Phytoplankton Patchiness Research Articles

Nutrient Patchiness, Phytoplankton Surge-Uptake, and Turbulent History: A Theoretical Approach and Its Experimental Validation

Despite ample evidence of micro- and small-scale (i.e., millimeter- to meter-scale) phytoplankton and zooplankton patchiness in the ocean, direct observations of nutrient distributions and the ecological importance of this phenomenon are still relatively scarce. In this context, we first describe a simple procedure to continuously sample nutrients in surface waters, and subsequently provide evidence of the existence of microscale distribution of ammonium in the ocean. We further show that ammonium is never homogeneously distributed, even under very high conditions of turbulence. Instead, turbulence intensity appears to control nutrient patchiness, with a more homogeneous or a more heterogeneous distribution observed under high and low turbulence intensities, respectively, under the same concentration in nutrient. Based on a modelling procedure taking into account the stochastic properties of intermittent nutrient distributions and observations carried out on natural phytoplankton communities, we introduce and verify the hypothesis that under nutrient limitation, the “turbulent history” of phytoplankton cells, i.e., the turbulent conditions they experienced in their natural environments, conditions their efficiency to uptake ephemeral inorganic nitrogen patches of different concentrations. Specifically, phytoplankton cells exposed to high turbulence intensities (i.e., more homogeneous nutrient distribution) were more efficient to uptake high concentration nitrogen pulses (2 µM). In contrast, under low turbulence conditions (i.e., more heterogeneous nutrient distribution), uptake rates were higher for low concentration nitrogen pulses (0.5 µM). These results suggest that under nutrient limitation, natural phytoplankton populations respond to high turbulence intensities through a decrease in affinity for nutrients and an increase in their transport rate, and vice versa.

Emergence of phytoplankton patchiness at small scales in mild turbulence

Phytoplankton often encounter turbulence in their habitat. As most toxic phytoplankton species are motile, resolving the interplay of motility and turbulence has fundamental repercussions on our understanding of their own ecology and of the entire ecosystems they inhabit. The spatial distribution of motile phytoplankton cells exhibits patchiness at distances of decimeter to millimeter scales for numerous species with different motility strategies. The explanation of this general phenomenon remains challenging. Furthermore, hydrodynamic cell-cell interactions, which grow more relevant as the density in the patches increases, have been so far ignored. Here, we combine particle simulations and continuum theory to study the emergence of patchiness in motile microorganisms in three dimensions. By addressing the combined effects of motility, cell-cell interaction, and turbulent flow conditions, we uncover a general mechanism: The coupling of cell-cell interactions to the turbulent dynamics favors the formation of dense patches. Identification of the important length and time scales, independent from the motility mode, allows us to elucidate a general physical mechanism underpinning the emergence of patchiness. Our results shed light on the dynamical characteristics necessary for the formation of patchiness and complement current efforts to unravel planktonic ecological interactions.

