Variability Of Surface Chlorophyll Research Articles

Analysis of surface chlorophyll a associated with sea surface temperature and surface wind in the South China Sea

In this study, the spatial and temporal variability in surface chlorophyll a (Chl-a) in the whole South China Sea (SCS) was investigated in detail by using 8-day, 4-km, gap-free MODIS-A data (2003–2016). Monthly climatology and empirical-orthogonal-functions analysis of Chl-a were performed in association with sea surface temperature and surface wind to aid in better understanding the physical mechanisms responsible for the Chl-a variability. The results are as follows: (1) Chl-a has out-of-phase variability between the coastal and open-sea regions due to different major factors controlling phytoplankton growth in each region; (2) in particular, Chl-a increases in the northern SCS during winter and in the western and southwestern SCS during summer mainly due to the effects of monsoons and orography; and (3) wind-driven coastal upwelling is stronger in the western SCS than in the eastern SCS. A wind-induced coastal upwelling not reported in the literature was detected along Palawan Island (8–12∘N, 117–120∘E) during winter. In the SCS, the Chl-a variability is influenced by the El Nino–Southern Oscillation with a time lag of 4–9 months, depending on the variability scales.

On the role of physical processes on the surface chlorophyll variability in the Northern Mozambique Channel

In the Indian Ocean regions under the influence of monsoons, two phytoplankton blooms characterize the seasonal cycle of surface chlorophyll, one during summer, and the other during winter. In the Northern Mozambique Channel, however, where the wind regime is an extension of the northern Indian Ocean monsoons, the annual cycle of chlorophyll concentrations is characterized by a single winter bloom. Wind stress and surface net heat flux modulate the seasonal cycle of the mixed layer depth with impacts on the surface chlorophyll. In order to evaluate the importance of these forcing fields on the seasonality of the mixed layer depth, and consequently, the surface chlorophyll variability, we used a suite of physical-biogeochemical model sensitivity experiments. Our results show that the seasonal cycle of surface chlorophyll is primarily modulated by the net heat flux while the wind field controls the amplitude. The winter bloom is triggered by negative surface heat fluxes, where cooling at the surface induces mixing and entrainment of nutrients at the base of the nutricline and light is not limiting. Winds enhance the winter bloom by uplifting additional nutrients and diluting subsurface chlorophyll into the surface layer. In the summertime, weaker wind stress and positive heat flux inhibit vertical mixing. As a consequence, the surface layer is depleted in nutrients and a deep chlorophyll maximum is formed. Analysis of top-down control on phytoplankton biomass reveals that zooplankton abundance increases in a near-linear proportion with phytoplankton biomass despite the deepening of the mixed layer depth. This suggests that the phytoplankton stock in the Northern Mozambique Channel is also controlled by grazing, given that zooplankton biomass is not directly affected by the deepening of the mixed layer during wintertime.

Occurrence and seasonal dynamics of metalimnetic Deep Chlorophyll Maximum (DCM) in a stratified meromictic tropical lake and its implications for zooplankton community distribution

Deep Chlorophyll Maxima (DCM) is an important feature of stratified lakes and oceans but very little is known about the phenomenon in tropical systems. Proximate factors accounting for DCM presence include light, thermal stratification and nutrients but biotic interactions such as zooplankton grazing can actively support DCM formation, structure and maintenance. We examined DCM characteristics in tropical Lake Bosumtwi at biweekly intervals between April and July of 2005 and 2006 and also assessed zooplankton herbivore interactions with the DCM. The onset and development of the seasonal DCM peak was based on stable water column, steep thermocline, availability of a nutricline and nutrient limitation in the epilimnion. Euphotic depth doubled during DCM formation and increasing transparency tripled DCM Chl a in the metalimnion just below the oxycline. DCM, however, disaggregated with seasonally induced deeper mixing. The range of variability in surface chlorophyll did not differ significantly between seasonal and aseasonal DCM period. There was about one‐fourth difference between mean surface and metalimnetic chlorophyll concentrations but this shift in resource location did not trigger a descent to deeper waters by herbivores. Herbivore grazing could not be linked to surface variations in water clarity and deep‐water irradiance needed for DCM formation and maintenance. DCM presence therefore was not enhanced by surface water grazing activities of zooplankton. This suppressed biotic influence indicated that DCM dynamics is driven by lake clarity, thermal and nutrient gradients which constitute key abiotic drivers. Direct consumption of the DCM by herbivorous zooplankton was not evident and the energy therein is possibly lost to the anoxic deep waters of the lake through sedimentation. The sinking organic matter could be returned to the pelagic food web via the microbial loop represented by bacteria, ciliates and heterotrophic/mixotrophic flagellates inhabiting the lake's deep layers.

