A northern Indian Ocean comparison between nutrient availability derived from AVHRR-SST and phytoplankton biomass/productivity based on SeaWiFS (original) (raw)

Phytoplankton distribution and nitrogen dynamics in the southwest indian subtropical gyre and Southern Ocean waters

Ocean Science, 2011

During the 1999 Marion Island Oceanographic Survey (MIOS 4) in late austral summer, a northbound and reciprocal southbound transect were taken along the Southwest Indian and Madagascar Ridge, between the Prince Edward Islands and 31 • S. The sections crossed a number of major fronts and smaller mesoscale features and covered a wide productivity spectrum from subtropical to subantarctic waters. Associated with the physical survey were measurements of size fractionated chlorophyll, nutrients and nitrogen (NO 3 , NH 4 and urea) uptake rates. Subtropical waters were characterised by low chlorophyll concentrations (max = 0.27.3 mg m −3 ) dominated by picophytoplankton cells (> 81%) and very low f-ratios (< 0.1), indicative of productivity based almost entirely on recycled ammonium and urea. Micro-phytoplankton growth was limited by the availability of NO 3 (< 0.5 mmol m −3 ) and Si(OH) 4 (< 1.5 mmol m −3 ) through strong vertical stratification preventing the upward flux of nutrients into the euphotic zone. Biomass accumulation of small cells was likely controlled by micro-zooplankton grazing. In subantarctic waters, total chlorophyll concentrations increased (max = 0.74 mg m −3 ) relative to the subtropical waters and larger cells became more prevalent, however smaller phytoplankton cells and low f-ratios (< 0.14) still dominated, despite sufficient NO 3 availability. The results from this study favour Si(OH) 4 limitation, light-limited deep mixing and likely Fe deficiency as the dominant mechanisms controlling significant new production by micro-phytoplankton.

Spatial variability of nutrient sources determining phytoplankton Chlorophyll-a concentrations in the Bay of Bengal

APN Science Bulletin

Primary production is a key marine ecosystem driver in the Bay of Bengal and is important for the societies and economies of the surrounding countries. Although the availability of inorganic nutrients is known to control primary production in this region, the specific nutrient sources that affect primary production in different parts of the bay have not been identified. In this study, we assess the importance of nutrients from different sources in determining chlorophyll-a concentration, an indicator of primary production, in the Bay of Bengal by conducting multiple linear regression of satellite-derived chlorophyll-a concentration, sea surface height anomaly, and sea surface temperature; modelled dust deposition; and in situ river discharge from 1997 to 2016. River-borne nutrients were important up to approximately 200 km from the coast. Deep-ocean nutrients influenced chlorophyll-a concentrations mainly in the south-western and western bay, whereas wind-borne nutrients were more i...

Dissolved nutrient dynamics along the southwest coastal waters of India during northeast monsoon: a case study

Chemistry and Ecology, 2017

Dissolved nutrients, Chl-a and primary productivity were measured from seven transects along the coastal waters of the southeastern Arabian Sea during northeast monsoon. Ten major estuaries were chosen to study the influence of estuarine discharge on the nutrient dynamics in the coastal waters. The mean water discharge of the estuaries in the north (64.8 ± 18 × 10 5 m 3 d −1) was found to be higher than those in the south (30.6 ± 21.4 × 10 5 m 3 d −1), whereas the nutrient concentrations were found to be higher in the estuaries of the south. The results from the offshore waters were discussed in accordance with the depth contour classification, that is, shelf (depth ≤ 30 m) and slope waters (depth ≥ 30 m). Our results suggest that the estuarine discharge plays a major role in the nutrient distribution in near shore shelf waters, whereas in shelf and slope waters, it was mainly controlled by in situ biological processes. The inorganic form of N to P ratios were found to be higher than Redfield ratio in slope waters when compared with shelf waters, suggesting that PO 4 3− (<0.15 µmol L −1) is a limiting nutrient for primary production. The multivariate statistical analysis revealed that the nutrient dynamics in the coastal waters was controlled by both biological and physical processes.

Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical South Indian Ocean

Biogeosciences Discussions, 2019

Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~30°S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of <5.9 µM NO3and <0.5 µM PO4 3-(<310 m; σ<26.4 kg/m³). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (δ 15 N and δ 18 O) indicate isotopic maxima of δ 15 N (>7 ‰) and δ 18 O (>4 ‰) centred at 400−500 m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N*<−1 µM) within the northern study area, accompanied by elevated isotopic ratios of nitrate (δ 15 N>7 ‰; δ 18 O>3 ‰). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N2-fixation within the Indian Ocean subtropical gyre (17° S−25° S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.

The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea

Deep Sea Research Part II: Topical Studies in Oceanography, 1999

Variations in the nutrient concentrations were studied during two cruises to the Arabian Sea. The situation towards the end of the southwest monsoon season (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the influence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringing colder water (24.2°C) into the surface waters of the coastal margin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this area also had cooler surface water (24.6°C). Further offshore ('1000 km) the average surface temperatures increased to '27°C. These waters were oligotrophic with no evidence of the upwelling effects observed further inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO "18 mol l\, PO "1.48 mol l\); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 mol l\ and a maximum phosphate concentration of 1.2 mol l\. Nitrate and phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentrations were 35.0 nmol l\ (0.035 mol l\) in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic waters, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column was enhanced to a distance of over 800 km offshore. During the intermonsoon period a stable surface mixed layer was established, with a well-defined thermocline and nitracline. Surface temperature was between 26.8 and 27.4°C for the entire transect from the coast to 1650 km offshore. Nitrate concentrations were typically between 2.0 and 0.4 mol l\ for the transect, to about 1200 km where the waters became oligotrophic, and nitrate concentrations were then typically : S 0 9 6 7 -0 6 4 5 ( 9 8 ) 0 0 1 1 8 -0 8-12 nmol l\. Ammonia concentrations for the oligotrophic waters were typically 130 nmol l\, and are reported for the first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios suggest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitation during the intermonsoon period.