Large-eddy simulation of subsurface phytoplankton dynamics: an optimum condition for chlorophyll patchiness induced by Langmuir circulations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 593:15-27 (2018) - DOI: https://doi.org/10.3354/meps12490 Large-eddy simulation of subsurface phytoplankton dynamics: an optimum condition for chlorophyll patchiness induced by Langmuir circulations Ashley Brereton1,*, Joseph Siddons2, David M. Lewis2 1National Oceanography Centre, Joseph Proudman Building, 6 Brownlow Street, Liverpool L3 5DA, UK 2Department of Mathematics, University of Liverpool, Liverpool L69 7ZL, UK *Corresponding author: ashbre@noc.ac.uk ABSTRACT: Phytoplankton patchiness occurs on a plethora of spatial and temporal scales, and can be extremely patchy in both horizontal and vertical directions. This patchiness directly affects the dynamics of the overall bloom; therefore, understanding the mechanisms behind the occurrence of patchiness on each scale is integral to the understanding of plankton bloom dynamics as a whole. This modelling study introduces a mechanism for patch formations, which has received little attention, but is ubiquitous to the oceanic mixed-layer, Langmuir circulations and their interaction with nutrient upwellings to induce patchiness in the plankton. By combining a large-eddy simulation, which resolves Langmuir circulations, with a nutrient-phytoplankton-zooplankton biological model, one can examine the horizontal and vertical patchiness that results from a flux of nutrients into the bottom of the mixed layer. Here, phytoplankton form significant horizontal patchiness at a depth interval where vertical currents from Langmuir cells are apparent and turbulent mixing is not; this comprises the lower region of the surface mixed layer. Aggregations have frequently been observed in lower regions of the surface mixed layer and have been attributed to the high nutrient flux associated with the pycnocline. This modelling study also shows patches occurring in this region and it is hypothesised that Langmuir cells are a catalyst for patchiness. The results clearly demonstrate that for certain levels of wind forcing, which are strong enough to introduce turbulent mixing only to the upper part of the mixed layer whilst inducing deeper Langmuir circulation, patchiness is greatly enhanced. KEY WORDS: Plankton patchiness · Oceanic mixed layer turbulence · Langmuir turbulence · Vertical mixing · Wind forcing · NPZ model · Nutrient upwelling Full text in pdf format PreviousNextCite this article as: Brereton A, Siddons J, Lewis DM (2018) Large-eddy simulation of subsurface phytoplankton dynamics: an optimum condition for chlorophyll patchiness induced by Langmuir circulations. Mar Ecol Prog Ser 593:15-27. https://doi.org/10.3354/meps12490 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 593. Online publication date: April 12, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Functional redundancy modifies species-area relationship for freshwater phytoplankton.

Although species–area relationship (SAR) is among the most extensively studied patterns in ecology, studies on aquatic and/or microbial systems are seriously underrepresented in the literature. We tested the algal SAR in lakes, pools and ponds of various sizes (10−2–108 m2) and similar hydromorphological and trophic characteristics using species‐specific data and functional groups. Besides the expectation that species richness increases monotonously with area, we found a right‐skewed hump‐shaped relationship between the area and phytoplankton species richness. Functional richness however did not show such distortion. Differences between the area dependence of species and functional richness indicate that functional redundancy is responsible for the unusual hump‐backed SAR. We demonstrated that the Small Island Effect, which is a characteristic for macroscopic SARs can also be observed for the phytoplankton. Our results imply a so‐called large lake effect, which means that in case of large lakes, wind‐induced mixing acts strongly against the habitat diversity and development of phytoplankton patchiness and finally results in lower phytoplankton species richness in the pelagial. High functional redundancy of the groups that prefer small‐scale heterogeneity of the habitats is responsible for the unusual humpback relationship. The results lead us to conclude that although the mechanisms that regulate the richness of both microbial communities and communities of macroscopic organisms are similar, their importance can be different in micro‐ and macroscales.

Patchiness of phytoplankton and primary production in Liaodong Bay, China.

A comprehensive study of water quality, phytoplankton biomass, and photosynthetic rates in Liaodong Bay, China, during June and July of 2013 revealed two large patches of high biomass and production with dimensions on the order of 10 km. Nutrient concentrations were above growth-rate-saturating concentrations throughout the bay, with the possible exception of phosphate at some stations. The presence of the patches therefore appeared to reflect the distribution of water temperature and variation of light penetration restricted by water turbidity. There was no patch of high phytoplankton biomass or production in a third, linear patch of water with characteristics suitable for rapid phytoplankton growth; the absence of a bloom in that patch likely reflected the fact that the width of the patch was less than the critical size required to overcome losses of phytoplankton to turbulent diffusion. The bottom waters of virtually all of the eastern half of the bay were below the depth of the mixed layer, and the lowest bottom water oxygen concentrations, 3–5 mg L–1, were found in that part of the bay. The water column in much of the remainder of the bay was within the mixed layer, and oxygen concentrations in both surface and bottom waters exceeded 5 mg L–1.

