Microphytobenthic Primary Production Research Articles

A new procedure for autonomous acquisition of photosynthesis-irradiance curves on a microphytobenthic biofilm

Despite their high productivity and their key role in coastal processes, microphytobenthic biofilm studies remain relatively scarce because in situ, meteorological hazards make it difficult to acquire reproducible measurements, also due to difficulties in properly reproducing field conditions in the laboratory. Therefore, in order to better understand the processes of microphytobenthic primary production, we have developed an automated laboratory system and procedure with variable light intensity, with a large number of replicates. This article aims to provide a description of the creation of a P-I curve based on a total of 128 vertical profiles recorded on a sediment core taken in situ, placed in the automated system and studied under controlled conditions of temperature and air humidity while light intensity was varied automatically, thus allowing to work in standard and replicable conditions. With measured production levels of up to 14.68 ± 3.70 mmol O2.m-2.h-1 and a productivity of 0.06 ± 0.01 mmol O2.m-2.h-1 per gram of Chl a corresponding to what is generally found in temperate environments, we have shown that our system is suitable for high frequency measurements and, by combining surficial measurements of modulated fluorescence and oxygen microprofiling in sediments, complementary information from a large dataset on photosynthetic and microphytobenthic migratory activity may be obtained under standard conditions. The development of this tool has made it possible to highlight a stabilization time for oxygen fluxes. For our study conducted in a temperate environment, we observed a time lag of a few minutes that should be considered when acquiring PE curves in the laboratory to study microphytobenthic photosynthetic capacities. This tool also allowed to describe microphytobenthic migration in response to light exposure, with successive steps observed through fluorescence and oxygen profiles. First, microphytobenthos migrated towards the surface until the optimal intensity of production at 475 µmol photons.m-2.s-1, then from this intensity as well as towards 780 µmol photons.m-2.s-1, downwards migratory movements were detected. This system is a working basis which can open interesting perspectives for the study of the effect of other abiotic (or biotic) parameters.

Effect of increased inundation time on intertidal ecosystem functions of South West Atlantic soft-bottom environments

Soft-bottom intertidal systems are valuable due to the ecosystem services they provide. Although these systems are normally subjected to highly variable environmental conditions, disturbances have become more frequent due to increases in anthropogenic pressure. Sea level rise (SLR) may generate changes in the ecosystem functioning of intertidal systems and in the ecosystem services they provide. In the Argentinean coast (SW Atlantic), SLR analysis shows trends similar to the global ones. It is not known how disturbances may affect the activity of benthic communities and the associated ecosystem functions. Therefore, through a field experiment at the marsh-mudflat edge in northeastern Argentina, we evaluated the effect of increased inundation time on bioturbation intensity, detritus decomposition and microphytobenthic primary production. Sediment physicochemical parameters (pH, temperature, water content and organic matter) were also measured. The results showed that increased inundation time had no effect on microphytobenthic primary production or bioturbation intensity in summer. Nevertheless, inundation time increased the burrowing activity of the intertidal crabNeohelice granulatain winter and spring and greatly decreased the variability inSpartina densifloradecomposition and organic matter content. In the context of a changing world due to global climate change, reduced variability in ecosystem functioning could indicate a decline in ecosystem resilience. As a consequence, vital ecosystem functions could be altered or lost, thus impacting the ecosystem services they deliver.

Potential Impact of Photoinhibition on Microphytobenthic Primary Production on a Large Intertidal Mudflat

AbstractMicrophytobenthos (MPB) are a key primary producer of intertidal mudflats. MPB face strong variability in incident irradiance during low tides. Despite photoprotection and photoacclimation, such variations can translate into the photoinhibition of MPB cells. This study explores the effect of photoinhibition on MPB primary production (PP) over a large and productive temperate mudflat (Brouage mudflat, NW France). We used a regional and high‐resolution tri‐dimensional hydrodynamic model coupled to an MPB model with or without photoinhibition. Photoinhibition leads to a 20% (−0.79 × t C) decrease of the simulated MPB PP over the entire mudflat. As the upper shore is exposed to light more frequently and longer than the lower shore, the decrease of MPB PP is higher on the upper shore (−29%) than on the lower shore (−5%). With the highest photosynthetically active radiation cumulated over the mudflat, the decrease of MPB PP due to photoinhibition is the highest during spring and spring tides (−22% and −23%, respectively). The model suggests MPB photoinhibition is sensitive to the photoacclimation status of MPB cells through the light saturation parameter. This first modeling attempt to account for MPB photoinhibition is highly constrained by our current theoretical knowledge and limitations on the MPB growth physiology, but it suggests that this process can have a substantial impact on the MPB PP. As such, assessing the MPB photosynthetic response to the highly variable environmental conditions that prevail in large and productive intertidal mudflats is a real challenge for quantifying MPB PP from a synoptic to inter‐annual time scale.

