Variation of phytoplankton assemblages along the Mozambique coast as revealed by HPLC and microscopy

This study is an integrated overview of pigment and microscopic analysis of phytoplankton communities throughout the Mozambican coast. Collected samples revealed notable patterns of phytoplankton occurrence and distribution, with community structure changing between regions and sample depth. Pigment data showed Delagoa Bight, Sofala Bank and Angoche as the most productive regions throughout the sampled area. In general, micro-sized phytoplankton, particularly diatoms, were important contributors to biomass both at surface and sub-surface maximum (SSM) samples, although were almost absent in the northern stations. In contrast, nano- and pico-sized phytoplankton revealed opposing patterns. Picophytoplankton were most abundant at surface, as opposed to nanophytoplankton, which were more abundant at the SSM. Microphytoplankton were associated with cooler southern water masses, while picophytoplankton were related to warmer northern water masses. Nanophytoplankton were found to increase their contribution to biomass with increasing SSM. Microscopy information on the genera and species level revealed the diatoms Chaetoceros spp., Proboscia alata, Pseudo-nitzschia spp., Cylindrotheca closterium and Hemiaulus haukii as the most abundant taxa of the micro-sized phytoplankton. Discosphaera tubifera and Emiliania huxleyi were the most abundant coccolithophores, nano-sized phytoplankton.

Theoretical analysis of ocean color radiances anomalies and implications for phytoplankton groups detection in case 1 waters

Past years have seen the development of different approaches to detect phytoplankton groups from space. One of these methods, the PHYSAT one, is empirically based on reflectance anomalies. Despite observations in good agreement with in situ measurements, the underlying theoretical explanation of the method is still missing and needed by the ocean color community as it prevents improvements of the methods and characterization of uncertainties on the inversed products. In this study, radiative transfer simulations are used in addition to in situ measurements to understand the organization of the signals used in PHYSAT. Sensitivity analyses are performed to assess the impact of the variability of the following three parameters on the reflectance anomalies: specific phytoplankton absorption, colored dissolved organic matter absorption, and particles backscattering. While the later parameter explains the largest part of the anomalies variability, results show that each group is generally associated with a specific bio-optical environment which should be considered to improve methods of phytoplankton groups detection.

Temporal changes in plankton of the North Sea: community shifts and environmental drivers

This paper analyses long-term and seasonal changes in the North Sea plankton community during the period 1970 to 2008. Based on Continuous Plankton Recorder (CPR) data covering 38 yr, major changes in both phytoplankton and zooplankton abundance and community structure were identified. Regime changes were detected around 1978, 1989 and 1998. The first 2 changes have been discussed in the literature and are defined as a cold episodic event (1978) and a regime shift towards a warm dynamic regime (1989). The effect of these 2 regime changes on plankton indicators was assessed and checked against previous studies. The 1998 change represents a shift in the abundance and seasonal patterns of dinoflagellates and the dominant zooplankton group, the neritic copepods. Furthermore, environmental factors such as air temperature, wind speed and the North Atlantic water inflow were identified as potential drivers of change in seasonal patterns, and the most-likely environmental causes for detected changes were assessed. We suggest that a change in the balance of dissolved nutrients driven by these environmental factors was the cause of the latest change in plankton community structure, which in turn could have affected the North Sea fish community.

Decadal fluctuations in North Atlantic water inflow in the North Sea between 1958-2003: impacts on temperature and phytoplankton populations

The circulation of Atlantic water along the European continental slope, in particular the inflow into the North Sea, influences North Sea water characteristics with consequent changes in the environment affecting plankton community dynamics. The long-term effect of fluctuating oceanographic conditions oil the North Sea, pelagic ecosystem is assessed. It is shown that (i) there are similar regime shifts in the inflow through the northern North Sea and in Sea, Surface Temperature, (ii) long-term phytoplankton trends are influenced by the inflow only in some North Sea regions, and (iii) the spatial variability in chemicophysical and biological parameters highlight the influence of smaller scale processes.