Detecting small–scale horizontal gradients in the upper ocean using wavelet analysis

Mesoscale and submesoscale eddies and fronts in the upper ocean are often closely coupled with biogeochemical processes. Improved instrumentation provides high–resolution data in both the horizontal and vertical capturing this large range of scales (1–100 km), but novel analysis methods are still needed to take full advantage of this advancement. A new method using wavelet analysis is therefore proposed to identify the horizontal scales at which biophysical interactions occur, defined by concurrent fluctuations in temperature and phytoplankton patchiness. The method is applied to temperature and chlorophyll–a fluorescence data measured in the North Sea’s German Bight during early spring using a towed undulating vehicle. The wavelet analysis identified the scale and location of individual features characterized by horizontal gradients of temperature and chlorophyll–a fluorescence. Applied to multiple transects, the method can also retrieve the statistics of relevant biophysical scales in a particular region. The combined analysis of seven transects suggests that physical and biogeochemical tracers tend to align at scales of 3–15 km in the German Bight, highlighting the likely relevance of submesoscale processes in this region. In general, the proposed wavelet analysis method is shown to be a robust tool for the analysis of biophysical interactions across a range of scales.

Plankton patchiness investigated using simultaneous nitrate and chlorophyll observations

AbstractThe complex patterns observed in marine phytoplankton distributions arise from the interplay of biological and physical processes, but the nature of the balance remains uncertain centuries after the first observations. Previous observations have shown a consistent trend of decreasing variability with decreasing length scale. Influenced by similar scaling found for the properties of the water that the phytoplankton inhabit, “universal” theories have been proposed that simultaneously explain the variability seen from meters to hundreds of kilometers. However, data on the distribution of phytoplankton alone have proved insufficient to differentiate between the many causal mechanisms that have been suggested. Here we present novel observations from a cruise in the North Atlantic in which fluorescence (proxy for phytoplankton), nitrate, and temperature were measured simultaneously at scales from 10 m to 100 km for the first time in the open ocean. These show a change in spectra between the small scale (10–100 m) and the mesoscale (10–100 km) which is different for the three tracers. We discuss these observations in relation to the current theories for phytoplankton patchiness.

Physical and optical properties of phytoplankton-rich layers in a coastal fjord: a step toward prediction and strategic sampling of plankton patchiness

Dense aggregations of phytoplankton in layers or patches alter the optical and physical properties of the water column and result in significant hetero- geneity in trophic and demographic rates of local plank- ton populations. Determining the factors driving patch formation, persistence, intensity, and dissipation is key to understanding the ramifications of plankton patchi- ness in marine systems. Regression and multi-parametric statistical analyses were used to identify the physical and optical properties associated with 71 phytoplank- ton-rich layers (PRLs) identified from 158 CTD profiles collected between 2008 and 2010 in East Sound, Wash- ington, USA. Generalized additive models (GAMs) were used to explore water column properties associated with and characterizing PRLs. Patch presence was associated with increasing water column stability represented by the Brunt-Vaisala frequency (N 2 ), Thorpe scale (Lt), and turbulent energy dissipation rate (e). A predictive re- gression identified patch presence with 100% accuracy when log10(N 2 ) = �1 and 70% of the cases when log10(e) = �3. A GAM of passively measured variables, which did not include fluorescence, was able to model patch inten- sity with considerable agreement (R 2 = 0.58), and the fit was improved by including fluorescence (R 2 = 0.69). Flu- orescence alone was an insufficient predictor of PRLs, due in part to the influence of non-photochemical quenching (NPQ) in surface waters and the wide range of fluorescence intensities observed. The results show that a multi-parametric approach was necessary to char- acterize phytoplankton patches and that physical struc- ture, resulting in steep gradients in bio-optical proper- ties, hold greater predictive power than bio-optical properties alone. Integration of these analytical ap- proaches will aid theoretical studies of phytoplankton patchiness but also improve sampling strategies in the field that utilize autonomous, in situ instrumentation.

A simple optical index shows spatial and temporal heterogeneity in phytoplankton community composition during the 2008 North Atlantic Bloom Experiment

