Abstract
An experiment with eight vertically stratified seawater enclosures of 27 m 3 (depth 9.3 m, diameter 2 m, 90% penetration of PAR) was run in order to test whether pulsed addition of nutrients may cause: 1, higher primary production; 2, higher build-up of phytoplankton biomass; 3, larger temporal mismatch between herbivores and phytoplankton biomass; and 4, higher sedimentation rates, distinguishing in each case between silicate and non-silicate fertilised systems. Nitrate and phosphate were added to all enclosures (NP), while silicate was added to four of the enclosures (NPS). Each enclosure received the same total amount of nutrients, but the nutrients were supplied at four different intervals ranging from one single load to continuous additions. Spring bloom-like systems developed where nutrients were added in one or two pulses as they were characterised by high primary production, high suspended biomass of chlorophyll a (Chl a) and particulate organic carbon (POC) and high sedimentation rates. In contrast, the seawater enclosures receiving nutrients about every third day or in a continuous supply resembled regenerated systems with low concentrations of suspended Chl a and POC and with low and stable loss rates. Due to a typical autumn inoculum with dominance of dinoflagellates and flagellates, diatoms did not dominate the NPS enclosures. The only significant effect of the silicate addition was higher vertical flux of particulate organic nitrogen in the NPS enclosures, and higher microzooplankton biomass. The mesozooplankton did not show responses to the different frequencies of nutrient additions. However, accumulation of mesozooplankton biomass was higher in the NP-mesocosms, probably reflecting better feeding conditions. We conclude that the frequency of nutrient additions had a stronger influence on the development of the phytoplankton and vertical flux of carbon than the +/- silicate treatment in this experiment.
Highlights
The relationship between pelagic productivity and sedimentation of organic matter is complex.Generally high sedimentation rates have been associated with areas or periods with high new production, like spring blooms and upwelling systems (Peinert et al, 1989). Wassmann (1990) suggested a positive, curvilinear relationship between total primary production and sedimentation based on the knowledge that new production systems are usually dominated by autotrophs and simple food web interactions
High sedimentation rates have been associated with areas or periods with high new production, like spring blooms and upwelling systems (Peinert et al, 1989)
The effect of nutrient pulsing on the vertical flux of biogenic matter has hardly been studied
Summary
The relationship between pelagic productivity and sedimentation of organic matter is complex.Generally high sedimentation rates have been associated with areas or periods with high new production, like spring blooms and upwelling systems (Peinert et al, 1989). Wassmann (1990) suggested a positive, curvilinear relationship between total primary production and sedimentation based on the knowledge that new production systems are usually dominated by autotrophs and simple food web interactions. High sedimentation rates have been associated with areas or periods with high new production, like spring blooms and upwelling systems (Peinert et al, 1989). The export potential of the bloom and the pelagic-benthic coupling over lengthy periods of time is determined by the availability of allochthonous, inorganic nutrients. Are the amount and ratio of macronutrients important for the coupling between primary production and export ratio, and the time span(s) through which new nutrients are available (constant or at regular/irregular intervals) may be important. The effect of nutrient pulsing on the vertical flux of biogenic matter has hardly been studied. Most of the experiments including nutrient pulsing are conducted within a single trophic level (phytoplankton), and vertical flux is only rarely investigated experimentally (but see Heiskanen et al, 1996; Svensen et al, 2001)
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