Microphytobenthic primary production on exposed coastal sandy sediments of the Southern Baltic Sea using ex situ sediment cores and oxygen optodes

The shallow coastal water zone of the tide-less southern Baltic Sea is dominated by exposed sandy sediments which are typically inhabited by microphytobenthic communities, but their primary production is poorly studied, and hence four stations between 3.0 and 6.2 m depth were investigated. Sediment cores were carefully taken to keep the natural layering and exposed in a controlled self-constructed incubator. Respiratory oxygen consumption and photosynthetic oxygen production were recorded applying planar oxygen optode sensors. We hypothesized that with increasing water depths the effects of wind- and wave-induced erosion and mixing of the upper sediment layer are dampened and expected higher microphytobenthic biomass and primary production in the incubated cores.Our data partly confirm this hypothesis, as cores sampled at the most exposed stations contained only 50% chlorophyll a m−2 compared to the deeper stations. However, primary production was highly variable, probably due to fluctuating sediment-disturbing conditions before the cores were taken. Due to these physical forces sand grains were highly mobile and rounded, and small epipsamic benthic diatoms dominated, which preferentially occurred in some cracks and crevices as visualized by scanning electron microscopy. The data fill an important gap in reliable production data for sandy sediments of the southern Baltic Sea, and point to the ecological importance and relevant contribution of microphytobenthic communities to the total primary production of this marine ecosystem. Oxygen planar optode sensor spots proved to be a reliable, sensitive and fast detection system for ex situ oxygen exchange measurements in the overlying water of intact sediment cores.

Water Column Turbidity Not Sediment Nutrient Enrichment Moderates Microphytobenthic Primary Production

Soft sediment intertidal habitats are under intense anthropogenic pressure resulting from increased land derived sediment and nutrient delivery. Long term, this can cause high water column turbidity and nutrient enrichment of sediment porewaters, which has cascading effects on coastal ecosystem functionality. However, how these stressors may interact and influence benthic productivity over alternating periods of submergence and emergence is largely unknown. This study investigates the effects of sediment nutrient enrichment (at three levels for 20 months) on benthic primary production at six sites in four New Zealand estuaries that spanned a gradient in water column turbidity. While nutrient enrichment had no detectable effect on microphytobenthic primary production, water column turbidity had a significant influence, explaining up to 40% of variability during tidal submergence, followed by temperature and sediment characteristics. In addition, negative net primary production (NPP) estimates and therefore net heterotrophy for the most turbid estuaries during tidal submergence resulted in an increased reliance on production during emerged periods, where NPP was positive across all sites. This study highlights the prominent role of water column turbidity over nutrient enrichment in moderating microphytobenthic production, and the increasing importance of emerged periods to maintain the health and functioning of coastal habitats.

Spatiotemporal variability in microphytobenthic primary production across bare intertidal flat, saltmarsh, and mangrove forest of Asia and Australia

The ecological role of intertidal microphytobenthos (MPB) is increasingly recognized in coastal production systems. MPB primary production (PP) measured in coastal wetlands of Korea, Cambodia, and Australia confirmed large variability at the global scale. Surprisingly, MPB biomass in mangrove forests almost doubled those measured in nearby bare tidal flats. However, MPB productivity (Pb) in vegetated habitats was significantly reduced (by ~50%) compared to that on bare tidal flats. Extensive measurements of MPB biomass, PP, and Pb across 12 Korean tidal flats revealed large spatiotemporal variations, suggesting complex sediment-MPB coupled dynamics. The key factors included sediment type, tide, bed elevation, irradiation, temperature, and vegetation. Winter MPB blooms and the elevated Pb seem to be unique characteristics of the Korean intertidal flats. The present study provides the baseline data of MPB PPs in mudflat, saltmarsh, and mangrove habitats in the highly productive zones of the Western Indo-Pacific Rim.

