu to Kaho'olawe. Porewater dissolved nutrient concentrations were comparable to those in nearshore permeable (sandy) sediments, with highly elevated concentrations at sediment depths available to algal holdfasts. Maximum observed concentrations were: 2.3 uM phosphate, 30 uM nitrate, 0.65 uM nitrite, 105 uM ammonium, and 88 uM silica. The sediment organic matter content was not correlated with the presence or absence of macroalgae. Vegetated sediments were consistently more oxidizing than non-vegetated sediments, judged from (nitrate + nitrite)/ammonium ratios. Halimeda-vegetated sediments had low DIN levels compared to other sites, suggesting algal uptake. These sands also exhibited dissolved inorganic carbon depletion at depth, perhaps also related to algal metabolism. In contrast, Udotea-vegetated sediments had high levels of DIN, indicating little or no algal DIN uptake and/or the presence of active N-fixation; these sands also had no DIC depletion at depth. Udotea-vegetated sediments also appeared highly oxidizing, evidenced by very high levels of dissolved nitrate and nitrite, along with low levels of ammonium. The calculated tracer N* was used to verify the conclusions regarding net DIN uptake; a similar calculated tracer, computed using Redfield-type N:Si ratios, was used to confirm the N* results. Unvegetated, nutrient- and organic-rich sandy mud along the flanks of Haleakala volcano showed near complete dissolved phosphate removal, perhaps due to adsorption onto freshly weathered iron oxy-hydroxides. Such phosphate removal was not seen at other sites, including a nearby non-vegetated non-muddy sand with elevated nutrient levels. Our results suggest that the presence of deepwater macroalgae may have significant effects on the geochemistry of permeable (sandy) sediments, in contrast to the widely held belief that macroalgae do not interact chemically with the sediment they are associated with.
DE: 0428 Carbon cycling (4806)
DE: 0456 Life in extreme environments
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
DE: 1051 Sedimentary geochemistry
DE: 4219 Continental shelf and slope processes (3002)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting