Community Primary Production Research Articles

Influence of climate change on algal community structure and primary productivity of Lake Malawi (East Africa) from the Last Glacial Maximum to present

Biomarkers of aquatic algae and compound‐specific carbon isotopes are used to examine past variations in algal community composition and primary productivity in Lake Malawi. We note major changes at the Pleistocene‐Holocene boundary. From the Last Glacial Maximum until ~11.8 calendar years before present (kyr B.P.), Lake Malawi was characterized by either low rates of primary productivity or algal productivity that was dominated by a group other than diatoms. At the onset of the Holocene, conditions similar to those of modern Lake Malawi were initiated, with diatoms and nitrogen‐fixing cyanobacteria becoming more important contributors to primary productivity. The transition at ~11.8 kyr B.P. is likely related to a shift in the dominant wind direction over Lake Malawi, resulting from a southward shift in the mean latitudinal position of the Intertropical Convergence Zone during the last glacial. Throughout the 23‐kyr B.P. record, the effects of wind‐induced upwelling are important and may be the main control on the carbon isotopic composition of algal lipids through delivery of 13C‐depleted CO2, derived from organic matter decomposition in deeper waters, to the photic zone. Relationships are also suggested between thermal stratification and primary productivity, with cool conditions at ~12 and 8 kyr B.P. resulting in weaker thermal stratification and increased primary productivity. The opposite situation occurs at 4.9 kyr B.P., when significantly warmer temperatures and decreased algal productivity are observed. Thus, in lentic systems such as Lake Malawi the major influence of climate on algal ecology appears to be through feedbacks in physical circulation mechanisms.

Direct and indirect effects of the fungicide azoxystrobin in outdoor brackish water microcosms

The effects of the strobilurin fungicide azoxystrobin were studied in brackish water microcosms, with natural plankton communities and sediment. Two experiments were conducted: Experiment 1 (nominal conc. 0, 15 and 60 microg/L, 24-L outdoor microcosms for 21 days) and a second, follow-up, Experiment 2 (nominal conc. 0, 3, 7.5, 15 microg/L, 4-L indoor microcosms for 12 days). The microcosms represent a simplified brackish water community found in shallow semi-enclosed coastal areas in agricultural districts in the Baltic Sea region. Measured water concentrations of the fungicide (Experiment 1) were, on average, 83 and 62% of nominal concentrations directly after application, and 25 and 30% after 21 days, for the low and high dose treatments, respectively, corresponding to mean DT50-values of 15.1 and 25.8 days, for low and high dose treatments, respectively. In Experiment 1, direct toxic effects on calanoid copepods at both test concentrations were observed. Similarly, in Experiment 2, the copepod abundance was significantly reduced at all tested concentrations. There were also significant secondary effects on zooplankton and phytoplankton community structure, standing stocks and primary production. Very few ecotoxicological studies have investigated effects of plant protection products on Baltic organisms in general and effects on community structure and function specifically. Our results show that azoxystrobin is toxic to brackish water copepods at considerably lower concentrations than previously reported from single species tests on freshwater crustaceans, and that direct toxic effects on this ecologically important group may lead to cascade effects altering lower food webs and ecosystem functioning.

Annual budget of benthic production in Mont Saint-Michel Bay considering cloudiness, microphytobenthos migration, and variability of respiration rates with tidal conditions

In order to provide an accurate annual rate of net benthic community production, community photosynthetic response to incident irradiance and respiration were measured at different times of the year, at mid-tide level on the muddiest part of the Mont Saint-Michel Bay. As the water turbidity prevented any photosynthesis by the microphytobenthos during immersion periods, primary production was measured only during emersion periods. In contrast respiration was expected to vary according to the tidal cycle and was measured during both emersion and immersion periods. Primary production and respiration rates under emersion were assessed using in situ infra-red gas analysis of CO2 exchange measured in a benthic chamber. Respiration rates under immersion were assessed through total CO2 concentration variations in incubated cores.When respiration rate was considered constant over a day and equal to the rate measured under emerged conditions, the community respiration was 54.56gCm−2yr−1. Taking into account the variations of respiration during the emersion/immersion cycle, community respiration increased to 101.76gCm−2yr−1 (86% increase). Assuming that community primary production varied as a function of irradiance from the time of exposure until flooding, GCP was 53.22gCm−2yr−1 when calculated with theoretical irradiance and 45.86gCm−2yr−1 after correction for cloudiness (14% decrease). Then, by integrating the effect of vertical migration of microphytobenthos, the rate was adjusted to 23.49gCm−2yr−1 (49% decrease). The net community production budget, calculated as the difference between the GCP and the CR budgets taking into account the corrections described above, was −78.27gCm−2yr−1. This example of annual budget calculation highlights the need to integrate several processes occurring over various time scales to accurately reflect the metabolic balance of a system. The results also confirmed the heterotrophic status generally suggested for intertidal muddy sediments.