Implications of Glacial Melt-Related Processes on the Potential Primary Production of a Microphytobenthic Community in Potter Cove (Antarctica)

The Antarctic Peninsula experiences a fast retreat of glaciers, which results in an increased release of particles and sedimentation and, thus, a decrease in the available photosynthetic active radiation (PAR, 400-700 nm) for benthic primary production. In this study, we investigated how changes in the general sedimentation and shading patterns affect the primary production by benthic microalgae, the microphytobenthos. In order to determine potential net primary production and respiration of the microphytobenthic community, sediment cores from locations exposed to different sedimentation rates and shading were exposed to PAR of 0-70 mu.mol photons m(-2)s(-1). Total oxygen exchange rates and microphytobenthic diatom community structure, density, and biomass were determined. Our study revealed that while the microphytobenthic diatom density and composition remained similar, the net primary production of the microphytobenthos decreased with increasing sedimentation and shading. By comparing our experimental results with in situ measured PAR intensities, we furthermore identified microphytobenthic primary production as an important carbon source within Potter Cove's benthic ecosystem. We propose that the microphytobenthic contribution to the total primary production may drop drastically due to Antarctic glacial retreat and related sedimentation and shading, with yet unknown consequences for the benthic heterotrophic community, its structure, and diversity.

Diel patterns of microphytobenthic primary production in intertidal sediments: the role of photoperiod on the vertical migration circadian rhythm

Diel primary production patterns of intertidal microphytobenthos (MPB) have been attributed to short-term physiological changes in the photosynthetic apparatus or to diel changes in the photoautotrophic biomass in the sediment photic layer due to vertical migration. Diel changes in primary production and vertical migration are entrained by external factors like photoperiod and tides. However, the role of photoperiod and tides has not been experimentally separated to date. Here, we performed laboratory experiments with sediment cores kept in immersion, in the absence of tides, with photoperiod or under continuous light. Measurements of net production, made with O2 microsensors, and of spectral reflectance at the sediment surface showed that, in intertidal sediments, the photoperiod signal was the major driver of the diel patterns of net primary production and sediment oxygen availability through the vertical migration of the MPB photoautotrophic biomass. Vertical migration was controlled by an endogenous circadian rhythm entrained by photoperiod in the absence of tides. The pattern progressively disappeared after 3 days in continuous light but was immediately reset by photoperiod. Even though a potential contribution of a subjective in situ tidal signal cannot be completely discarded, Fourier and cross spectral analysis of temporal patterns indicated that the photosynthetic circadian rhythm was mainly characterized by light/dark migratory cycles.

Radiative Energy Budgets in a Microbial Mat Under Different Irradiance and Tidal Conditions.

Irradiance and temperature variations during tidal cycles modulate microphytobenthic primary production potentially by changing the radiative energy balance of photosynthetic mats between immersion and emersion and thus sediment daily net metabolism. To test the effect of tidal stages on the radiative energy budget, we used microsensor measurements of oxygen, temperature, and scalar irradiance to estimate the radiative energy budget in a coastal photosynthetic microbial mat during immersion (constant water column of 2cm) and emersion under increasing irradiance. Total absorbed light energy was higher in immersion than emersion, due to a lower reflectance of the microbial mat, while most (> 97%) of the absorbed light energy was dissipated as heat irrespective of tidal conditions. During immersion, the upward heat flux was higher than the downward one, whereas the opposite occurred during emersion. At highest photon irradiance (800μmolphotonm-2s-1), the sediment temperature increased ~ 2.5°C after changing the conditions from immersion to emersion. The radiative energy balance showed that less than 1% of the incident light energy (PAR, 400-700nm) was conserved by photosynthesis under both tidal conditions. At low to moderate incident irradiances, the light use efficiency was similar during the tidal stages. In contrast, we found an ~ 30% reduction in the light use efficiency during emersion as compared to immersion under the highest irradiance likely due to the rapid warming of the sediment during emersion and increased non-photochemical quenching. These changes in the photosynthetic efficiency and radiative energy budget could affect both primary producers and temperature-dependent bacterial activity and consequently daily net metabolism rates having important ecological consequences.