The emergence of automated high-frequency flow cytometry: revealing temporal and spatial phytoplankton variability

Phytoplankton observation is the product of a number of trade-offs related to sampling processes, required level of diversity and size spectrum analysis capabilities of the techniques involved. Instruments combining the morphological and high-frequency analysis for phytoplankton cells are now available. This paper presents an application of the automated high-resolution flow cytometer Cytosub as a tool for analysing phytoplanktonic cells in their natural environment. High resolution data from a temporal study in the Bay of Marseille (analysis every 30 min over 1 month) and a spatial study in the Southern Indian Ocean (analysis every 5 min at 10 knots over 5 days) are presented to illustrate the capabilities and limitations of the instrument. Automated high-frequency flow cytometry revealed the spatial and temporal variability of phytoplankton in the size range 1−∼50 μm that could not be resolved otherwise. Due to some limitations (instrumental memory, volume analysed per sample), recorded counts could be statistically too low. By combining high-frequency consecutive samples, it is possible to decrease the counting error, following Poisson’s law, and to retain the main features of phytoplankton variability. With this technique, the analysis of phytoplankton variability combines adequate sampling frequency and effective monitoring of community changes.

Effect of phytoplankton size classes on bio-optical properties of phytoplankton in the Western Iberian coast: Application of models

Chlorophyll-a satellite products are routinely used in oceanography, providing a synoptic and global view of phytoplankton abundance. However, these products lack information on the community structure of the phytoplankton, which is crucial for ecological modelling and ecosystem studies. To assess the usefulness of existing methods to differentiate phytoplankton functional types (PFT) or phytoplankton size classes from satellite data, in-situ phytoplankton samples collected in the Western Iberian coast, on the North-East Atlantic, were analysed for pigments and absorption spectra. Water samples were collected in five different locations, four of which were located near the shore and another in an open-ocean, seamount region. Three different modelling approaches for deriving phytoplankton size classes were applied to the in situ data. Approaches tested provide phytoplankton size class information based on the input of pigments data (Brewin et al., 2010), absorption spectra data (Ciotti et al., 2002) or both (Uitz et al., 2008). Following Uitz et al. (2008), results revealed high variability in microphytoplankton chlorophyll-specific absorption coefficients, ranging from 0.01 to 0.09 m2 (mg chl)− 1 between 400 and 500 nm. This spectral analysis suggested, in one of the regions, the existence of small cells (< 20 μm) in the fraction of phytoplankton presumed to be microphytoplankton (based on diagnostic pigments). Ciotti et al. (2002) approach yielded the highest differences between modelled and measured absorption spectra for the locations where samples had high variability in community structure and cell size. The Brewin et al. (2010) pigment-based model was adjusted and a set of model coefficients are presented and recommended for future studies in offshore water of the Western Iberian coast.

Satellite estimates of net community production indicate predominance of net autotrophy in the Atlantic Ocean

There is ongoing debate as to whether the oligotrophic ocean is predominantly net autotrophic and acts as a CO2 sink, or net heterotrophic and therefore acts as a CO2 source to the atmosphere. This quantification is challenging, both spatially and temporally, due to the sparseness of measurements. There has been a concerted effort to derive accurate estimates of phytoplankton photosynthesis and primary production from satellite data to fill these gaps; however there have been few satellite estimates of net community production (NCP). In this paper, we compare a number of empirical approaches to estimate NCP from satellite data with in vitro measurements of changes in dissolved O2 concentration at 295 stations in the N and S Atlantic Ocean (including the Antarctic), Greenland and Mediterranean Seas. Algorithms based on power laws between NCP and particulate organic carbon production (POC) derived from 14C uptake tend to overestimate NCP at negative values and underestimate at positive values. An algorithm that includes sea surface temperature (SST) in the power function of NCP and 14C POC has the lowest bias and root-mean square error compared with in vitro measured NCP and is the most accurate algorithm for the Atlantic Ocean. Nearly a 13 year time series of NCP was generated using this algorithm with SeaWiFS data to assess changes over time in different regions and in relation to climate variability. The North Atlantic subtropical and tropical Gyres (NATL) were predominantly net autotrophic from 1998 to 2010 except for boreal autumn/winter, suggesting that the northern hemisphere has remained a net sink for CO2 during this period. The South Atlantic subtropical Gyre (SATL) fluctuated from being net autotrophic in austral spring-summer, to net heterotrophic in austral autumn–winter. Recent decadal trends suggest that the SATL is becoming more of a CO2 source. Over the Atlantic basin, the percentage of satellite pixels with negative NCP was 27%, with the largest contributions from the NATL and SATL during boreal and austral autumn–winter, respectively. Variations in NCP in the northern and southern hemispheres were correlated with climate indices. Negative correlations between NCP and the multivariate ENSO index (MEI) occurred in the SATL, which explained up to 60% of the variability in NCP. Similarly there was a negative correlation between NCP and the North Atlantic Oscillation (NAO) in the Southern Sub-Tropical Convergence Zone (SSTC),which explained 90% of the variability. There were also positive correlations with NAO in the Canary Current Coastal Upwelling (CNRY) and Western Tropical Atlantic (WTRA)which explained 80% and 60% of the variability in each province, respectively. MEI and NAO seem to play a role in modifying phases of net autotrophy and heterotrophy in the Atlantic Ocean.