Abstract. The ratio of two in situ optical measurements – chlorophyll fluorescence (Chl F) and optical particulate backscattering (bbp) – varied with changes in phytoplankton community composition during the North Atlantic Bloom Experiment in the Iceland Basin in 2008. Using ship-based measurements of Chl F, bbp, chlorophyll a (Chl), high-performance liquid chromatography (HPLC) pigments, phytoplankton composition and carbon biomass, we found that oscillations in the ratio varied with changes in plankton community composition; hence we refer to Chl F/bbp as an "optical community index". The index varied by more than a factor of 2, with low values associated with pico- and nanophytoplankton and high values associated with diatom-dominated phytoplankton communities. Observed changes in the optical index were driven by taxa-specific chlorophyll-to-autotrophic carbon ratios and by physiological changes in Chl F associated with the silica limitation. A Lagrangian mixed-layer float and four Seagliders, operating continuously for 2 months, made similar measurements of the optical community index and followed the evolution and later demise of the diatom spring bloom. Temporal changes in optical community index and, by implication, the transition in community composition from diatom to post-diatom bloom communities were not simultaneous over the spatial domain surveyed by the ship, float and gliders. The ratio of simple optical properties measured from autonomous platforms, when carefully validated, provides a unique tool for studying phytoplankton patchiness on extended temporal scales and ecologically relevant spatial scales and should offer new insights into the processes regulating patchiness.

Gyrotactic trapping in laminar and turbulent Kolmogorov flow

Phytoplankton patchiness, namely the heterogeneous distribution of microalgae over multiple spatial scales, dramatically impacts marine ecology. A spectacular example of such heterogeneity occurs in thin phytoplankton layers (TPLs), where large numbers of photosynthetic microorganisms are found within a small depth interval. Some species of motile phytoplankton can form TPLs by gyrotactic trapping due to the interplay of their particular swimming style (directed motion biased against gravity) and the transport by a flow with shear along the direction of gravity. Here we consider gyrotactic swimmers in numerical simulations of the Kolmogorov shear flow, both in laminar and turbulent regimes. In the laminar case, we show that the swimmer motion is integrable and the formation of TPLs can be fully characterized by means of dynamical systems tools. We then study the effects of rotational Brownian motion or turbulent fluctuations (appearing when the Reynolds number is large enough) on TPLs. In both cases, we show that TPLs become transient, and we characterize their persistence.

The consequences of internal waves for phytoplankton focusing on the distribution and production of Planktothrix rubescens.

Consequences of internal wave motion for phytoplankton and in particular for the distribution and production of the harmful and buoyant cyanobacterium Planktothrix rubescens were investigated based on data from two field campaigns conducted in Lake Ammer during summer 2009 and 2011. In both years, P. rubescens dominated the phytoplankton community and formed a deep chlorophyll maximum (DCM) in the metalimnion. Internal wave motions caused vertical displacement of P. rubescens of up to 6 m and 10 m, respectively. Vertical displacements of isotherms and of iso-concentration lines of P. rubescens from the same depth range coincided, suggesting that P. rubescens did not or could not regulate its buoyancy to prevent wave-induced vertical displacements. Diatoms dominated the phytoplankton community in the epilimnion and were vertically separated from P. rubescens. The thickness of the diatom layer, but not the diatom concentrations within the layer, changed in phase with the changes in the thickness of the epilimnion caused by internal wave motions. Seiche induced vertical displacements of P. rubescens caused fluctuations in the light intensity available at the depth of the P. rubescens layer. The interplay between seiche induced vertical displacements of the P. rubescens layer and the daily cycle of incident light lead to differences in the daily mean available light intensity between lake ends by up to a factor of ∼3. As a consequence, the daily mean specific oxygen production rate of P. rubescens differed by up to a factor of ∼7 between lake ends. The horizontal differences in the specific oxygen production rate of P. rubescens were persistent over several days suggesting that the associated production of P. rubescens biomass may lead to phytoplankton patchiness. The effect of internal seiches on the spatial heterogeneity and the persistence of horizontal differences in production, however, depend on the timing and the synchronization between internal wave motion and the daily course of incident light intensity. Vertical displacements caused by internal waves could be distinguished from other factors influencing the distribution of P. rubescens (e.g. active buoyancy control, production, vertical mixing) by a temperature-based data transformation. This technique may be of general use for separating wave-induced transport from other processes (e.g. sedimentation, vertical mixing) that affect the distributions of dissolved substances and suspended particles.