Satellite chlorophyll off the British Columbia Coast, 1997-2010

We examine the spatial and temporal variability of satellite-sensed sea surface chlorophyll off the west coast of North America from 1997 to 2010, with focus on coastal British Columbia. The variability in surface chlorophyll is complex. Whereas the spring bloom generates the highest phytoplankton concentration for coastal Alaska, the north and east coasts of Haida Gwaii, Queen Charlotte Sound, the Strait of Georgia, and coastal Oregon and California, it is the fall bloom that normally generates the highest concentration for the west coast of Vancouver Island, Juan de Fuca Strait, and the west coast of Washington. The highest satellite-sensed chlorophyll concentrations occur in the Strait of Georgia, where mean values are at least 2 times higher than elsewhere in the northeast Pacific. Moreover, the annual average surface chlorophyll concentration increased significantly in the Strait of Georgia during this period, with highest concentration observed during the near neutral ENSO conditions of the spring of 2007. The next highest concentrations occur off southwest Vancouver Island but have no statistically significant trend. The lowest average peak chlorophyll concentration is observed off Southern California. The timing of the highest chlorophyll concentration is latest off the coast of Washington and earliest off the coast of Southern California. Small increasing concentration trends are observed off the Washington and California coasts.

Seasonal and intraseasonal variability of surface chlorophyll a concentration in the South China Sea

Seasonal and intraseasonal variability of surface chlorophyll a concentration in the South China Sea is explored using the NASA standard Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) monthly and 8-day level-3 products from 1997 to 2007. The link between the seasonal and intraseasonal variability of the chlorophyll and physical forcing is revealed. Continuous wavelet transform indicates that chlorophyll in the South China Sea presents pronounced seasonal, as well as intraseasonal, variability. Chlorophyll a variability shows significant correlation with wind speed in all five sub-regions, while significant correlation with sea surface temperature only appears in the northern Sea. Intraseasonal variability of chlorophyll appears mainly in the winter, with spectral peaks around 32–64 days. It shows high commonalities for some periods in the intraseasonal cycle between chlorophyll a and sea surface temperature, wind stress curl or wind speed using cross wavelet transform.

Assessment of chlorophyll variability along the Louisiana coast using multi-satellite data

Variability in surface chlorophyll (Chl) concentrations derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) were examined in conjunction with river discharge, QuikSCAT satellite-derived winds, and sea surface height (SSH) anomaly data along the Louisiana coast, USA. Surface Chl distributions exhibited rapid response to strong northerly winds following a frontal passage. A comparison of time series (1998–2010) river discharge and monthly Chl data indicated Chl variability to be well correlated to seasonal river discharge only for locations near the two river deltas, while offshore, enhancements in Chl during fall–winter was likely due to cross-shelf transport or mixing associated with strong northerly wind stress. Variance in Chl examined using wavelet analysis applied to nearly 10 years (1998–2007) of SeaWiFS data indicated patterns of significant Chl variability due to combined enhanced wind and river discharge, offshore flows associated with Ekman transport and coastal wind convergence, and the effect of Hurricane Rita in 2005. Instances of significant Chl variance were also observed to occur during years of large hypoxic zone size suggesting potential linkages to hypoxia. SSH anomaly imagery indicated the presence of warm-core eddies that were responsible for the offshore dispersal of elevated Chl observed in the monthly SeaWiFS imagery. Overall, the use of multi-satellite data better described the forcing and patterns of Chl distributions along the river-dominated Louisiana coast and shelf.

