Abstract
Abstract. The effects of inorganic and/or organic nutrient inputs on phytoplankton and heterotrophic bacteria have never been concurrently assessed in open ocean oligotrophic communities over a wide spatial gradient. We studied the effects of potentially limiting inorganic (nitrate, ammonium, phosphate, silica) and organic nutrient (glucose, aminoacids) inputs added separately as well as jointly, on microbial plankton biomass, community structure and metabolism in five microcosm experiments conducted along a latitudinal transect in the Atlantic Ocean (from 26° N to 29° S). Primary production rates increased up to 1.8-fold. Bacterial respiration and microbial community respiration increased up to 14.3 and 12.7-fold respectively. Bacterial production and bacterial growth efficiency increased up to 58.8-fold and 2.5-fold respectively. The largest increases were measured after mixed inorganic-organic nutrients additions. Changes in microbial plankton biomass were small as compared with those in metabolic rates. A north to south increase in the response of heterotrophic bacteria was observed, which could be related to a latitudinal gradient in phosphorus availability. Our results suggest that organic matter inputs will result in a predominantly heterotrophic versus autotrophic response and in increases in bacterial growth efficiency, particularly in the southern hemisphere. Subtle differences in the initial environmental and biological conditions are likely to result in differential microbial responses to inorganic and organic matter inputs.
Highlights
The nature of nutrient limitation of phytoplankton and bacterial production in open ocean waters is known to vary over spatial and temporal scales (Cullen et al, 1992; Arrigo 2005; Church 2008; Saito et al, 2008)
Our results suggest that organic matter inputs will result in a predominantly heterotrophic versus autotrophic response and in increases in bacterial growth efficiency, in the southern hemisphere
The depth of the nutricline, which is a proxy for nutrient supply into the euphotic layer, was significantly (GPower 3.1.0. correction was applied when necessary as explained in Material and Methods section) and negatively correlated with chlorophyll a concentration (r = −0.87, p = 0.06; n = 5), primary production
Summary
The nature of nutrient limitation of phytoplankton and bacterial production in open ocean waters is known to vary over spatial and temporal scales (Cullen et al, 1992; Arrigo 2005; Church 2008; Saito et al, 2008). Nitrogen is the proximal limiting nutrient of phytoplankton growth in the oligotrophic tropical and subtropical Atlantic over physiological and/or ecological time scales (Graziano et al, 1996; Mills et al, 2004, 2008; Moore et al, 2008), whereas P and Fe, as limiting nutrients for N2 fixation (Falkowski 1997; Tyrrell 1999), are responsible for N-limitation of primary production at geological time scales. It has been suggested that increasing atmospheric inputs together with enhanced nitrogen fixation rates may lead to phosphorus limitation in the tropical North Atlantic Ocean (Wu et al, 2000, Ammerman et al, 2003; Mather et al, 2008). Recent studies suggest that atmospheric water-soluble organic nitrogen entering central ocean regions accounts for up to 30% of the total atmospheric nitrogen inputs into these marine areas
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