High-resolution view of the spring bloom initiation and net community production in the Subantarctic Southern Ocean using glider data

In the Southern Ocean, there is increasing evidence that seasonal to subseasonal temporal scales, and meso- to submesoscales play an important role in understanding the sensitivity of ocean primary productivity to climate change. This drives the need for a high-resolution approach to re- solving biogeochemical processes. In this study, 5.5 months of continuous, high-resolution (3 h, 2 km horizontal resolution) glider data from spring to summer in the Atlantic Subantarctic Zone is used to investigate: (i) the mechanisms that drive bloom initiation and high growth rates in the region and (ii) the seasonal evolution of water column production and respiration. Bloom initiation dates were analysed in the context of upper ocean boundary layer physics highlighting sensitivities of different bloom detection methods to different environmental processes. Model results show that in early spring (September to mid-November) increased rates of net community production (NCP) are strongly affected by meso- to submesoscale features. In late spring/early summer (late-November to mid-December) seasonal shoaling of the mixed layer drives a more spatially homogenous bloom with maximum rates of NCP and chlorophyll biomass. A comparison of biomass accumulation rates with a study in the North Atlantic highlights the sensitivity of phytoplankton growth to fine-scale dynamics and emphasizes the need to sample the ocean at high resolution to accurately resolve phytoplankton phenology and improve our ability to estimate the sensitivity of the biological carbon pump to climate change.

Photoacclimation of natural phytoplankton communities

Phytoplankton regulate internal pigment concentrations in response to light and nutrient availability. Chlorophyll to carbon ratios (Chl:Cphyto) are commonly reported as a function of growth irradiance (Eg) for evaluating the photoacclimation response of phytoplankton. In contrast to most culture experiments, natural phytoplankton communities experience fluctuating environmental conditions making it difficult to compare field and lab observations. Observing and understanding photoacclimation in nature is important for deciphering changes in Chl:Cphyto resulting from environmental forcings and for accurately estimating net primary production (NPP) in models which rely on a parameterized description of photoacclimation. Here we employ direct analytical measurements of Cphyto and parallel high-resolution biomass estimates from particulate backscattering (bbp) and flow cytometry to investigate Chl:Cphyto in natural phytoplankton communities. Chl:Cphyto observed over a wide range of Eg in the field was consistent with photoacclimation responses inferred from satellite observations. Field-based photoacclimation observations for a mixed natural community contrast with laboratory results for single species grown in continuous light and nutrient replete conditions. Applying a carbon-based net primary production (NPP) model to our field data for a north-south transect in the Atlantic Ocean results in estimates that closely match 14C depth-integrated NPP for the same cruise and with historical records for the distinct biogeographic regions of the Atlantic Ocean. Our results are consistent with previous satellite and model observations of cells growing in natural or fluctuating light and showcase how direct measurements of Cphyto can be applied to explore phytoplankton photophysiology, growth rates, and production at high spatial resolution in-situ.