Propagule supply controls grazer community structure and primary production in a benthic marine ecosystem

Early theories of species diversity proposed that communities at equilibrium are saturated with species. However, experiments in plant communities suggest that many communities are unsaturated and species richness can be increased by adding propagules of new species. We experimentally tested for community saturation and measured the effects of propagule supply on community structure in a benthic marine system. We manipulated propagule supply (arrival of individuals of numerous species) of mobile grazers in experimental mesocosms over multiple generations and, unlike previous tests, we examined the cascading effects of propagule supply on prey (macroalgae) biomass. We found little evidence for saturation, despite the absence of processes such as disturbance and predation that are thought to alleviate saturation in nature. Increasing propagule supply increased the total number of species and made rare species more abundant. Perhaps surprisingly, given the strong effect of propagule supply on species richness, supply-related changes in body size and composition suggest that competitive interactions remained important. Grazer supply also had strong cascading effects on primary production, possibly because of dietary complementarity modified by territorial behavior. Our results indicate that propagule supply can directly influence the diversity and composition of communities of mobile animals. Furthermore, the supply of consumer propagules can have strong indirect effects on prey and fundamental ecosystem properties.

Effects of N : P loading ratios on phytoplankton community composition, primary production and N fixation in a eutrophic lake

Summary 1. The aim of this study was to assess the effects of different nitrogen (N) to phosphorus (P) loading ratios on phytoplankton community composition and primary production in a naturally eutrophic lake. Furthermore, the sources of N fuelling primary production were estimated using 15N stable isotope tracers. 2. A mesocosm experiment was performed with the same amount of P added to all mesocosms (similar to internal loading rates) but with a range of N additions (0–86 μm N), resulting in a gradient of N : P supply ratios. 3. Low N : P supply ratios resulted in a significant shift in the phytoplankton assemblage to a community dominated by N‐fixing cyanobacteria and a supply of atmospheric N2 estimated to be up to 60% of total supply. 4. The N : P loading ratio had no significant effect on primary production, total nitrogen (TN) concentration or particulate N concentration. 5. Our results imply that a reduced N : P ratio of the nutrient load does not necessarily result in a lower TN concentration and downstream N export due to compensation by N‐fixing cyanobacteria.

Propagule supply controls grazer community structure and primary production in a benthic marine ecosystem

Propagule supply controls grazer community structure and primary production in a benthic marine ecosystem

Impact of the Phaeocystis globosa spring bloom on the intertidal benthic compartment in the eastern English Channel: A synthesis

From 1999 to 2005, studies carried out in the frame of regional and national French programs aimed to determine whether the Phaeocystis globosa bloom affected the intertidal benthic communities of the French coast of the eastern English Channel in terms of composition and/or functioning. Study sites were chosen to cover most of the typical shore types encountered on this coast (a rocky shore, an exposed sandy beach and a small estuary). Both the presence of active Phaeocystis cells and their degradation product (foam) did have a significant impact on the studied shores. The primary production and growth rates of the kelp Saccharina latissima decreased during the bloom because of a shortage of light and nutrient for the macroalgae. On sandy sediments, the benthic metabolism (community respiration and community primary production), as well as the nitrification rate, were enhanced during foam deposits, in relation with the presence of bacteria and active pelagic cells within the decaying colonies. In estuarine sediments, the most impressive impact was the formation of a crust at the sediment surface due to drying foam. This led to anoxic conditions in the surface sediment and resulted in a high mortality among the benthic community. Some organisms also tended to migrate upward and were then directly accessible to the higher trophic level represented by birds. Phaeocystis then created a shortcut in the estuarine trophic network. Most of these modifications lasted shortly and all the systems considered came back to their regular properties and activities a few weeks after the end of the bloom, except for the most impacted estuarine area.

