High-nutrient Low-chlorophyll Region Research Articles

Biomass, growth rates and limitation of Equatorial Pacific diatoms

Biomass, growth and species composition of siliceous phytoplankton were studied in the Equatorial Pacific during October 1994. Experiments were carried out in different nutrient conditions along the Equator. An oligotrophic area, with nitrate concentrations as low as 10 nM in the upper layer, was encountered in the western part of the transect (166°E–170°W). The concentration of biogenic silica varied from 10 nmol 1 −1 in the surface layer up to 40 nmol 1 −1 in the deep chlorophyll maximum located near the nutracline. Biogenic silica production, measured by the 32Si method, showed a similar vertical pattern in the nitrate-depleted water, and the mean assimilation rate for Si was 0.4±0.2 nmol l −1 h −1 (integrated mean value: 63 μmol m −2 h −1). In contrast, nitrate concentration ranged from 2 to 4 μM in the surface layer in the high-nutrient low-chlorophyll (HNLC) area located from 170 to 150°W and biogenic silica increased to 200 nmol 1 −1. The Si assimilation rate was 1.7±1.0 nmol 1 −1 h −1 (integrated mean value: 162 μmol m-Z h −1). In both areas, 80% of Si biomass was concentrated in larger cells ( > 10 μm). Scanning electron microscopy was used to estimate diatom numbers and cell surface areas. This latter parameter correlates well with biogenic silica and warrants a discussion of the contribution of different species to the total biogenic silica. The measured values for specific uptake rate of Si never reached the optimum uptake or growth rate deduced from environmental parameters and kinetic constants reported in the literature. In addition the mean growth rate (0.9±0.3 doubling per day) for the nitrate-depleted water does not differ from the mean value (0.8±0.2 doubling per day) in the HNLC area. Therefore it can be concluded that diatom growth is severely limited in both regions. This agrees well with the nutrient balance study. In the oligotrophic area, N supply appears to be the limiting factor in the upper layer. In the HNLC region, the results of this study are consistent with the hypothesis that diatom growth might be limited by a micro nutrient such as iron. Carbon production by diatoms was estimated to be 31 ± 4 mmol Cm −2 d −1, which is one-third of the total carbon production of the nutrient-enriched area.

Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis

Recent studies in the central equatorial Pacific allow a comprehensive assessment of phytoplankton regulation in a high-nutrient, low-chlorophyll (HNLC) ecosystem. Elemental iron enters the euphotic zone principally via upwelling and is present at concentrations (530 PM) well below the estimated half-saturation constant (120 PM) for the large cells that bloom with iron enrichment. In addition, the meridional trend in quantum yield of photosynthesis suggests that even the dominant small phytoplankton are held below their physiological potential by iron deficiency. Grazing by microzooplankton dominates phytoplankton losses, accounting for virtually all of the measured phytoplankton production during El Nina conditions and -66% during normal upwelling conditions, with mesozooplankton grazing and lateral advection closing the balance. Nitrate uptake is strongly correlated with the pigment biomass of diatoms, which increase in relative abundance during normal upwelling conditions. Nonetheless, the f-ratio remains low (0.07-0.12) under all conditions. Iron budgets are consistent with the notions that new production is determined by the rate of new iron input to the system while total production depends on efficient iron recycling by grazers. Although the limiting substrates differ, the interactions of resource limitation and grazing in HNLC regions are conceptually similar to the generally accepted view for oligotrophic subtropical regions. In both systems, small dominant phytoplankton grow at rapid, but usually less than physiologically maximal, rates; they are cropped to low stable abundances by microzooplankton; and their sustained high rates of growth depend on the remineralized by-products of grazing.