Phytoplankton patchiness during spring intermonsoon in western coast of South China Sea

Jet-like phytoplankton blooms usually occur off the southwestern coast of the South China Sea (SCS) caused by strong winds during summer monsoons. However a jet-like phytoplankton patch was observed in the western SCS in the spring intermonsoon of 2010 in both field and remote sensing data. The present study investigated the biological processes associated with this spring phytoplankton patchiness. The data showed that chlorophyll a concentrations increased in the surface water, extending out to the SCS, and the depth of the subsurface chlorophyll maximum uplifted from 75m to 50m depth; low dissolved oxygen, low pH and nutrient enrichment (nitrate+nitrite and soluble reactive phosphate) were observed in the subsurface water (50 to ~200m depth). Data analysis showed that variations in chlorophyll a, nutrients and temperature in the water column were related to wind-stress curl: the spatial distribution pattern and vertical structure of the phytoplankton patchiness were controlled by vertical flux of nutrients caused by curl-driven upwelling through Ekman pumping. There was a high correlation between chlorophyll a concentration and wind-stress curl where the influence of nutrient influx from the coast was limited. This study shows the importance of wind-stress curl in providing nutrients to support phytoplankton growth during the spring intermonsoon along the western coast of SCS. It may help to better understand the role of wind in marine biological processes.

Phytoplankton patchiness in Winam Gulf, Lake Victoria: a study using principal component analysis of in situ fluorescent excitation spectra

Summary1. In order to characterise phytoplankton patchiness at fine scales, a profiling multiwavelength fluorometer was cast at numerous locations throughout Winam Gulf in Lake Victoria to measure fluorescent excitation spectra, which are indicators of both phytoplankton diversity and coloured dissolved organic matter (CDOM).2. Processing the spectral data with principal component analysis (PCA) revealed that linear combinations of four fundamental ‘base’ spectra could explain almost all of the variation in spectral measurements. Three of the base spectra were associated with spatially distinct patches of phytoplankton containing different species assemblages, while the fourth base spectrum was due to CDOM fluorescence.3. The locations of the phytoplankton patches were traced to the south‐east of Winam Gulf, the western end of the Rusinga Channel and the open waters of Lake Victoria adjacent to Winam Gulf, respectively. The high CDOM fluorescence was traced mainly to relatively deep water in the Rusinga Channel.4. The phytoplankton and CDOM patchiness were interpreted in the context of physical and chemical gradients that were measured at the site at the same scale as the spectral data. Strong relationships were found between the gradients in spectral data and other environmental variables, which suggested several underlying explanations for the phytoplankton and CDOM patchiness.

Spatial Variation and Community Composition of Phytoplankton along the Pahang Estuary, Malaysia

The aim of this study was to examine the phytoplankton patchiness, as expressed by community composition and their diversity during the monsoon and Non-monsoon seasons along the Pahang estuary, Malaysia during 2008. Three zones in the estuary were sampled on both the seasons covering both upstream and downstream of the estuary. Among the identified 42 genera of phytoplankton, the most dominant phytoplankton were Leptocylindrus sp. (19.05%), Lauderia sp. (11.02%) and Skeletonema sp. (10.32%) of total abundance. The least dominant or rare phytoplankton were Ditylum sp. (0.05%), Asterionella sp. (0.04%), Gyrosigma sp. (0.05%), Gymnodinium sp. (0.04%) and Prorocentrum sp. (0.07%) of total abundance. The statistical analysis showed that there was no significant difference in diversity of phytoplankton between different zones (p>0.05). Result indicated that many of the phytoplankton species inhabiting in Pahang estuary were sensitive to salinity fluctuation. The increased diversity during monsoon might be due to the runoff water that probably brought the phytoplankton of other freshwater sources. Physicochemical parameters showed significant variation between different zones (p<0.05) except for temperature. Present study clearly showed that phytoplankton community in Pahang estuary was quiet diverse during monsoon and dominant during non monsoon season.

Multifractal analysis of oceanic chlorophyll maps remotely sensed from space

Abstract. Phytoplankton patchiness has been investigated with multifractal analysis techniques. We analyzed oceanic chlorophyll maps, measured by the SeaWiFS orbiting sensor, which are considered to be good proxies for phytoplankton. The study area is the Senegalo-Mauritanian upwelling region, because it has a low cloud cover and high chlorophyll concentrations. Multifractal properties are observed, from the sub-mesoscale up to the mesoscale, and are found to be consistent with the Corssin-Obukhov scale law of passive scalars. This result indicates that, in this specific region and within this scale range, turbulent mixing would be the dominant effect leading to the observed variability of phytoplankton fields. Finally, it is shown that multifractal patchiness can be responsible for significant biases in the nonlinear source and sink terms involved in biogeochemical numerical models.