Predicting plant nutrient concentrations from temperature and sigma-t in the upper kilometer of the world ocean

Deep Sea Research Part A. Oceanographic Research Papers, 1986

A National Oceanographic Data Center data set comprised of 230,202 oceanographic stations representing all regions of the world ocean was analyzed statistically for temperature and sigma-; (a,) relationships with nitrate, phosphate or silicic acid concentrations in the upper kilometer of the water column. Six cubic regressions were computed for each 10° square of latitude and longitude containing adequate data. World maps display the locations that allow the prediction of plant nutrient concentrations from temperature or o, within the limits of selected subjective and objective criteria. Geographic coverage improves along the sequence: nitrate, phosphate and silicic acid, and is better for rj, than for temperature. A percentile analysis of the temperature or o, at which less than a selected amount of plant nutrient occurs provided a method to examine incipient nutrient limitation. Contour maps display the approximate temperatures above or ci, values below which nitrate, phosphate or silicic acid routinely approach unmeasurable concentrations in the world ocean as determined by classical plant nutrient analyses. The results summarize the global potential to predict plant nutrient concentrations from remotely sensed temperature or a, and emphasize the latitudinally and longitudinally changing phytoplankton growth environment based on temperature, a, and plant nutrients in present and past oceans.

Phytoplankton composition and biomass across the southern Indian Ocean

Deep Sea Research Part I: Oceanographic Research Papers, 2011

Phytoplankton composition and biomass was investigated across the southern Indian Ocean. Phytoplankton composition was determined from pigment analysis with subsequent calculations of group contributions to total chlorophyll a (Chl a) using CHEMTAX and, in addition, by examination in the microscope. The different plankton communities detected reflected the different water masses along a transect from Cape Town, South Africa, to Broome, Australia. The first station was influenced by the Agulhas Current with a very deep mixed surface layer. Based on pigment analysis this station was dominated by haptophytes, pelagophytes, cyanobacteria, and prasinophytes. Sub-Antarctic waters of the Southern Ocean were encountered at the next station, where new nutrients were intruded to the surface layer and the total Chl a concentration reached high concentrations of 1.7 mg Chl a L À 1 with increased proportions of diatoms and dinoflagellates. The third station was also influenced by Southern Ocean waters, but located in a transition area on the boundary to subtropical water. Prochlorophytes appeared in the samples and Chl a was low, i.e., 0.3 mg L À 1 in the surface with prevalence of n Corresponding author. Tel.: þ45 4516 9557; fax: þ45 4516 9292.

Impact of intra-seasonal oscillations of Indian summer monsoon on biogeochemical constituents of North Indian Ocean

Theoretical and Applied Climatology, 2018

The intra-seasonal perturbations in the atmospheric weather are closely related to the variability in the ocean circulation. NASA Ocean Biogeochemical Model (NOBM) couples the oceanic general circulation and the radiative forcing. The NOBM model products of nitrate, total chlorophyll, and mixed layer depth (MLD) collected during the period from 1998 to 2007 as well as the sea surface temperature (SST), precipitation, outgoing long wave radiation (OLR), and wind are considered in this study to identify the influence of intra-seasonal oscillation (ISO) of Indian summer monsoon (ISM) on the biogeochemical constituents of Bay of Bengal (BOB) (6°-22°N; 80°-100°E) and Arabian Sea (AS) (3°-17°N; 55°-73.5°E) of North Indian Ocean (NIO). The active and break phases are the most significant components of ISO during ISM. The result of the study reveals that the upper ocean biology and chemistry significantly vary during the said phases of ISM. The nitrate, total chlorophyll, and MLD are observed to be strongly correlated with the ISO of ISM. The result shows that, during ISO of ISM, the concentration of nitrate and chlorophyll is strongly and positively correlated both in BOB and AS. However, the correlation is more in AS, endorsing that the Arabian Sea is more nutrient reach than Bay of Bengal. Nitrate and MLD, on the other hand, are strongly but negatively correlated in the said basins of North Indian Ocean (NIO). The forcing behind the variability of the biogeochemical constituents of BOB and AS during active and break phases of ISM is identified through the analyses of SST, precipitation, OLR, and wind.

A quantitative method for describing the seasonal cycles of surface chlorophyll in the Indian Ocean

Proceedings of SPIE - The International Society for Optical Engineering

The seasonal cycles of surface chlorophyll (SCHL) in the Indian Ocean (IO) are regionally described by means of 6 parameters: the timing of the bloom onset and of the bloom peak, and the integrated SCHL value in between these two extrema for both winter and summer blooms. This description, based on a climatology constructed from 7 years of SeaWiFS data, provides a regional image of the influence of the two monsoons on phytoplankton blooms. Over a large part of the basin, the seasonal cycle is characterized by two distinct growth periods, one in summer during the South West Monsoon (SWM), the other in winter during the North East Monsoon (NEM). However, in some specific areas such as the southwestern coast of India, there is no maxima during the NEM. The bloom areas during the SWM and the NEM show totally different regional patterns. Important lags in the timing of the blooms are identified, and are also associated with distinct regional patterns. The next step in the understanding o...