Nitrogenous nutrient uptake rates and primary production of phytoplankton in the Southern Bight of the North Sea during the spring of 1997
Gichuki, J.W. (1997). Nitrogenous nutrient uptake rates and primary production of phytoplankton in the Southern Bight of the North Sea during the spring of 1997. MSc Thesis. VUB: Brussel. 75 pp.
The uptake of nitrate, ammonium and urea was investigated experimentally during the spring of 1997 in the Southern Bight of the North Sea, with particular attention placed on the role of ammonium on the nitrogenous nutrient uptake regime. The phytoplankton assemblage was dominated by the colonial prymnesiophyte Phaeocystis pouchetii. Nitrate concentrations ranged from 16.87 µM representing 91% of the total dissolved N at the beginning of the spring bloom season and decreased steadily to almost below detection limits at 0.11 µM at the peak of the spring bloom in mid April before increasing to 1.83 µM in early May. Ammonium concentrations ranged from 0.97 µM at the beginning of the bloom to 6.24 µM at the wane of the bloom. Urea concentrations ranged from 0.9 µM at the start of the spring bloom and decreased rapidly to 0.18 µM in early April before increasing to 1.24 µM in early May. The C/N ratio was persistently higher than the Redfield ratio (6.6) over the whole sampling period. Elevated ammonium concentration inhibited total absolute N uptake, and the response appeared to be that of exponential decrease with increasing ammonium availability. When data was subjected to Spearman Rank correlation, the correlation between absolute nitrate uptake and ammonium concentration was highly significant at the level of 5 % (Spearman R = -0.59, p = 0.009). On average, absolute nitrate uptake rates decreased from 0.0537 ± 0.048 µM. h-1 in natural sea water to 0.021 ± 0.024 µM .h-1 in ammonium enriched sea water representing a mean decrease of 59.71% (range: 29.92 -89.50%). Absolute urea uptake rates decreased from 0.013 ± 0.007 µM h-1 in natural sea water to 0.0058 ± 0.005 µM h-1 in ammonium enriched sea water representing a mean decrease of 62.75% (range: 33.92 -91.58%). Total N uptake rates decreased from 0.054 ± 0.049 µM .h-1 in natural sea water to 0.029 ± 0.027µM. h-1 in ammonium enriched sea water representing a mean decrease of 46.38% (range: 14.08 -70.93%). Conversely enrichment with ammonium on natural sea water enhanced absolute ammonium uptake rates by 50% (range:13.83-86.17%) from 0.02 ± 0.024 µM h-1 in natural sea water to 0.033 ± 0.018 µM h-1 in ammonium enriched sea water. On average, PON increased from 8.96 ± 3.6 to 9.78 ± 4.1 µM in natural sea water corresponding to an increase of 9% and to 9.14 ± 4.1 µM in ammonium enriched sea water after 24 hours representing 2% increase. POC increased from 83.08 ± 45.8 µM to 99.25 ± 56 µM in natural sea water corresponding to an increase of 19.5% and to 85.62 ± 43.7 µM in ammonium enriched sea water representing an increase of 3%. The differences between increase of POC and PON in natural and enriched sea water were not statistically significant at the level of 5% (t-test, p = 0.69 and 0.54 for PON and POC respectively) Overall, these results suggest that elevated ammonium concentration has a leading role in regulating the removal of nitrate and urea and consequently, a reducing effect on the primary production in this margin ecosystem.
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