Phosphorus-limited bacterioplankton growth in the Sargasso Sea

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 13:141-149 (1997) - doi:10.3354/ame013141 Phosphorus-limited bacterioplankton growth in the Sargasso Sea Cotner JB, Ammerman JW, Peele ER, Bentzen E Oceanic central gyres cover large areas of the earth and contribute significantly to global productivity. Oceanic phytoplankton production is believed to be limited by nitrogen (N) in central gyres and iron (Fe) in high-nutrient low-chlorophyll regions. Bacterioplankton have been less studied but are believed to be limited by organic carbon. We report here that bacterioplankton in the Sargasso Sea were phosphorus (P) limited on cruises in 1992 and 1993. This assertion is supported by measurements of high dissolved and particulate N:P and C:P ratios, high alkaline phosphatase activity and phosphate uptake rates, and bacterioplankton growth rate responses in bioassays where inorganic P was added. Particulate C:P ratios were always higher than the Redfield ratio (106:1) and occasionally greater than 400:1. N:P ratios were 75:1 and 46:1 on 2 cruises and time-series data indicated that ratios were always greater than 24:1 over nearly a 2 yr span. Phosphate concentrations were extremely low in the euphotic zone (<10 nM) and biomass-normalized alkaline phosphatase activities indicated moderate to severe P limitation, with most severe limitation occurring in the spring. Bioassays indicated that heterotrophic bacteria may be P limited in the northwestern Sargasso Sea, especially in the spring. Limitation by P and not dissolved organic carbon may explain why dissolved organic carbon accumulates in the water column at that time. Bacterioplankton · Phosphorus · Biogeochemistry · Carbon · Phytoplankton Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 13, No. 2. Publication date: August 21, 1997 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1997 Inter-Research.

Iron limitation and the cyanobacterium Synechococcus in equatorial Pacific waters

Iron enrichments in bottle experiments in high nutrient, low chlorophyll (HNLC) surface waters typically stimulate the net growth of nanophytoplankton (2.0–20 µm) but not picophytoplankton (<2.0 µm), suggesting either that picophytoplankton are not Fe limited or that any increased picophytoplankton growth is balanced by increased microzooplankton grazing rates. Using a novel approach in which we diminish iron availability in seawater with the fungal siderophore deferriferrioxamine B, we demonstrate that growth of the abundant picoplanktonic cyanobacterium Synechococcus is not strongly rate limited by Fe in the HNLC equatorial Pacific Ocean. However, experiments at the equator and 5°S show an unsuspected non-uniformity in iron nutrition within this HNLC region. Furthermore, our results indicate that a major portion of “dissolved” (<0.4 µm) iron in these waters is unavailable to Synechococcus, providing the first evidence that a significant fraction of dissolved iron occurs in forms other than the simple hydroxy species predicted by thermodynamic models. Deferriferrioxamine B affords a powerful new tool for probing the iron nutrition of marine phytoplankton and can provide unique insight into the role of iron in marine biogeochemical cycles.

Low specific nitrate uptake rate: A common feature of high-nutrient, low-chlorophyll marine ecosystems

We have searched for common features of three high-nutrient, low-chlorophyll (HNLC) regions of interest—the Southern Ocean, the eastern equatorial Pacific, and Station P in the northeast Pacific. In each of these areas, the rates of specific NO3 uptake, whether normalized to particulate organic nitrogen (PON) or chlorophyll, are low compared to coastal upwelling systems with comparable nutrient concentrations. When maximum values of NO3 concentration and maximum values of PON-specific 15NO3 uptake, V15NO3, are plotted together with values from coastal upwelling systems, the data diverge into a high V15NO3 track for the coastal systems, and a low V15NO3 track for the three HNLC regions which have V15NO3 values consistent with oligotrophic regions and so are functionally oligotrophic. These values of V15NO3 are too low to allow biomass accumulation and the formation of blooms of diatoms. One possible reason for the lack of high V15NO3 values in the HNLC regions is that seeding of the large, fast-growing, fast-sinking diatoms is inadequate and primarily due to the lack of a bottom or other recirculation system to assure a supply of these diatoms to the surface regions. Grazing control limits biomass development and may function to hold V15NO3 to low values resulting in conditions certain to appear as HNLC. Comparison with model results suggests that deep mixed layers in the Southern Ocean and at Station P may limit V15NO3 and that in much of the eastern equatorial Pacific NO3 concentrations are too low for VNO3 to develop to coastal upwelling values. Strong dependence on temperature of N and C uptake in psychrophilic Antarctic phytoplankton and sea-ice algae may be a dominant factor in constraining V15NO3 to low values in the Southern Ocean. Inadequate levels of available Fe could be expected to lead to low specific 15NO3 uptake. However, experiments conducted to date in all three HNLC regions fail to show a response ofV15NO3to Fe additions within the first few days of enclosure experiments. Si(OH)4 concentrations are low with respect to half-saturation values, Ks for Si(OH)4) in the eastern equatorial Pacific and in the Southern Ocean where surprisingly high Ks values for Si(OH)4 have been reported.