Climatology of surface chlorophyll a, autumn‐winter and spring blooms in the southwest Pacific Ocean

Ocean color data from the 13 year Sea‐viewing Wide Field‐of‐view Sensor mission are used to examine the distribution of surface chlorophyll a in the southwest Pacific Ocean. The mean surface chlorophyll field is similar to that found by previous workers, with elevated levels in the Subtropical Front and around the subantarctic islands that have an associated shelf. The annual cycle in surface chlorophyll shows a ubiquitous summer bloom in subantarctic water, with autumn, winter, and spring blooms variously in subtropical water and across the Subtropical Front. The autumn blooms progress equatorward with time at the same rate as wind stress. This supports the idea that the autumn bloom develops in response to increased wind stress, with a likely mechanism for the bloom being mixing to the surface of the deep chlorophyll maximum, and/or increased production due to entrainment of nutrients. The spring bloom progresses poleward with time. It starts after the mixed layer reaches its deepest, and its timing appears to be linked to the reduction in wind stress in spring. Under the assumption that all tracers are well mixed to the seasonal thermocline in autumn and winter, we conclude that vertically integrated chlorophyll increases at all latitudes in subtropical water during autumn and winter. Unfortunately, carbon‐to‐chlorophyll ratios are not known well enough to determine whether the same is true for vertically integrated carbon biomass. Individual spring blooms show significant spatial structure and are different from year to year. This leads to low spatial coherence for the temporal variability in surface chlorophyll.

Biophysical coupling in remotely-sensed wind stress, sea surface temperature, sea ice and chlorophyll concentrations in the South Indian Ocean

Satellite records of chlorophyll, sea-surface temperature, sea-ice concentration and wind-stress curl, together with reanalysis wind fields, have been analysed to identify connections between the physical environment and phytoplankton growth. The study focusses on the BROKE-West survey region in the Indian Ocean sector of the Southern Ocean: 20° to 90°E, 70° to 50°S. Correlation and regression analyses showed that, of the parameters that can be routinely monitored from space, wind stress and sea surface temperature were most strongly correlated with chlorophyll across the BROKE-West survey area during the period of the survey. Each of these factors, as well as sea ice concentration, was found to be strongly correlated with chlorophyll at different locations. Three distinct regions were identified: The continental shelf and slope (max. depth=3000 m), where satellite data were most obstructed by ice and clouds, supported high concentrations of chlorophyll throughout the growth season (October to April), and although the most important factors determining chlorophyll were likely to be those not observable using remote sensing, close to 100% of variability in surface chlorophyll could be predicted by sea-surface temperature and sea ice concentration. Offshore to the west of 45°E, the eastern Weddell Gyre was found to retain sea-ice later into the growth season and support chlorophyll concentrations on the order of 0.1 mg m -3. Wind-driven advection of sea ice to the north and south of the Antarctic Divergence, located at 64°S, produced a strong positive correlation between chlorophyll concentrations along the landward and off-shore edges of the seasonal ice zone and the Southern Annular Mode. Offshore to the east of 45°E, chlorophyll concentrations encountered during the BROKE-West cruise were found to be unusually high. This was attributed to a southerly excursion of the Antarctic Circumpolar Current corresponding to negative Southern Annular Mode indices during the cruise. Significant, regionally varying correlations were found between the physical and biological parameters examined and climatological indices, including the Southern Annular Mode and the tropical El Nino-Southern Oscillation signal. © 2010 Elsevier Ltd.

The seasonal cycle of surface chlorophyll in the Peruvian upwelling system: A modelling study

The seasonal variability of surface chlorophyll in the northern Humboldt Current System is studied using satellite data, in situ observations and model simulations. The data show that surface chlorophyll concentration is highest in austral summer and decreases during austral winter, in phase opposition with coastal upwelling intensity. A regional model coupling ocean dynamics and biogeochemical cycles is used to investigate the processes which control this apparently paradoxical seasonal cycle. Model results suggest that the seasonal variability of the mixed layer depth is the main controlling factor of the seasonality. In winter, the mixed layer deepening reduces the surface chlorophyll accumulation because of a dilution effect and light limitation. In summer, biomass concentrates near the surface in the shallow mixed layer and nitrate limitation occurs, resulting in a biomass decrease in the middle of summer. Intense blooms occur during the spring restratification period, when winter light limitation relaxes, and during the fall destratification period, when the surface layer is supplied with new nutrients. Model sensitivity experiments show that the seasonal variations in insolation and surface temperature have little impact on the surface chlorophyll variability.