Dynamic responses of the benthic bacterial community at the Western English Channel observatory site L4 are driven by deposition of fresh phytodetritus

The impact of the seasonal deposition of phytoplankton and phytodetritus on surface sediment bacterial abundance and community composition was investigated at the Western English Channel site L4. Sediment and water samples were collected from January to September in 2012, increasing in frequency during periods of high water column phytoplankton abundance. Compared to the past two decades, the spring bloom in 2012 was both unusually long in duration and contained higher than average biomass. Within spring months, the phytoplankton bloom was well mixed through the water column and showed accumulations near the sea bed, as evidenced by flow cytometry measurements of nanoeukaryotes, water column chlorophyll a and the appearance of pelagic phytoplankton at the sediment. Measurements of chlorophyll and chlorophyll degradation products indicated phytoplankton material was heavily degraded after it reached the sediment surface: the nature of the chlorophyll degradation products (predominantly pheophorbide, pyropheophorbide and hydroxychlorophyllone) was indicative of grazing activity. The abundance of bacterial 16S rRNA genes g−1 sediment (used as a proxy for bacterial biomass) increased markedly with the onset of the phytoplankton bloom, and correlated with measurements of chlorophyll at the surface sediment. Together, this suggests that bacteria may have responded to nutrients released via grazing activity. In depth sequencing of the 16S rRNA genes indicated that the composition of the bacterial community shifted rapidly through-out the prolonged spring bloom period. This was primarily due to an increase in the relative sequence abundance of Flavobacteria.

An unusually large phytoplankton spring bloom drives rapid changes in benthic diversity and ecosystem function

In 2012, the Western English Channel experienced an unusually large and long-lived phytoplankton spring bloom. When compared with data from the past 20 years, average phytoplankton biomass at Station L4 (part of the Western Channel Observatory) was approximately 3× greater and lasted 50% longer than any previous year. Regular (mostly weekly) box core samples were collected from this site before, during and after the bloom to determine its impact on macrofaunal abundance, diversity, biomass, community structure and function. The spring bloom of 2012 was shown to support a large and rapid response in the majority of benthic taxa and functional groups. However, key differences in the precise nature of this response, as well as in its timing, was observed between different macrofauna feeding groups. Deposit feeders responded almost instantly at the start of the bloom, primarily thorough an increase in abundance. Suspension feeders and opportunistic/predatory/carnivorous taxa responded slightly more slowly and primarily with an increase in biomass. At the end of the bloom a rapid decline in macrobenthic abundance, diversity and biomass closely followed the decline in phytoplankton biomass. With suspension feeders showing evidence of this decline a few weeks before deposit feeders, it was concluded that this collapse in benthic communities was driven primarily by food availability and competition. However, it is possible that environmental hypoxia and the presence of toxic benthic cyanobacteria could also have contributed to this decline. This study shows evidence for strong benthic–pelagic coupling at L4; a shallow (50 m), coastal, fine-sand habitat. It also demonstrates that in such habitats, it is not just planktonic organisms that demonstrate clear community phenology. Different functional groups within the benthic assemblage will respond to the spring bloom in specific manner, with implications for key ecosystem functions and processes, such as secondary production and bioturbation. Only by taking integrated benthic and pelagic observations over such fine temporal scales (weekly) was the current study able to identify the intimate structure of the benthic response. Similar studies from other habitats and under different bloom conditions are urgently needed to fully appreciate the strength of benthic–pelagic coupling in shallow coastal environments.

Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles

Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs); six types of phytoplankton, three types of zooplankton, and heterotrophic bacteria. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing zooplankton, and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean High Nutrient Low Chlorophyll (HNLC) region during summer. When model simulations do not represent crustacean macrozooplankton grazing, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there was no iron deposition from dust. When model simulations included the developments of the zooplankton component, the simulation of phytoplankton biomass improved and the high chlorophyll summer bias in the Southern Ocean HNLC region largely disappeared. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community rather than iron limitation. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.