Water column dissolved silica concentration limits microphytobenthic primary production in intertidal sediments.

Primary production of microphytobenthos (MPB) contributes significantly to the total production in shallow coastal environments. MPB is a diverse community in which diatoms are usually the main microalgal group. Diatoms require N, P, and other nutrients as with other autotrophs, but in addition require silicate to create their outer cell wall. Therefore, dissolved silica (DSi) might be a potential limiting factor for benthic primary production in areas with reduced freshwater input. To test this hypothesis, a microcosm experiment was conducted using intact sediment cores collected from an intertidal mudflat in the Bay of Cádiz and supplied with increasing concentrations of DSi (0, 5, 10, 25, and 45μmol·L-1 ). After 7d of enrichment, we determined chlorophyll a and c (Chl a, c) contents, metabolic rates (Net [Pn ] and Areal Gross [PgA ] Production and Light [RL ] and Dark [RD ] Respiration), as well as fluxes of inorganic nutrients across the sediment-water interface. Chl a and c contents increased significantly with respect to the initial conditions but no differences between treatments were found. Both Pn and PgA showed a saturating-like pattern with silicate concentration, reaching maximum rates at a DSi concentration of 45μmol·L-1 . The addition of DSi also resulted in an increase of DSi and ammonium uptake by the sediment, which was significantly higher in light than in darkness. Our results clearly show that water column DSi concentrations have a direct impact on benthic primary production, also controlling other related processes such as inorganic nutrient fluxes.

Development of temperature-based algorithms for the estimation of microphytobenthic primary production in a tidal flat: A case study in Daebu mudflat, Korea

This study presents the results of field experiments that were designed to investigate the photophysiological characteristics of microphytobenthos (MPB) and to estimate primary production (PP) in Daebu mudflat, which is located at the west coast of Korea. A typical seasonal (or monthly) fluctuation of intertidal MPB PP was found in association with biotic (benthic Chl-a) and/or abiotic parameters (irradiance and temperature) over a period of three years. From a series of field-laboratory experiments using the oxygen micro-profiling method (totaling 28 surveys), three consistent phenomena were observed: 1) winter to early spring algal blooms, 2) seasonal changes in Q10, and 3) temperature dependent MPB photosynthesis-irradiance (P-I). In particular, both the chlorophyll-specific maximum photosynthetic capacity (Pbmax) and the saturated light intensity (Ik), derived from 126 P-I curves (1870 data sets of oxygen micro-profiling in the sediment), were significantly correlated with sediment temperature (p < 0.01). To develop an empirical MPB PP model, the relationships between P-I parameters and environmental variables were parameterized following established exponential forms (e.g., Q10). It was possible to estimate the MPB PP in Daebu mudflat area by using easily accessible explanatory factor, suitable to be used for future explorations of parameters such as sediment temperature, irradiance, chlorophyll concentration, and tidal height. The estimated annual MPB PP based on the empirical PP model were found to be greater than that in the Wadden Sea and average annual PP in the temperate zones of the world. Authors believe that the present approach of the MPB PP estimation could be combined with remote-sensing techniques (e.g., satellites) to support coastal ecosystem management.

A model to assess microphytobenthic primary production in tidal systems using satellite remote sensing

Quantifying spatial variability in intertidal benthic productivity is necessary to guide management of estuaries and to understand estuarine ecological processes, including the amount of benthic organic carbon available for grazing, burial and transport to the pelagic zone.We developed a model to assess microphytobenthic (MPB) primary production using (1) remotely sensed information on MPB biomass and remotely sensed information on sediment mud content, (2) surface irradiance and ambient temperature (both from local meteorological observations), (3) directly-measured photosynthetic parameters and (4) a tidal model. MPB biomass was estimated using the normalized-difference vegetation index (NDVI) and mud content was predicted using surface reflectance in the blue and near-infrared, both from Landsat 8 satellite imagery. The photosynthetic capacity (maximum photosynthesis rate normalized to MPB chl-a) was estimated from ambient temperature, while photosynthetic efficiency and the light saturation parameter were derived from in situ fluorometry-based production measurements (PAM). The influence of tides (submergence by turbid water) on MPB production was accounted for in the model. The method was validated on several locations in two temperate tidal basins in the Netherlands (Oosterschelde and Westerschelde). Model based production rates (mg C m−2 h−1) matched well with an independent set of in situ (PAM) measurement based production rates (Oosterschelde: RMSE = 9.7, mean error = 1.5, χ = 0.57; Westerschelde: RMSE = 46.7, mean error = −17.6, χ = 0.9). The relationship between photosynthetic capacity and temperature shows considerable variation and may be improved by using sediment surface temperature instead of ambient temperature. A sensitivity analysis revealed that emersion duration and mud content determine most of the variability in MPB production. Our results demonstrate that it is possible to derive a satellite remote sensing-based overview of average hourly and daily MPB primary production rates at the macro scale. As the proposed model is generic, the model can be applied to other tidal systems to assess spatial variability in MPB primary production at the macro scale after calibration at the site of interest. Model calibration, results and possible applications for regular monitoring of MPB production are discussed below.

