Spring bloom dynamics in Kongsfjorden,Svalbard: nutrients,phytoplankton, protozoans and primary production

The marine ecosystem in Kongsfjorden (79°N), a glacial fjord in Svalbard, is to a large extent well known with regard to hydrography, mesozooplankton and higher trophic levels. Research on primary production and lower trophic levels is still scare and especially investigations from winter and spring periods. The spring bloom dynamics in Kongsfjorden were investigated in 2002. The development in nutrient conditions, phytoplankton, protozoans and primary production were followed from 15 April until 22 May. The winter/spring in 2002 was categorized as a cold year with sea ice cover and water masses dominated by local winter-cooled water. The spring bloom started around 18 April and lasted until the middle of May. The bloom probably peaked in late April, but break-up of sea ice made it impossible to sample frequently in this period. Diatoms dominated the phytoplankton assemblage. We estimated the total primary production during the spring bloom in 2002 to range 27–35 g C m⁻². There was a mismatch situation between the mesozooplankton and the phytoplankton spring bloom in 2002.     

Qualitative importance of the microbial loop and plankton community structure in a eutrophic lake during a bloom of cyanobacteria

Plankton community structure and major pools and fluxes of carbon were observed before and after culmination of a bloom of cyanobacteria in eutrophic Frederiksborg Slotssø, Denmark. Biomass changes of heterotrophic nanoflagellates, ciliates, microzooplankton (50 to 140 μm), and macrozooplankton (larger than 140 μm) were compared to phytoplankton and bacterial production as well as micro- and macrozooplankton ingestion rates of phytoplankton and bacteria. The carbon budget was used as a means to examine causal relationships in the plankton community. Phytoplankton biomass decreased and algae smaller than 20 μm replacedAphanizomenon after the culmination of cyanobacteria. Bacterial net production peaked shortly after the culmination of the bloom (510 μg C liter^?1 d^?1 and decreased thereafter to a level of approximately 124 μg C liter^?1 d^?1. Phytoplankton extracellular release of organic carbon accounted for only 4–9% of bacterial carbon demand. Cyclopoid copepods and small-sized cladocerans started to grow after the culmination, but food limitation probably controlled the biomass after the collapse of the bloom. Grazing of micro- and macrozooplankton were estimated from in situ experiments using labeled bacteria and algae. Macrozooplankton grazed 22% of bacterial net production during the bloom and 86% after the bloom, while microzooplankton (nauplii, rotifers and ciliates larger than 50 μm) ingested low amounts of bacteria and removed 10–16% of bacterial carbon. Both macro-and microzooplankton grazed algae smaller than 20 μm, although they did not control algal biomass. From calculated clearance rates it was found that heterotrophic nanoflagellates (40–440 ml^?1) grazed 3–4% of the bacterial production, while ciliates smaller than 50 μm removed 19–39% of bacterial production, supporting the idea that ciliates are an important link between bacteria and higher trophic levels. During and after the bloom ofAphanizomenon, major fluxes of carbon between bacteria, ciliates and crustaceans were observed, and heterotrophic nanoflagellates played a minor role in the pelagic food web.     

Protozooplankton in the Weddell Sea,Antarctica: Abundance and distribution in the ice-edge zone

Summary Protozooplankton were sampled in the iceedge zone of the Weddell Sea during the austral spring of 1983 and the austral autumn of 1986. Protozooplankton biomass was dominated by flagellates and ciliates. Other protozoa and micrometazoa contributed a relatively small fraction to the heterotrophic biomass. During both cruises protozoan biomass, chlorophyll a concentrations, phytoplankton production and bacterial biomass and production were low at ice covered stations. During the spring cruise, protozooplankton, phytoplankton, and bacterioplankton reached high concentrations in a welldeveloped ice edge bloom 100 km north of the receding ice edge. During the autumn cruise, the highest concentrations of biomass were in open water well-separated from the ice edge. Integrated protozoan biomass was <12% of the biomass of phytoplankton during the spring cruise and in the autumn the percentages at some stations were >20%. Bacterial biomass exceeded protozooplankton biomass at ice covered stations but in open water stations during the fall cruise, protozooplankton biomass reached twice that of bacteria in the upper 100m of the water column. The biomass of different protozoan groups was positively correlated with primary production, chlorophyll a concentrations and bacterial production and biomass, suggesting that the protozoan abundances were largely controlled by prey availability and productivity. Population grazing rates calculated from clearance rates in the literature indicated that protozooplankton were capable of consuming significant portions of the daily phyto- and bacterioplankton production.     

