Nitrogen uptake and regeneration pathways in the equatorial Pacific: a basin scale modeling study

It is well known that most primary production is fueled by regenerated nitrogen in the open ocean. Therefore, studying the nitrogen cycle by focusing on uptake and regeneration pathways would advance our understanding of nitrogen dynamics in the marine ecosystem. Here, we carry out a basin-scale modeling study, by assessing model simulations of nitrate and ammonium, and rates of nitrate uptake, ammonium uptake and regeneration in the equatorial Pacific. Model-data comparisons show that the model is able to reproduce many observed features of nitrate, ammonium, such as the deep ammonium maximum (DAM). The model also reproduces the observed de-coupling of ammonium uptake and regeneration, i.e., regeneration rate greater than uptake rate in the lower euphotic zone. The de-coupling largely explains the observed DAM in the equatorial Pacific Ocean. Our study indicates that zooplankton excretion and remineralization of organic nitrogen play a different role in nitrogen regeneration. Rates of zooplankton excretion vary from <0.01 mmol m−3 d−1 to 0.1 mmol m−3 d−1 in the upper euphotic zone while rates of remineralization fall within a narrow range (0.015–0.025 mmol m−3 d−1 . Zooplankton excretion contributes up to 70% of total ammonium regeneration in the euphotic zone, and is largely responsible for the spatial variability of nitrogen regeneration. However, remineralization provides a steady supply of ammonium in the upper ocean, and is a major source of inorganic nitrogen for the oligotrophic regions. Overall, ammonium generation and removal are approximately balanced over the top 150 m in the equatorial Pacific.

Nitrogen uptake and regeneration pathways in the equatorial Pacific: a basin scale modeling study

It is well known that most primary production is fueled by regenerated nitrogen in the open ocean. Therefore, studying the nitrogen cycle by focusing on uptake and regeneration pathways would advance our understanding of nitrogen dynamics in the marine ecosystem. Here, we carry out a basin-scale modeling study, by assessing model simulations of nitrate and ammonium, and rates of nitrate uptake, ammonium uptake and regeneration in the equatorial Pacific. Model-data comparisons show that the model is able to reproduce many observed features of nitrate, ammonium, such as the deep ammonium maximum (DAM). The model also reproduces the observed de-coupling of ammonium uptake and regeneration, i.e. regeneration rate greater than uptake rate in the lower euphotic zone. The de-coupling largely explains the observed DAM in the equatorial Pacific Ocean. Our study indicates that zooplankton excretion and remineralization of organic nitrogen play a different role in nitrogen regeneration. Rates of zooplankton excretion vary from &amp;lt0.01 mmol m−3 d−1 to 0.1 mmol m−3 d−1 in the upper euphotic zone while rates of remineralization fall within a narrow range (0.015–0.025 mmol m−3 d−1). Zooplankton excretion contributes up to 70% of total ammonium regeneration in the euphotic zone, and is largely responsible for the spatial variability of nitrogen regeneration. However, remineralization provides a steady supply of ammonium in the upper ocean, and is a major source of inorganic nitrogen for the oligotrophic regions. Overall, ammonium generation and removal are approximately balanced over the top 150 m in the equatorial Pacific.

Nitrogen Transformations in Lake Ontario

Concentrations of ammonium plus nitrite in Lake Ontario were highly correlated with ammonium regeneration from zooplankton excretion (r = 0.966), inferring that elevated nitrite concentrations result from nitrification. Nitrapyrin-sensitive dark 14C-labeled bicarbonate assays confirmed high rates of nitrification by chemoautotrophic bacteria. 15N-labeled nitrate experiments showed that nitrate, not ammonium, was the principal form of N used for total microbial protein synthesis. Size fractionation experiments also suggested that small cells were responsible for most of the ammonium uptake, while large cells used mostly nitrate. Nitrate depletion in the surface waters during summer stratification resulted from movement to particulate N, nitrite, and ammonium as well as losses in particulate N due to sedimentation. At least one third, however, was unaccounted for (i.e. 30 mg N∙m−2∙d−1) and may have been converted to protein which would move up the food chain to larger organisms (e.g. fish) not sampled during conventional water chemistry. Nitrous oxide profiles showed that nitrate losses through denitrification are unlikely to occur. Consequently, unless nitrate loading to Lake Ontario is reduced, nitrate concentrations should be expected to continue to increase.

Ammonia and phosphate excretion by zooplankton from the inshore waters of the Great Barrier Reef. II. Their in situ contributions to nutrient regeneration

Zooplankton excretion was estimated by combining biomass data with experimental laboratory data on excretion rates at three stations, characterized by low nutrient levels, in the inshore waters of the Great Barrier Reef. Hourly ammonia excretion (as nitrogen) by net zooplankton (>205 �) was calculated to range from 11.9 to 22.6 � m-3 or from 1.6 to 5.0 g m-2 per year, and hourly phosphate excretion (as phosphorus) from 1.4 to 2.8 � m-3 or from 0.16 to 0.63 g m-2 per year. Hourly excretion of ammonia by microzooplankton ( < 205 �) was calculated to be 2.6 � m-3 (as nitrogen) and of phosphate 0.55 � m-3 (as phosphorus), values that were 15 and 27% of the excretion by net zooplankton, respectively. Combined excretion rates by net zooplankton and microzooplankton could supply only 9.0-29.2% of the nitrogen and 6.6-25.6% of the phosphorus for an assumed yearly primary production of carbon of 100 g m-2 (= 17.6 g m-2 of nitrogen and 2.5 g m-2 of phosphorus). Calculations from an empirical equation relating temperature to oxygen consumption by a bottom community indicated a high potential for benthic nutrient regeneration in reef inshore waters (27.1 g m-2 per year, as nitrogen). The bottom community therefore appears to be the most important source of nutrient regeneration within the area studied.

Nutrient stimulation of carbon fixation in summertime English Channel phytoplankton assemblages

The effects of nutrient additions and zooplankton excretion products upon carbon fixation rates in the phytoplankton present at Station L 4 in the English Channel during the summer and autumn of 1979 have been studied. Nitrate, ammonium, urea, phosphate, glucose-6-phosphate and the excretion products when added individually all caused photosynthesis to be stimulated, and the result of the simultaneous addition of nitrate and phosphate indicated that their effects were additive. Germanic acid, which inhibits photosynthesis mainly in diatoms, removed the stimulatory effect of the nitrogen supplements, indicating that they were utilized mostly by the diatoms; the higher fixation rates caused by the phosphate enrichments were, however, decreased by the same proportion as the unenriched controls when germanic acid was present, suggesting that the whole of the phytoplankton population was phosphorus-limited. This was supported by the finding that glucose-6-phosphate stimulated carbon fixation in all of the phytoplankton.The excretion products, even at concentrations likely to be produced in the sea, stimulated carbon fixation, and it has been calculated that zooplankton-regenerated nitrogen and phosphorus compounds could supply the amounts needed to maintain primary production during the summer period.Nutrient additions and zooplankton excretion products had little effect upon carbon fixation in the autumn samples, presumably because the higher nutrient levels then present in the water exceeded the requirements of the phytoplankton.It has been concluded that the predominance of the sub-10 μ microflagellates in the summertime is probably due to their ability to utilize more efficiently than the other types of phytoplankton the low levels of nutrients which become available due to regeneration.