Biological Response to Ocean Macronutrient Fertilisation

Abstract

The aim of this study was to examine biological response to ocean macronutrient fertilisation on the oligotrophic waters offshore Sydney, Australia. This research investigates nutrient uptake, phytoplankton biomass growth, microzooplankton grazing and zooplankton and bacteria changes. After inoculating the sample with macronutrients there was a period of latency followed by rapid growth of phytoplankton until a maximum concentration was reached and subsequently decreased to a value similar to the initial concentration. Once the maximum concentration of phytoplankton decreased to a value similar to the initial concentration, another fertilisation (re-fertilisation) was conducted using the same seawater samples and the same concentration of macronutrients N & P. Results indicate that macronutrients N & P were initially limiting phytoplankton growth. The phytoplankton concentration maximum varied from 4 µg/l to 20 µg/l when add to an initial concentration of nutrients of 16 µmol N and 1 µmol P, while microzooplankton grazing varied from 30% to 77% of the total growth of phytoplankton biomass. It was observed that high phytoplankton concentration (20 µg/l) occurred where there was low grazing pressure (33 %) whereas low phytoplankton concentration (5.5 µg/l) occurred where there was high grazing pressure (71%). This suggests that phytoplankton concentration maximum was limited by microzooplankton grazing pressure rather than only macronutrient availability. The initial predominant zooplankton taxon group found in the samples are copepod cyclopod and calanoid. The species distribution was weakly affected by fertilisation and re-fertilisation. DNA sequencing results show that the initial predominant marine bacteria found in the samples are prochlorococcus and the total number of bacteria has doubled after fertilisation but remained similar after re-fertilisation. No remineralisation of particulate matter was observed after the macronutrients were exhausted.