Surface Chlorophyll Maximum Research Articles

Environmental controls on the interannual variability in chlorophyll and phytoplankton community structure within the seasonal sub surface chlorophyll maximum in the western English channel

The subsurface chlorophyll maximum (SCM) is increasingly recognised as an important but understudied locus of primary production particularly in shelf seas. Here we report the results of a 4 year, repeat station, summer sampling programme (2013–2016) of a seasonally recurrent SCM in the Western English Channel. Interannual variability in phytoplankton community structure and chlorophyll distribution and intensity was strongly related to water column stability at the depth interval of the SCM and also to water temperature. The phytoplankton community was statistically distinct in each year. High stability, as evidenced by large Richardson numbers and a well-developed strong thermocline appeared to favour the growth of larger dinoflagellates (autotrophs or mixotrophs) and diatoms. Such conditions led to development of the most intense SCMs and these were sometimes dominated by a single or a few key species most prominently in 2015 with near monospecific concentrations of the dinoflagellate Tripos fusus with average peak SCM chlorophyll concentrations of 7.3 ± 4.4 μg l−1. By contrast, in years with low water column stability and intermittent turbulence at the thermocline (2014, 2016) there was greater chlorophyll dispersal and less intense SCM. In these low stability conditions, red fluorescent nano-phytoplankton, such as naked dinoflagellates, chlorophytes and prymnesiophytes, made a greater contribution to the community, possibly as a result of the advantages that motility and enhanced light utilisation efficiency confer within an SCM exposed to turbulence. It is also likely that turbulence disrupted the stability required by the larger dinoflagellates and diatoms. Several of the key SCM taxa were absent from surface waters including the dinoflagellates Tripos fusus, Tripos lineatus, and most of the Rhizosolenia/Proboscia diatoms, consistent with adaptations more suited to survival at depth in stratified waters. These traits include luxury nutrient uptake and storage and survival in low light (both groups) and mixotrophy (dinoflagellates). On the other hand, in 2013, diatoms including Pseudo-nitzschia spp. were abundant in both surface, SCM and bottom waters. The relatively cooler waters (11.6–12.1 °C on average in 2013 and 2016) were characterised by smaller diatoms (Chaetoceros spp. and Pseudo-nitzschia spp.) whereas the warmer waters (13.1 °C on average in 2014) contained larger diatoms (large Rhizosolenia spp., Lauderia annulata and Leptocylindrus danicus). There did not appear to be continuity of key species between years, other than for the dinoflagellate Tripos lineatus, which was significant in both 2013 and 2014 and present in 2015. In any given year, there was no correspondence between the key spring bloom phytoplankton species as monitored in the nearby Western Channel Observatory L4 station and the key SCM taxa.

Regulation of Algal Bloom Hotspots Under Mega Estuarine Constructions in the Changjiang River Estuary

Massive large-scale engineering projects have been built in river estuaries around the world, but their effects on environments in the surrounding coastal waters were less emphasized compared to those due to the watershed projects. In this study, we used the Changjiang River Estuary as an example to show that a significant consequence can be resulted in such a situation. Through analyzing the harmful algal bloom events data and the chlorophyll satellite data, we investigate the spatiotemporal variations of algal blooms in the estuary and its adjacent water. The results indicate that the location of algal bloom hotspot changed over the period of the estuarine constructions. Furthermore, using a well-validated numerical model, we explored the mechanisms responsible for such an ecosystem regime shift. It was found that after the estuarine constructions were built, the surface chlorophyll maximum was attenuated and part of it migrated landward north of the river mouth but was strengthened south of the river mouth and extended seaward. Alternations of the nutrient concentration distribution and turbidity distribution induced by river plume deviation are responsible for the redistribution of the high chlorophyll concentration area. By contrast, the direct impact of the Three Gorges Dam through changing the runoff and sediments flux, which has been highlighted in numerous studies, was less important than expected. Given the fact that Three Gorges Dam and mega estuarine constructions were built in a similar period, any observed regime shift of hydrodynamic and ecological status outside the estuary should be interpreted with particular caution.

Phytoplankton succession and the formation of a deep chlorophyll maximum in a hypertrophic volcanic lake

Deep chlorophyll maxima (DCM) are a common feature of poorly mixed, low-productivity lakes, but are less commonly observed in eutrophic systems. This study investigates the transition from a late-spring surface chlorophyll maximum (SCM) to an early summer DCM in a hypertrophic lake, identifying the phenology and mechanisms responsible for the transition and how the DCM was maintained in this hypertrophic system. Data and samples were collected weekly during summer and monthly in winter at five sampling stations in monomictic Lake Ōkaro, North Island, New Zealand. Additional samples were collected on two occasions at c. 3–5 h intervals over 24 h at a single station, corresponding to periods with a SCM and DCM, respectively. The mean DCM depth was correlated with mean thermocline depth, and euphotic depth (z eu) continuously exceeded the depths of both the DCM and thermocline. The decline in the SCM and the transition to a DCM population were coincident with a gradual depletion of dissolved inorganic nutrients in surface waters and increased light penetration into the nutrient-rich hypolimnion. This study demonstrated that there are similarities between DCMs in lakes of high and low productivity, relating to the interplay between light, nutrients and thermal stratification.

Dynamics of microplankton communities at the ice-edge zone of the Lazarev Sea during a summer drogue study

Microzooplankton grazing and community structure were investigated in the austral summer of 1995 during a Southern Ocean Drogue and Ocean Flux Study (SODOFS) at the ice-edge zone of the Lazarev Sea. Grazing was estimated at the surface chlorophyll maximum (5–10 m) by employing the sequential dilution technique. Chlorophyll a concentrations were dominated by chainforming microphytoplankton (>20 μm) of the genera Chaetoceros and Nitzschia. Microzooplankton were numerically dominated by aloricate ciliates and dinoflagellates (Protoperidinium sp., Amphisoleta sp. and Gymnodinium sp.). Instantaneous growth rates of nanophytoplankton (<20 μm) varied between 0.019 and 0.080 day−1, equivalent to between 0.03 and 0.12 chlorophyll doublings day−1. Instantaneous grazing rates of microzooplankton on nanophytoplankton varied from 0.012 to 0.052 day−1. This corresponds to a nanophytoplankton daily loss of between 1.3 and 7.0% (mean = 3.76%) of the initial standing stock, and between 45 and 97% (mean = 70.37%) of the daily potential production. Growth rates of microphytoplankton (>20 μm) were lower, varying between 0.011 and 0.070 day−1, equivalent to 0.015–0.097 chlorophyll doublings day−1. At only three of the 10 stations did grazing by microzooplankton result in a decrease in microphytoplankton concentration. At these stations instantaneous grazing rates of microzooplankton on microphytoplankton ranged between 0.009 and 0.015 day−1, equivalent to a daily loss of <1.56% (mean = 1.11%) of initial standing stock and <40% (mean = 28.55%) of the potential production. Time series grazing experiments conducted at 6 h intervals did not show any diel patterns of grazing by microzooplankton. Our data show that microzooplankton grazing at the ice edge were not sufficient to prevent chlorophyll a accumulation in regions dominated by rnicrophytoplankton. Here, the major biological routes for the uptake of carbon therefore appear to be grazing by metazoans or the sedimentation of phytoplankton cells. Under these conditions, the biological pump will be relatively efficient in the drawdown of atmospheric CO2.