Nutrient and phytoplankton trends on the western Black Sea shelf in response to cultural eutrophication and climate changes

Long-term trends (1960s–1990s) in surface winter nutrients and summer oxygen concentration of the euphotic zone, as well as seasonal and interannual variability in surface chlorophyll a (chl a) and Secchi depth ( Z d) were investigated for different shelf regions (depth <50 m) of the western Black Sea. Increasing phosphate and nitrate levels and changes in the summer oxygen concentration on the shelf before the mid-1980s corresponded well with the increase in riverine nutrient inputs. At the same time, decreasing silicate levels resulted almost equally from enhanced diatom stripping and trapping of silicate in the numerous dams constructed on the Danube River. The associated decrease in the Si:N ratio caused a shift towards more non-siliceous phytoplankton blooms. A decoupling of winter nutrient levels and summer oxygen concentration on the shelf after the mid-1980s suggests that other sources of inputs, such as regenerated nutrients from shelf sediments and/or upwelling, may have increased substantially. Large variations in the regional climate during the 1980s and 1990s could potentially account for increases from either or both of these sources and the resulting high summer primary production despite decreasing winter nutrients. The seasonal pattern in chl a within the Ukrainian NW shelf is similar to the open Black Sea, with low chl a in summer and high concentrations in cold months. The seasonal chl a variations on the Romanian and Bulgarian shelves also show high concentrations in May/June, most likely related to the Danube River maximum discharge during spring. As a result, chl a annual means in these two regions are significantly higher than – in the Ukrainian NW shelf.

Suppression and enhancement of the spring bloom in the southwestern East Sea/Japan Sea

Abstract We investigated the variability in surface chlorophyll a in the southwestern region of the East Sea/Japan Sea using OCTS (1997) and SeaWiFS (1999) data, together with in situ data from bimonthly oceanographic surveys. In 1997, the spring bloom appeared to be suppressed in the Ulleung Basin (UB) region, whereas the bloom in the subpolar front (SF) was intensified and continued for four weeks. In 1999, the spring bloom was observed throughout the southwestern region (both UB and SF), but the bloom in the SF region lasted for only one week. Consequently, the overall productivity was higher in 1997 than in 1999, while the spatial difference was much greater in 1997 than in 1999. In 1997, after about seven weeks, copepods became significantly more abundant in the bloom than in the non-bloom area. We relate all these differences to the variability in the volume transport and direction of the Tsushima Current (TC), which is the major input to the East Sea. The volume transport in April–June 1997 was unusually low as compared with that in 1999. The nutrient-poor surface layer of the TC in the UB was much thicker in 1997 and seemed to suppress the spring bloom. However, the front was stationary for more than four weeks and a high-density bloom continued in the SF region for four weeks in 1997. In 1999, the front moved northward rapidly; as a result, the bloom in the SF lasted for one week. In addition, concurrent differences were found in sea-surface height, yearly mean sea level, and the mean position of the SF. Concurrent changes in a suite of oceanographic properties suggest that the unusual conditions in 1997 were related to global variability. A possible relation with ENSO is discussed.

Suppression and enhancement of the spring bloom in the southwestern East Sea/Japan Sea

We investigated the variability in surface chlorophyll a in the southwestern region of the East Sea/Japan Sea using OCTS (1997) and SeaWiFS (1999) data, together with in situ data from bimonthly oceanographic surveys. In 1997, the spring bloom appeared to be suppressed in the Ulleung Basin (UB) region, whereas the bloom in the subpolar front (SF) was intensified and continued for four weeks. In 1999, the spring bloom was observed throughout the southwestern region (both UB and SF), but the bloom in the SF region lasted for only one week. Consequently, the overall productivity was higher in 1997 than in 1999, while the spatial difference was much greater in 1997 than in 1999. In 1997, after about seven weeks, copepods became significantly more abundant in the bloom than in the non-bloom area. We relate all these differences to the variability in the volume transport and direction of the Tsushima Current (TC), which is the major input to the East Sea. The volume transport in April–June 1997 was unusually low as compared with that in 1999. The nutrient-poor surface layer of the TC in the UB was much thicker in 1997 and seemed to suppress the spring bloom. However, the front was stationary for more than four weeks and a high-density bloom continued in the SF region for four weeks in 1997. In 1999, the front moved northward rapidly; as a result, the bloom in the SF lasted for one week. In addition, concurrent differences were found in sea-surface height, yearly mean sea level, and the mean position of the SF. Concurrent changes in a suite of oceanographic properties suggest that the unusual conditions in 1997 were related to global variability. A possible relation with ENSO is discussed.