A simple, user friendly tool to readjust raw PAM data from field measurements to avoid over- or underestimating of microphytobenthos photosynthetic parameters

Intertidal mudflats are among the most productive ecosystems and microphytobenthic (MPB) biofilms play a key role in primary production. MPB primary production varies at short spatial and temporal scales. Accurate measurements thus require rapid non-intrusive methods like pulse amplitude modulate (PAM) fluorescence. However, the effect of granulometry and chl a concentration profile in light attenuation on irradiance and on fluorescence signal in the photic layer need to be taken into account when primary production is estimated using PAM. We propose a tool to readjust raw photosynthetic parameters (rETRmax, α, Ik) estimated from PAM measurements on the field, to avoid over- or underestimation. To develop the tool, we used models previously designed by Kühl and Jørgensen (1992), Serôdio (2004) and Forster and Kromkamp (2004) by integrating the chl a distribution profiles and sediment granulometry from pure sand to pure mud. The sensitivity of the correction to sediment granulometry and the shape of chl a profile were evaluated theoretically using a typical fluorescence data set obtained using PAM measurements. Our results confirm the importance of accounting for both the chl a profile and sediment granulometry when estimating a light attenuation coefficient. We show that, with the same chl a profile, the photosynthetic parameters are more underestimated in mud than in a sandy environment. Thus, granulometry and the chl a profile need to be systematically quantified and used to correct raw data measured in field studies using PAM before estimating photosynthetic parameters. The numerical tool is available as an e-document that is simple and easy to apply to any PAM data.

Benthic fluxes of oxygen and nutrients in sublittoral fine sands in a north-western Mediterranean coastal area

Traditionally, benthic metabolism in sublittoral permeable sands have not been widely studied, although these sands can have a direct and transcendental impact in coastal ecosystems. This study aims to determine oxygen and nutrient fluxes at the sediment–water interface and the study of possible interactions among environmental variables and the benthic metabolism in well-sorted fine sands. Eight sampling campaigns were carried out over the annual cycle in the eastern coast of Spain (NW Mediterranean) at 9m depth station with permeable bottoms. Water column and sediment samples were collected in order to determine physico-chemical and biological variables. Moreover, in situ incubations were performed to estimate the exchange of dissolved solutes in the sediment–water interface using dark and light benthic chambers. Biochemical compounds at the sediment surface ranged between 160 and 744µgg−1 for proteins, 296 and 702µgg−1 for carbohydrates, and between 327 and 1224µgCg−1 for biopolymeric carbon. Chloroplastic pigment equivalents in sediments were mainly composed by chlorophyll a (1.81–2.89µgg−1). These sedimentary organic descriptors indicated oligotrophic conditions according to the biochemical approach used. In this sense, the most abundant species in the macrobenthic community were sensitive to organic enrichment. In dark conditions, benthic fluxes behaved as a sink of oxygen and a source of nutrients. Oxygen fluxes (between −26,610 and −10,635μmolm−2d−1) were related with labile organic fraction (r=−0.86, p<0.01 with biopolymeric carbon; r=−0.91, p<0.01 with chloroplastic pigment equivalents). Daily fluxes of dissolved oxygen, that were obtained by adding light and dark fluxes, were only positive in spring campaigns (6966μmolm−2d−1) owing to the highest incident irradiance levels (r=0.98, p<0.01) that stimulate microphytobenthic primary production. Microphytobenthos played an important role on benthic metabolism and was the main primary producer in this coastal ecosystem. However, an average annual uptake of 31mmolm−2d−1 of oxygen and a release of DIN and Si(OH)4 (329 and 68mmolm−2d−1 respectively) were estimated in these bottoms, which means heterotrophic conditions.