Spatio-temporal variability of intertidal benthic primary production and respiration in the western part of the Mont Saint-Michel Bay (Western English Channel, France)

In situ measurements of both community metabolism (primary production and respiration) and PAM fluorometry were conducted during emersion on intertidal sediments in the Mont Saint-Michel Bay, in areas where oysters and mussels were cultivated. Results highlighted a low benthic metabolism compared to other intertidal areas previously investigated with the same methods. Comparisons between gross community primary production and relative electron transport rates confirmed this statement. More specifically, primary productivity remained very low all over the year, whereas the associated microalgal biomass was estimated to be high. We suggest that the microphytobenthic community studied was characterized by a self-limitation of its primary productivity by its own biomass, as previously shown in Marennes-Oleron Bay for example. The almost permanent high biomass would represent a limiting factor for micromigration processes within the first millimetres of the sediment. This could be explained by very low resuspension processes occurring in the western part of the bay, enhanced by the occurrence of numerous aquaculture structures that could decrease tidal currents in the benthic boundary layer

Photosynthetic activity of intertidal microphytobenthic communities during emersion: in situ measurements of chlorophyll fluorescence (PAM) and CO2 flux (IRGA)1

Photosynthetic microphytobenthic activity has increasingly been examined using pulse‐amplitude‐modulated (PAM) fluorescence techniques. Nevertheless, estimating carbon production rates from fluorescence measurements implies the establishment of reliable relationships. The aim of this study was to determine such a relationship from field measurements of both PAM fluorescence and CO2 fluxes. Three study sites of varying sedimentary features were investigated in different seasons. Both linear and with plateau relationships were obtained between the fluorescence parameter (relative electron transport rate [rETR]) and the community‐level carbon‐fixation rate (gross community primary production rate [GCP] in mg C · m−2 · h−1). The correlation calculated from the whole data set (i.e., all sites and all seasons) was very strong (n = 106; r = 0.928). Significant correlations were also obtained for light‐curve parameters assessed with the two methods: Pm (n = 8; r = 0.920) and Ik (n = 8; r = 0.818). Total community‐level carbon fixation for the emersion period was calculated from fluorescence measurements according to the relationship established between GCP and rETR, and between light‐curve parameters, and the results were compared to the estimation obtained directly from GCP measurements. The agreement between the two estimations was quite good for both ways of calculation (with a mean discrepancy of 30% for the first one and −2% for the second one). These results suggest the potential application of PAM measurements to calculate carbon‐fixation rates at large spatial and temporal scales, provided that a set of experiments coupled with CO2‐flux measurements are performed.

Estrogen-receptor independent effects of two ubiquitous environmental estrogens on Melosira varians Agardh, a common component of the aquatic primary production community

Estrogenic compounds have been discovered in many surface water samples in many anthropogenically altered surface waters. Wastewater effluent has been identified as a major pathway of contamination and found to revert much of the metabolic products of these biologically active compounds back to their original form. This presentation explains methodology for determining exposure effects through a newly developed bioassay, examining the physiological response of a diatom to these compounds. Diatoms represent an important aspect of the primary production community. They are a desirable food source over other members of the primary production community through storage of photosynthetically produced sugars in the form of lipids rather than starch. Therefore, many members of higher trophic levels selectively feed on diatoms when present with other members of the primary production community. This study examines the effects of 17β-estradiol and 4-nonylphenol on the physiological development of the diatom species Melosira varians. Clearly, unicellular protists such as diatoms are not susceptible to these contaminants in a manner directly analogous to that expressed in vertebrates. However, estradiol and nonylphenol are lipophilic making them particularly effective in entering the diatom cell membrane. Melosira varians was selected because it commonly occurs in most freshwater environments and has been the subject of other toxicological studies. An adequate literature base also exists for evaluating results of this experiment. The species grows rapidly and is easy to maintain in culture. Comparing cell density, chl- a, and lipid content in control and exposed cultures allowed interpretation of how the species responded to varying compound concentrations. Results of this study revealed differences in responses to each compound. 17β-estradiol appears to have no detrimental effect on M. varians, while 4-nonylphenol results in cell mortality with sufficient dosage. Similar results with 17β-estradiol are reported for other autotrophic organism, but the 4-nonylphenol response is not as uniform in other photosynthetic groups. The use of a phytoplankton species as a test organism complements other investigations involving vertebrate models at or near the apex of the trophic hierarchy by considering effects at the base of the food web.