Sufficient noise and turbulence can induce phytoplankton patchiness

Phytoplankton patchiness ubiquitously obser- ved in marine ecosystems is a simple phy- sical phenomenon. Only two factors are required for its formation: one is persistent variations of inhomogeneous distributions in the phytopl- ankton population and the other is turbulent stirring by eddies. It is not necessary to assume continuous oscillations such as limit cycles for realization of the first factor. Instead, a certain amount of noise is enough. Random fluctua-tions by environmental noise and turbulent ad-vection by eddies seem to be common in open oceans. Based on these hypotheses, we pro-pose seemingly the simplest method to simulate patchiness formation that can create realistic images. Sufficient noise and turbulence can induce patchiness formation even though the system lies on the stable equilibrium conditions. We tentatively adopt the two-component model with nutrients and phytoplankton, however, the choice of the mathematical model is not essen-tial. The simulation method proposed in this study can be applied to whatever model with stable equilibrium states including one-com-ponent ones.

Short-term variability in the phytoplankton and physico-chemical variables in a high-tidal regime, Bahía Blanca Estuary, Argentina

Corresponding autor: vguinder@criba.edu.ar Hydrological processes such as advection, resuspension, estuarine circulation, internal waves, tidal stirring, vertical mixing and stratification are the main physical forces affecting the phytoplankton distribution in estuaries (DENMAN; GARGETT, 1983; CLOERN et al., 1989; CUSHING 1989; DE JONGE; VAN BEUSEKOM, 1992). Vertical and horizontal transport of phytoplankton in coastal areas could be induced by winds, water mass intrusions from the open sea, turbulence, freshwater inflow and tides (CLOERN et al., 1989; LI; SMAYDA, 2001; BRUNET; LIZON, 2003; HAGY III et al., 2005; POPOVICH; MARCOVECCHIO, 2008; POPOVICH et al., 2008b) and these sorts of transport are related to biomass accumulation and dispersion. Many studies have revealed intratidal variations in phytoplankton dynamics (e.g. DEMERS; LEGENDRE, 1979; DEMERS et al., 1986; DUSTAN; PINCKNEY, 1989; BRUNET; LIZON, 2003) and even differences in biomass between micro and macrotidal estuaries (MONBET, 1992). Moreover, phytoplankton patchiness in estuaries has been associated with the physical influence of tides (DUSTAN; PINCKNEY, 1989). In estuaries characterized by high-tidal energy, a steady phytoplankton composition over short time periods might be expected owing to the intense mixing processes continuously occurring in the water column. However, heterogeneous distributions of phytoplankton cells in the water column on different temporal and spatial scales have been observed (e.g. CUSHING, 1989; BRUNET; LIZON, 2003). Many studies have used the biomass (estimated by the chlorophyll a concentration) as the main intrinsic property to describe the phytoplankton community dynamics during tidal cycles (DENMAN, 1977; FRECHETTE; LEGENDRE, 1982, CLOERN et al., 1989; DUSTAN; PINCKNEY, 1989; MONBET 1992), without taking into consideration the species-specific patterns related to these regular short-time events. Hence, studies based only on phytoplankton biomass variability throughout tidal cycles may overlook the short-term temporal changes in the species composition. The Bahia Blanca Estuary constitutes a mesotidal system with a semidiurnal cycle, the tides being the single most important input of energy (PERILLO; PICCOLO, 1991; PERILLO et al., 2004). The temporal and spatial dynamics of the phytoplankton in the estuary have been extensively studied (GAYOSO, 1998, 1999; POPOVICH; MARCOVECCHIO, 2008; POPOVICH et al. 2008a, 2008b) but none of these works has considered the short-term variability associated with the tidal cycle. The main goal of this study was to describe the temporal phytoplankton and physico-chemical dynamics over a short-time period at a fixed station under strong tidal influence. This study presents the first integrated description of daily phytoplankton-community dynamics together with physical and chemical variables in the Bahia Blanca Estuary. Beside its local relevance, it is also a contribution to the understanding of the short-term variability in estuaries characterized by high tidal energy. Bahia Blanca is a coastal plain estuary (38°45´S; 62°22´W) on the Atlantic coast of Argentina (Fig. 1). Tidal currents are reversible with maximum surface velocities of about 1.3 m s