The Relationship between Primary Production of Microphytobenthos and Tidal Cycle on the Hwaseong Mudflat, West Coast of Korea

Kwon, B.-O.; Koh, C.-H.; Khim. J. S.; Park, J.; Kang, S-G., and Hwang, J.H., 2014. The relationship between primary production of microphytobenthos and tidal cycle on the Hwaseong mudflat, west coast of Korea.The present study examined variation in sediment temperature and irradiance with respect to microphytobenthos primary production at the Hwaseong mudflat, west coast of Korea, under different tidal cycles. The study was carried out during March and April 2009, which encompassed two spring tides and one neap tide. It was not possible to continuously measure primary production in situ across the tidal cycles; hence, photosynthesis–irradiance (P-I) curves at different temperatures were obtained experimentally in the laboratory. These readings were then used to calculate primary production, by projecting in situ mooring records of irradiance and temperature onto the P-I curves. The sediment temperature and irradiance were measured continuously, and the averages of 10-minute intervals from the mooring records were used. The results show that changes in primary production simulated from the P-I curves were strongly associated with the tidal cycle. During the two spring tides that were recorded, mean daily primary production was 1005 and 1125 mg C m−2 d−1, respectively. However, primary production was lower during the neap tide (898 mg C m−2 d−1). Temperature appeared to influence these differences in mean daily production. In situ mean daily temperatures during exposure were 14°C and 20°C during the first and second spring tides, respectively. In contrast, mean daily total irradiance was noticeably higher in the first spring tide compared with the second spring tide. During the neap tide, the site was exposed in the morning and afternoon, when light intensity and temperature were relatively low. In contrast, during the spring tide, the site was exposed at noon, when light intensity and temperature were high. The current study provided new insights about how changes in environmental parameters during tidal cycles affect microphytobenthic primary production, indicating the complexity of this process and the requirement for further fine-scale research.

Characterization of the top sediment layer in coastal intertidal mudflats from medium-to-coarse resolution satellite imagery and field measurements

Intertidal sediments are critically important in controlling intertidal mudflat microphytobenthic primary productivity and the functioning of intertidal ecosystems. This paper demonstrates the possibility of deriving different intertidal sediment properties from coarse-to-medium resolution remote sensing imagery. Supervised and image based classification methods were used to map different substrate types based on the Spectral Angle Mapper (SAM) algorithm. The algorithm characterized different sediment properties from remote sensing data based on field collected and image-extracted endmembers. The results demonstrate that, different substrate types can be derived from coarse-to-medium resolution images using SAM algorithm. Supervised and image-based classification methods performed well in deriving intertidal sediment properties. From the results, sand sediments cover a wide area in extent than clay whereas Normalized Difference Vegetation Index (NDVI) validation results indicate that, clay sediments have higher NDVI values as compared to sand sediments. We conclude that, intertidal sediment properties can be successfully derived from coarse-to-medium resolution satellite imagery. Key words: Endmember, microphytobenthos, spectral signature, substrates, trios ramses, wadden sea.

Microphytobenthic production estimated by in situ oxygen microprofiling: short-term dynamics and carbon budget implications

Purpose Short-term temporal variability of microphytobenthic primary production is suspected to be of the same magnitude as seasonal variability, but data remain very scarce due to methodological limitations. In this context, a 6-day in situ high-frequency survey was performed in a temperate intertidal mudflat using an automated microprofiling system.

