The influence of changes in nitrogen: silicon ratios on diatom growth dynamics

Gilpin, L.C.; Davidson, K.; Roberts, E.

Abstract

Nitrate loading to coastal waters has increased over recent decades while silicon loading has remained relatively constant or decreased. As the N:Si ratio in coastal waters shifts due to these anthropogenic influences, silicate limitation of diatom biomass may become a feature of the biogeochemistry in coastal waters especially in regions of reduced exchange. Two sets of nutrient enrichment mesocosm experiments were conducted in successive years using a natural planktonic assemblage obtained from the Trondheimsfjord, Norway. The inorganic nutrient concentrations at the start of the experiments were manipulated to give a variety of N:Si concentrations at ratios representative of current and possible future values, should N loading continue. In June 1999 experiments were conducted with a gradient of inorganic N:Si ratios (1:2, 1:1, 2:1, 4:1) to investigate the influence of low and high N:Si ratio conditions and to determine the conditions that would generate Si limitation of diatom growth. In June 2000, based on 1999 data, highly replicated experiments were conducted at N:Si ratios of 1:1 and 4:1 which were expected to result in N and Si limitation of diatom growth, respectively; statistical differences in cellular composition were recorded. N limitation of diatom biomass increase was observed under the three lowest N:Si ratios: particulate carbon (C) accumulation continued to occur following N exhaustion resulting in an increase in the organic C:N ratio. Silicate limitation of diatom biomass increase only occurred at the highest N:Si ratio of 4:1. Silicate exhaustion was followed by continued nitrate uptake for several days, at a slower rate than previously. The resulting increase in organic N was accompanied by an increase in organic C such that the C:N ratio of the organic material at the highest N:Si ratio failed to increase to the extent observed under the N limited conditions. Statistically significant differences in chlorophyll-a yield per unit nitrate, C:chlorophyll-a ratios, C:N ratio and diatom cell yield per unit nitrate or Si were observed in Si compared to N limited conditions. All mesocosms became dominated numerically and in terms of biomass by the diatom Skeletonema costatum. The potential implications of changing N and Si regimes in coastal waters are discussed.

• Journal Article Type: Article
• Acceptance Date: May 20, 2003
• Online Publication Date: Jan 21, 2004
• Publication Date: 2004-02
• Deposit Date: Jul 4, 2008
• Journal: Journal of Sea Research
• Print ISSN: 1385-1101
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 51
• Issue: 1
• Pages: 21-35
• DOI: https://doi.org/10.1016/j.seares.2003.05.005
• Keywords: Nitrate:silicon ratios; N:Si; Diatoms; C:N; Nutrient limitation
• Public URL: http://researchrepository.napier.ac.uk/id/eprint/1686
• Publisher URL: http://dx.doi.org/10.1016/j.seares.2003.05.005