Community primary production and calcification in a NW Mediterranean ecosystem dominated by calcareous macroalgae

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 334:37-45 (2007) - doi:10.3354/meps334037 Community primary production and calcification in a NW Mediterranean ecosystem dominated by calcareous macroalgae Nathaniel Bensoussan1,*, Jean-Pierre Gattuso2 1Laboratoire Diversité, évolution et écologie fonctionnelle marine, CNRS—Université de la Méditerranée, Centre d’Océanologie de Marseille, Station Marine d’Endoume, rue Batterie des Lions, 13007 Marseille, France 2Laboratoire d’Océanographie de Villefranche, CNRS—Université de Paris 6, BP 28, 06234 Villefranche-sur-mer Cedex, France *Email: nathaniel.bensoussan@univmed.fr ABSTRACT: The community metabolism of a shallow infralittoral ecosystem dominated by the calcareous macroalgae Corallina elongata was investigated in Marseilles (NW Mediterranean), by monitoring hourly changes of seawater pH and total alkalinity over 6 d in February 2000. Fair weather conditions prevailed over the study period as indicated by oceanographic (temperature, salinity, and current velocity and direction) and meteorological variables, which validated the standing water hypothesis. This temperate ecosystem exhibited high community gross primary production (GPP = 519 ± 106 mmol C m–2 d–1, n = 6) and also supported high rates of community respiration (R). As a result, the system was slightly autotrophic (net community production, NCP = 20 mmol C m–2 d–1), with a GPP/R ratio of 1.06. NCP exhibited circadian variations with 2- to 3-fold changes in community respiration, both in the light and in the dark. Rates of net community calcification also exhibited circadian variations, with positive rates (up to 24 mmol CaCO3 m–2 h–1) for irradiance values >300 W m–2 (about 1380 µmol photon m–2 s–1). Below this irradiance threshold, net community dissolution prevailed. Daily net calcification (G) was on average 8 mmol CaCO3 m–2 d–1. CO2 fluxes generated by primary production, respiration, and calcification suggest that the study site was a potential atmospheric CO2 sink of 15 mmol CO2 m–2 d–1 at the time of measurement. KEY WORDS: Ecosystem metabolism · Primary production · Respiration · Calcification · Calcareous macroalgae · Corallina elongata · pH · Total alkalinity · TA Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 334. Online publication date: March 26, 2007 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2007 Inter-Research.

The significance of nitrogen fixation to new production during early summer in the Baltic Sea

Abstract. Rates of dinitrogen (N2) fixation and primary production were measured during two 9 day transect cruises in the Baltic proper in June–July of 1998 and 1999. Assuming that the early phase of the bloom of cyanobacteria lasted a month, total rates of N2 fixation contributed 15 mmol N m−2 (1998) and 33 mmol N m−2 (1999) to new production (sensu Dugdale and Goering, 1967). This constitutes 12–26% more new N than other annual estimates (mid July–mid October) from the same region. The between-station variability observed in both total N2 fixation and primary productivity greatly emphasizes the need for multiple stations and seasonal sampling strategies in biogeochemical studies of the Baltic Sea. The majority of new N from N2 fixation was contributed by filamentous cyanobacteria. On average, cyanobacterial cells >20 µm were able to supply a major part of their N requirements for growth by N2 fixation in both 1998 (73%) and 1999 (81%). The between-station variability was high however, and ranged from 28–150% of N needed to meet the rate of C incorporation by primary production. The molar C:N rate incorporation ratio (C:NRATE) in filamentous cyanobacterial cells was variable (range 7–28) and the average almost twice as high as the Redfield ratio (6.6) in both years. Since the molar C:N mass ratio (C:NMASS) in filamentous cyanobacterial cells was generally lower than C:NRATE at a number of stations, we suggest that the diazotrophs incorporated excess C on a short term basis (carbohydrate ballasting and buoyancy regulation), released nitrogen or utilized other regenerated sources of N nutrients. Measured rates of total N2 fixation contributed only a minor fraction of 13% (range 4–24) in 1998 and 18% (range 2–45) in 1999 to the amount of N needed for the community primary production. An average of 9 and 15% of total N2 fixation was found in cells <5 µm. Since cells <5 µm did not show any detectable rates of N2 fixation, the 15N-enrichment could be attributed to regenerated incorporation of dissolved organic N (DON) and ammonium generated from larger diazotroph cyanobacteria. Therefore, N excretion from filamentous cyanobacteria may significantly contribute to the pool of regenerated nutrients used by the non-diazotroph community in summer. Higher average concentrations of regenerated N (ammonium) coincided with higher rates of N2 fixation found during the 1999 transect and a higher level of 15N-enrichment in cells <5 µm. A variable but significant fraction of total N2 fixation (1–10%) could be attributed to diazotrophy in cells between 5–20 µm.