Short-term variability of microphytobenthic primary production associated with in situ diel and tidal conditions

The short-term variability in microphytobenthos (MPB) production, measured by the oxygen microprofiling method, found under different experimental conditions indicated an endogenous production response reflecting in situ diel and tidal conditions. MPB production was measured for submerged core samples (collected from Daebu mudflat, Korea) at a fixed irradiance and temperature in the laboratory under the conditions of (1) constant light (data-I), (2) light–dark incubation (data-II), and (3) in situ reflected (data-III). The experimental design aimed to characterize within-day, across-day, and long term changes in MPB production. Our results showed that, under constant light conditions for 72 h, temporal fluctuations in MPB production (day:night = 2.4:1) were clearly present for three consecutive days (data-I), indicating a diel rhythm in production. Production increased at the beginning of light exposure, and dramatically decreased at the time of submersion, indicating tide-dependent rhythm in production. Furthermore, over a 10 d period under the same light and temperature conditions, a weakening (declining) trend in production was observed, which was logarithmic with diel fluctuation (r2 = 0.995, p < 0.01). This diel rhythm in production was also observed under an alternating light–dark (L14 h/D10 h) incubation period across an additional 18 d of measurement (data-II). The decline in production was slower, and more linear (r2 = 0.930, p < 0.01) under this condition, as the period of dark incubation (D10 h) seemed to allow the community to recover to a certain level of production. Finally, the effects of tidal condition (spring tide vs. neap tide) and biomass (dense vs. lesser dense) on the short-term (<hourly) variability of MPB production (data-III) appeared to be negligible when time integrated the production.

Microalgal productivity in an estuarine lake during a drought cycle: The St. Lucia Estuary, South Africa

The St. Lucia estuarine lake on the north coast of KwaZulu-Natal, South Africa, is one of the largest estuarine systems in Africa and of unique importance for the adjacent marine and terrestrial ecosystems. The area regularly experiences periods of drought, resulting in hypersaline conditions in its shallow lakes and the closure of the estuarine mouth. This study aimed to assess the primary production rates of phytoplankton and microphytobenthos throughout an annual cycle of this drought phase. Primary production rates were assessed at representative sites, namely the Mouth, Narrows, South and North Lakes from June 2006 to May 2007. Because of the drought, the salinity gradient from the mouth to the head of the estuary was reversed by comparison to estuarine systems with a steady freshwater inflow and regular marine exchange. In March 2007, during the study, the mouth opened as a result of rough seas, and the marine influence broke the existing reversed gradient, producing a marine salinity throughout the system. Microphytobenthic primary productivity varied between 0 and 34 mg C m −2 h −1 and showed strong correlations with salinity, DIN:DIP ratios and irradiance. Benthic productivity was high across the system after breaching of the mouth. Pelagic primary productivity (between 0 and 180 mg C m −2 h −1), showed a correlation with temperature and irradiance and was highest across the system in February 2007 when the mouth was still closed. There was no significant correlation between production rates and biomass (chl-a) in either the benthic or pelagic habitats. The negative correlation between DIN:DIP ratio and benthic primary productivity indicated that phosphorus was the limiting nutrient. This study shows that salinity, along with seasonally dependent parameters such as temperature and irradiance, correlates with the rate of microalgal production. Hence, in these shallow lakes, the largest primary productivity can occur in either the pelagic or benthic subsystems, depending on prevailing conditions at the time.

Different fixatives and chloridric acid concentrations in microphytobenthic primary production estimates using radiolabeled carbon: their use and misuse

In literature, different fixatives have been used to stop microphytobenthic photosynthetic activity estimated using the 14C technique, and different concentrations of HCl have been used to remove the excess labeled bicarbonate. To standardize these two steps of the 14C method, we designed a series of experiments using sublittoral muddy sediments colonized by microphytobenthos. The first aim was to identify which fixative among formaldehyde, glutaraldehyde, and HCl was most effective in arresting photosynthetic activity. Formaldehyde increases cell membrane permeability leading to a loss of assimilated carbon because an underestimate of disintegrations per minute (DPM) of up to 64% was obtained when compared with HCl values. Glutaraldehyde led to an overestimate of DPM due to its autofluorescence. We propose that the best way to stop photosynthetic activity is by using HCl. The second aim of the article was to define which HCl concentration from 0.1N to 5N was sufficient to remove the excess labeled bicarbonate. The residual inorganic 14C led to an overestimate of DPM values when mild acids were added. None of the acid treatments completely removed the inorganic labeled carbon because of the sediment matrix effect. However, a further pH decrease, caused by too strong acid normalities, could digest the organic carbon pool. Besides dark estimates, blank production estimates should be assessed before each primary production experiment to consider the sediment matrix effect.