Nutrient Availability to Marine Macroalgae in Siliciclastic versus Carbonate-Rich Coastal Waters

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Abstract
Abundant populations of frondose epilithic macroalgae from a variety of carbonate-rich tropical waters were significantly depleted in phosphorus relative to carbon and nitrogen when compared to macroalgae from temperate siliciclastic waters. Percent carbon (C) and percent nitrogen (N) dry weight contents were similar between tissues from the siliciclastic and carbonate environments (means of 22.6% vs. 20.1% and 1.0% vs. 1.2%, respectively), but phosphorus (P) levels were two-fold lower (0.15% vs. 0.07%) in the carbonate-rich systems. Accordingly, the molar C:N tissue ratios were comparable between macroalgae from the siliciclastic and carbonate sites (mean of 29.2 vs. 23.1), whereas large differences were observed for the C:P (mean of 430 vs. 976) and N:P ratios (mean of 14.9 vs. 43.4). In addition, alkaline phosphatase activity was low and often undetectable in the macroalgae from siliciclastic habitats (mean of 7.3 μM PO4 3− released g dry wt−1 h−1) compared to seven-fold higher rates (52.5 μM PO4 3− released g dry wt−1 h−1) observed in the macroalgae from carbonate systems. Seawater samples taken adjacent to benthic macroalgae from the carbonate-rich tropical waters contained relatively high levels of dissolved inorganic nitrogen with low concentrations of soluble reactive phosphorus (SRP), and showed elevated N:SRP ratios (mean=36) compared to siliciclastic environments (mean <3). These data support the precept that availability of N limits the productivity of macroalgae in temperate siliciclastic waters but, conversely, suggest that availability of P, rather than N, may be of paramount importance in limiting primary production of macroalgae in carbonate-rich tropical waters.

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