Unexpected underestimation of primary productivity by 18O and 14C methods in a lake: Implications for slow diffusion of isotope tracers in and out of cells

By use of in situ bottle incubations, we determined three variables for the characterization of plankton community primary productivity in Lake Kinneret, Israel: (1) H218O‐based primary productivity (18OP), measured by 34O2 (18O16O) evolution; (2) radiocarbon‐uptake‐based primary productivity (14CP); and (3) net O2 production (NOP), calculated from the rate of O2 : Ar change. Six experiments were conducted in the fall, a period that is characterized by thermocline deepening, erosion of the hypolimnion, and eventual introduction of high concentration of reduced substances into the epilimnion. An additional experiment was conducted in spring. In comparison to net O2 evolution, the tracer‐based methods severely underestimated primary productivity in the fall. This is indicated by unusually high NOP: 18OP and NOP: 14CP ratios (1–2.2 and 2–4, respectively). The latter is considerably different than the typical values of aquatic environments (~1.4). In contrast to these ratios, the 18OP: 14CP appears normal (1.6–2.9) and not significantly different from previous results in the lake. Ammonium was the major nitrogen source in the fall experiments, and there is no reason to assume that the cellular composition of phytoplankton was exceptional. It is therefore unlikely that the high NOP: 14CP ratio represents genuine ratios of oxygen and carbon metabolism. We suggest that an explanation accounting for the high values of both NOP: 18O P and NOP: 14CP can be slow diffusion of the tracers in and out of phytoplankton cells.

Phytoplankton community structure and primary production in small intertidal estuarine-bay ecosystem (eastern English Channel, France)

From May 2002 to October 2003, a fortnightly sampling programme was conducted in a restricted macrotidal ecosystem in the English Channel, the Baie des Veys (France). Three sets of data were obtained: (1) physico-chemical parameters, (2) phytoplankton community structure illustrated by species composition, biovolume and diversity, and (3) primary production and photosynthetic parameters via P versus E curves. The aim of this study was to investigate the temporal variations of primary production and photosynthetic parameters in this bay and to highlight the potential links with phytoplankton community structure. The highest level of daily depth-integrated primary production Pz (0.02–1.43 g C m−2 d−1) and the highest maximum photosynthetic rate PB max (0.39–8.48 mg C mg chl a−1 h−1) and maximum light utilization coefficient αB [0.002–0.119 mg C mg chl a−1 h−1 (μmol photons m−2 s−1)] were measured from July to September. Species succession was determined based on biomass data obtained from cell density and biovolume measurements. The bay was dominated by 11 diatoms throughout the year. However, a Phaeocystis globosa bloom (up to 25 mg chl a m−3, 2.5 × 106 cells l−1) was observed each year during the spring diatom bloom, but timing and intensity varied interannually. Annual variation of primary production was due to nutrient limitation, light climate and water temperature. The seasonal pattern of microalgal succession, with regular changes in composition, biovolume and diversity, influenced the physico-chemical and biological characteristics of the environment (especially nutrient stocks in the bay) and thus primary production. Consequently, investigation of phytoplankton community structure is important for developing the understanding of ecosystem functioning, as it plays a major role in the dynamics of primary production.