Intense benthic grazing of phytoplankton in a coral reef

Grazing on phytoplankton by a section of the fringing coral reef in Eilat, the Gulf of Aqaba, Red Sea, was studied using the control‐volume approach, a direct, nonintrusive technique common in engineering studies. Concentrations of chlorophyll a were measured using four vertical arrays of pumps, 10 pumps on each, that defined an imaginary box (i.e., the control volume), extending from the bed to surface, overlying a section of the sloping fore‐reef &gt;100 m2 in size. Concurrent velocity measurements were made at the center of the box using a vertical array of acoustic current meters. Together, the concentration and velocity measurements allowed us to compute phytoplankton fluxes in and out of the control volume. While phytoplankton patchiness and other turbulent variations in concentration introduced significant variability, mass balances of fluxes into and out of the box indicate a flow‐dependent grazing rate of approximately 10 m d−1 to 20 m d−1. This value was larger than, but not statistically different from, values derived from an "a priori estimate" (calculated based on the density and feeding rates of benthic grazers) and an estimate computed from the observed near‐bed gradient of Chl a. Phytoplankton grazing is a principal pathway through which allochthonous nutrients are imported to the reef community from the flowing water.

Sub meso scale phytoplankton distribution in the North East Atlantic surface waters determined with an automated flow cytometer

Abstract. Phytoplankton cells in the size range ~1–50 μm were analysed in surface waters using an automated flow cytometer, the Cytosub (http://www.cytobuoy.com), from the Azores to the French Brittany during spring 2007. The Cytosub records the pulse shape of the optical signals generated by phytoplankton cells when intercepted by the laser beam. A total of 6 distinct optical groups were resolved during the whole transect, and the high frequency sampling (15 min) provided evidence for the cellular cycle (based on cyclic changes in cell size and fluorescence) and distribution changes linked to the different water characteristics crossed in the North East Atlantic provinces. Nutrient concentrations and mixed layer depth varied from west to east, with a decrease in the mixed layer depth and high nutrient concentrations in the middle of the transect as well as near the French coast. Data provided a link between the sub meso scale processes and phytoplankton patchiness, some abundance variations due to the cellular cycle can be pointed out. The high frequency spatial sampling encompasses temporal variations of the phytoplankton abundance, offering a better insight into phytoplankton distribution.

Phytoplankton patchiness and their role in the modelled productivity of a large, seasonally stratified lake

Phytoplankton concentration in Lake Kinneret (Israel) has varied up to 10-fold in space and time, with horizontal patches ranging from a couple of kilometres to a basin scale. Previous studies have used a 1D model to reproduce the temporal evolution of physical and biogeochemical variables in this lake. The question that arises then is how appropriate is a 1D approach to represent the dynamic of a spatially heterogeneous system, where there are non-linear dependencies between variables. Field data, a N-P-Z model coupled to both a 1D and a 3D hydrodynamic model, a 1D diffusion-reaction equation and scaling analysis are used to understand the role of spatial variability, expressed as phytoplankton patchiness, in the modelling of primary production. The analysis and results are used to investigate the effect of horizontal variability in the forcing and in the free mechanisms that affect the growth of patterns. The study shows that the use of averaged properties in a 1D approach may produce misleading results in the presence of localised patches, in terms of both concentration and composition of phytoplankton. The reason lies in the fact that the calibration process of ecological parameters in the 1D model appears to be site and process specific. That is, it depends on the pattern's characteristics and the underlying physical processes causing them. And this is a critical point for the success of numerical simulations under spatial variability. In this study, it is also shown that a length scale based on diffusion and growth rate of phytoplankton could be used as a criterion to assess the appropriateness of the 1D assumption.