Nitrate and chlorophyll distributions in relation to thermohaline and current structures in the western tropical Pacific during 1985–1989

Fourteen transects carried out from 1985 to 1989 permit us to describe the nitrate and chlorophyll a distributions and their temporal variability in the tropical western Pacific (165°E, 20°S-10°N). Sections representative of the moderate 1986–1987 El Niño, the strong 1988–1989 La Niña and an equatorial westerly wind burst were compared to the January 1986 transect, which we define as a “reference” section. Along 165°E, there appears to be four characteristic distributions of nitrate and chlorophyll relative to the thermohaline structure. (1) In the south (20-17°S), a seasonal cycle was observed in both nitrate and chlorophyll distributions. Variability of surface chlorophyll and depth of the deep chlorophyll maximum (DCM) was associated with variations in the vertical mixing and available amount of light. (2) In the north (6–10°N), seasonal and interannual changes remained in the subsurface layer because of the strong stratification. Ekman pumping is one cause of the changes of the DCM depth. (3) In the 2°S–2°N band, the seasonal variability of the DCM depth was associated with variations in precipitation and eastward advection of low salinity water. During the 1986–1987 El Niño, elevation and intensification of the pycnocline and shoaling of the nutrient reservoir were the result of basin-wide changes. Consequences included abnormally low surface chlorophyll concentrations and an increase of the primary production. Intra-El Niño variations of the DCM depth were associated with changes of the thickness of the low salinity surface layer. During the subsequent La Niña, upwelling developed and vertical nitrate and chlorophyll distributions were strongly modified. Surface nitrate and chlorophyll concentrations for this period were the highest of the dataset, and primary production values were about twice the El Niño values. The main differences between upwelling in the western and central Pacific are attributed to the existence of two atmospheric convergence zones in the western basin, and especially to the tendency of the Intertropical Convergence Zone to migrate south of the equator. Intraseasonal Kelvin waves influence the variations of nutrient concentration and phytoplankton biomass. During the December 1989 westerly wind burst, there were strong modification of the thermohaline and nitrate distributions, associated with a geostrophic adjustment, which were not echoed on the chlorophyll distribution. (4) At 10°S, a doming of the different properties was frequently observed. Transient surface enrichments in nitrate or chlorophyll result from combined effects of the divergence between the South Equatorial Counter Current and the southern branch of the South Equatorial Current, Ekman pumping favourable to upwelling and local wind events. In all cases, nitrate and chlorophyll distributions were closely linked to the density structure for which salinity can be the controling factor. Low-frequency variability (seasonal, intraseasonal and interannual scales) of the nitrate and chlorophyll distributions were determined primarily by local or remote physical processes which controlled stability of the water column that governs the vertical displacements of phytoplankton.

Limnology of Lake Waikaremoana with special reference to littoral and pelagic primary producers

Abstract Lake Waikaremoana, the North Island's deepest lake (248 m), lies in a natural forested catchment, but the lake itself has been modified for hydro‐electric power generation and by the introduction of trout, smelt, and adventive aquatic plants. The lake is a warm monomictic water body of low conductivity (82 μS cm‐1) and a high seasonal water column stability. The waters are oligotrophic, with epilimnetic dissolved reactive phosphorus concentrations typically < 1 mg m‐3. The concentration of NO3‐N is seasonally variable but generally high in winter and spring with maximum epilimnetic values approaching 70 mg m 3. This contrasts with other central North Island lakes. Horizontal variability in surface chlorophyll a is low as are the absolute values (< 1–2 mg m‐3). A notable feature is the formation of a deep chlorophyll maximum within the metalim‐nion comprised largely of Sphaewcystis schweteri as opposed to diatoms and flagellates which normally dominate the epilimnion. Vascular macro‐phytes (maximum biomass 659 g m2 dry weight) extended to 9 m and characeans (maximum biomass 447 g m2dry weight) to 16 m. Total phyto‐plankton primary production was calculated as 4524 tCy‐1 and macrophyte production as 578 t C y‐1. The proportion of macrophyte to phy‐toplankton production (0.14) is higher than in the other deep lakes of the central North Island.