Short-term variability of intertidal benthic community production during emersion and the implication in annual budget calculation

Short-term variability in benthic community primary production was studied during the course of emersion on a study site located in the Bay of Somme (eastern English Channel, France). Primary production was estimated at the community level through in situ measurements of carbon dioxide fluxes (infra-red analysis) in benthic chambers. Throughout the emersion period, gross com- munity production (GCP) exhibited strong variations that were strongly correlated with natural vari- ations in light. Light response curves explained the variations in GCP during the greater part of the emersion period, and vertical migrations of the microphytobenthos were suggested to explain the remaining variations (i.e. just before or just after immersion of the study site). A time lag was observed between the start of emersion and the maximum GCP, which was positively correlated with the time lag between emersion and local noon. The introduction of this time lag within the annual budget calculation enhanced production variability at a 2 wk time scale (i.e. tidal cycle), but light variability at the small time scale (i.e. cloudiness) remained the major factor inducing production vari- ability. Results highlighted that omitting the time lag between emersion time and the maximum GCP within annual budget calculations led to large overestimations of annual GCP, since the annual bud- get for the study site was recalculated as being 67 g C m -2 yr -1 compared to 110 g C m -2 yr -1 when the short-term variability was not taken into account.

Identification of a Diagnostic Marker To Detect Freshwater Cyanophages of Filamentous Cyanobacteria

Cyanophages are viruses that infect the cyanobacteria, globally important photosynthetic microorganisms. Cyanophages are considered significant components of microbial communities, playing major roles in influencing host community diversity and primary productivity, terminating cyanobacterial water blooms, and influencing biogeochemical cycles. Cyanophages are ubiquitous in both marine and freshwater systems; however, the majority of molecular research has been biased toward the study of marine cyanophages. In this study, a diagnostic probe was developed to detect freshwater cyanophages in natural waters. Oligonucleotide PCR-based primers were designed to specifically amplify the major capsid protein gene from previously characterized freshwater cyanomyoviruses that are infectious to the filamentous, nitrogen-fixing cyanobacterial genera Anabaena and Nostoc. The primers were also successful in yielding PCR products from mixed virus communities concentrated from water samples collected from freshwater lakes in the United Kingdom. The probes are thought to provide a useful tool for the investigation of cyanophage diversity in freshwater environments.

A generalized model of pelagic biogeochemistry for the global ocean ecosystem. Part II: Numerical simulations

This paper presents a global ocean implementation of a multi-component model of marine pelagic biogeochemistry coupled on-line with an ocean general circulation model forced with climatological surface fields (PELAgic biogeochemistry for Global Ocean Simulations, PELAGOS). The final objective is the inclusion of this model as a component in an Earth System model for climate studies. The pelagic model is based on a functional stoichiometric representation of marine biogeochemical cycles and allows simulating the dynamics of C, N, P, Si, O and Fe taking into account the variation of their elemental ratios in the functional groups. The model also includes a parameterization of variable chlorophyll/carbon ratio in phytoplankton, carrying chl as a prognostic variable. The first part of the paper analyzes the contribution of non-local advective–diffusive terms and local vertical processes to the simulated chl distributions. The comparison of the three experiments shows that the mean chl distribution at higher latitudes is largely determined by mixing processes, while vertical advection controls the distribution in the equatorial upwelling regions. Horizontal advective and diffusive processes are necessary mechanisms for the shape of chl distribution in the sub-tropical Pacific. In the second part, the results have been compared with existing datasets of satellite-derived chlorophyll, surface nutrients, estimates of phytoplankton community composition and primary production data. The agreement is reasonable both in terms of the spatial distribution of annual means and of the seasonal variability in different dynamical oceanographic regions. Results indicate that some of the model biases in chl and surface nutrients distributions can be related to deficiencies in the simulation of physical processes such as advection and mixing. Other discrepancies are attributed to inadequate parameterizations of phytoplankton functional groups. The model has skill in reproducing the overall distribution of large and small phytoplankton but tends to underestimate diatoms in the northern higher latitudes and overestimate nanophytoplankton with respect to picoautotrophs in oligotrophic regions. The performance of the model is discussed in the context of its use in climate studies and an approach for improving the parameterization of functional groups in deterministic models is outlined.

Benthic primary production during emersion: In situ measurements and potential primary production in the Seine Estuary (English Channel, France)

A two year survey of benthic primary production during periods of emersion was performed on two stations of an intertidal mudflat (a muddy-sand station and a muddy station) in the Seine Estuary (English Channel, France). The goals of this study were to investigate the seasonal variations of metabolism, to estimate daily potential primary production variation at the annual scale and to estimate the annual potential primary production of the mudflat. Primary production and respiration were estimated by in situ measurements of carbon dioxide fluxes. Chlorophyll a concentration exhibited a great variability on both locations. Gross community production ranged from ca. 0 to 77 mg C m −2 h −1 at the muddy-sand location and from ca. 0 to 122 mg C m −2 h −1 at the muddy location. Community respiration showed a seasonal trend following temperature variations (up to 28.51 mg C m −2 h −1 in the muddy-sand and up to 23.40 mg C m −2 h −1 in the mud). Daily potential primary production was calculated, according to seasonal variations of photosynthetic parameters calculated using three photosynthesis versus irradiance curves obtained for the muddy location. The annual gross community primary production was 135 g C m −2 yr −1, leading to a low autotrophic annual budget, considering an annual community respiration of 110 g C m −2 yr −1.

CONTROLS OF ALGAL ABUNDANCE AND COMMUNITY COMPOSITION DURING ECOSYSTEM STATE CHANGE

Shallow lake ecosystems can shift between clear-water, macrophyte-rich conditions and turbid states with abundant phytoplankton. However, little is known about the controls of algal community composition and primary production before, during, and after ecosystem state change, because long time series that monitor biological change through the transition are scarce. Using proxy data sets derived from sediment cores from two shallow hypertrophic lakes in Denmark, variance-partitioning analysis (VPA) was used to determine the relative importance of changes in total phosphorus (diatom inferred), planktivorous fish density (zooplankton inferred), and submerged macrophyte communities (as macrofossil abundance) as determinants of algal abundance and community composition (as sedimentary pigments) over ecosystem state transitions since 1750 (CE) for Lake Lading and 1900 for Lake Søbygaard. Past variation in densities of planktivorous fish explained 12.3% and 18.2% of historical algal community change in lakes Lading and Søbygaard, respectively, while a further 22.3% and 6% of algal variability was explained by variation in macrophyte abundance. Total phosphorus (TP) alone explained nonsignificant amounts of variance (1.5%, 3.6%) but had a significant effect in combination with macrophytes and fish (27%, 13.4%). State transitions occurred ca. 1940 but were preceded by increases in benthic diatoms and macrophytes, suggesting that transitions were gradual rather than instantaneous. In contrast, green, colonial blue-green, and cryptophyte algae were abundant only during turbid states after ca. 1960 and were correlated to changes in planktivorous fish or fish–TP interactions. Contrary to expectations, the shift from predominantly benthic to pelagic algal production represented only a change in habitat and did not result in an increase in total abundance of primary producers.

Dependence of light-saturated photosynthesis on temperature and community structure

Using data collected in the Arabian Sea and on the Nova Scotian Shelf we examined the relationships between temperature, phytoplankton taxonomic composition (indexed by pigment composition) and the size structure (indexed by chlorophyll-specific absorption coefficient at 676 nm), Chlorophyll- a biomass and the light-saturated photosynthesis parameter P m B . Our results show that changes in phytoplankton community structure for the Scotian Shelf closely follow changes in temperature. In the Arabian Sea the relationship between phytoplankton community structure and temperature was less striking and fluctuations in phytoplankton pigment composition were strongly related to Chlorophyll- a biomass. The light-saturated photosynthesis parameter P m B was correlated with temperature for the Scotian Shelf. When we compared data collected on the Scotian Shelf with the temperature-dependent function of Eppley (1972. Fishery Bulletin 70, 1063–1085), the model fitted the data remarkably well. For the Arabian Sea data, however, only a weak temperature dependence in P m B was observed, which may be a result of a strong negative correlation between ambient nitrate concentration and temperature. Our results also show that for both the Scotian Shelf and the Arabian Sea, diatom-dominated waters had similarly low P m B values even though the temperatures of the two systems were markedly different. The absorptive characteristics of phytoplankton on the Scotian Shelf were well correlated with P m B , thus providing another potential predictor of P m B that may, in the future, be routinely accessible by remote sensing. The contrasting relationships among community structure, temperature, and primary production observed in the two study regions suggest that primary production algorithms incorporating such information could be developed and applied on a domain-specific basis. These algorithms should be centred on knowledge of the relationships between physical forcing, community structure and phytoplankton photophysiology.