The structure of the plankton community of the Öregrundsgrepen (southwest Bothnian Sea)

Taxonomic composition and variations in density and biomass of the plankton community in the Öregrundsgrepen, a shallow coastal area, were investigated from June 1972 to November 1973. The phytoplankton biomass was large in spring but small during the rest of the year. The spring bloom was dominated by diatoms and dinoflagellates, especially byThalassiosira spp. which were also important during other seasons. Small forms, such asCryptomonas spp.,Rhodomonas spp. and monads, dominated during summer. Blue-green algae were never of any major importance. During the summer, the trophogenic layer exceeded 10 m in thickness. The metazoan fauna was of lower diversity than the plankton flora. The dominating species, the copepodsAcartia bifilosa andEurytemora affinis, constituted on the average 83% of the standing crop. The low salinities, 5–6 S, were regarded as the principal pertinent limiting factor. The metazoan fauna reached large biomass values from July to October. The protozoan fauna (in the case of ciliates), obtained biomass maxima during the spring bloom. It is suggested that the Öregrundsgrepen represents an area of elevated productivity within a region of low overall production, presumably due to local upwelling. From June 1972 to May 1973, the average biomasses were: phytoplankton 0.464 g C m^–2, ciliates 0.040 g C m^–2, copepod nauplii 0.010 g C m^–2, micro-rotifers 0.004 g C m^–2, and mesozooplankton (larger than 0.2 mm) 0.312 g C m^–2. It is estimated that about than 60% of the phytoplankton production is consumed by the microzooplankton (<0.2 mm).     

Seasonal succession of ciliates in lake constance

We found a recurrent seasonal pattern in abundance and composition of planktonic ciliates in Lake Constance, FRG, over a three-year period. Abundance peaks occurred in early spring and summer/autumn, while ciliate numbers were low in late spring (clear-water phase) and winter. Prostomatida and Oligotrichida dominated in early spring. They responded immediately to the phytoplankton spring bloom, while Haptorida, Peritrichida, and large Scuticociliatida (Histiobalantium) were delayed by 1 to 2 weeks. The spring community broke down at the onset of the clear-water phase.Pelagohalteria viridis containing symbiontic algae appeared shortly after this event. A highly diverse community was recorded in summer/autumn. Peritrichida, small Oligotrichida, and large Scuticociliatida reached their maxima during this season. Small Scuticociliatida were rare throughout the year and contributed moderately to total ciliate numbers only during the cold season. The observed seasonal sequence of pelagic ciliates in Lake Constance is discussed in relation to simultaneously collected data on potential food organisms and grazers.     

Seasonal,inter‐annual and long term variation in top–down versus bottom–up regulation of primary production

Environmental change strongly affects primary productivity of ecosystems via modifying bottom–up and top–down regulation of primary producers. Here we present a novel approach to quantify the relative importance of regulating factors in natural systems over various time scales: we calculated daily effect sizes of major factors affecting phytoplankton growth during the spring bloom period during almost three decades of lake oligotrophication using numerical experiments with a data based simulation model. We show that with oligotrophication the regulation of spring phytoplankton shifts from primarily top–down to bottom–up, and that the changes in regulation are non‐linearly related to the nutrient (phosphorus) concentrations. Our findings indicate that long‐term changes in top–down regulation cannot be understood without considering multiple herbivore taxa, here, microzooplankton (ciliates) and mesozooplankton (daphnids). We further demonstrate that bottom–up and top–down regulation are not independent from each other and that their interaction is time‐scale dependent.     

Subsurface phytoplankton blooms fuel pelagic production in the North Sea

The seasonal phytoplankton biomass distribution pattern in stratifiedtemperate marine waters is traditionally depicted as consistingof spring and autumn blooms. The energy source supporting pelagicsummer production is believed to be the spring bloom. However,the spring bloom disappears relatively quickly from the watercolumn and a large proportion of the material sedimenting tothe bottom following the spring bloom is often comprised ofintact phytoplankton cells. Thus, it is easy to argue that thespring bloom is fueling the energy demands of the benthos, butmore difficult to argue convincingly that energy fixed duringthe spring bloom is fueling the pelagic production occurringduring summer months. We argue here that periodic phytoplanktonblooms are occurring during the summer in the North Sea at depthsof >25 m and that the accumulated new production [sensu (Dugdaleand Goering, Limnol. Oceanogr., 12, 196–206, 1967)] occurringin these blooms may be greater than that occurring in the springbloom in the same regions. Thus, such blooms may explain apparentdiscrepancies in production yields between different temperatemarine systems.     

Seasonal microbial processes in a high-latitude fjord (Kongsfjorden, Svalbard): I. Heterotrophic bacteria, picoplankton and nanoflagellates

Temporal dynamics of the microbial food web in the Barents Sea and adjacent water masses in the European Arctic are to a large extent unknown. Seasonal variation in stocks and production rates of heterotrophic bacteria and phototrophic and heterotrophic picoplankton and nanoflagellates was investigated in the upper 50 m of the high-latitude Kongsfjorden, Svalbard, during six field campaigns between March and December 2006. Heterotrophic bacteria, picoplankton and nanoflagellates contributed to ecosystem structure and function in all seasons. Activity within the microbial food web peaked during spring bloom in April, parallel to low abundances of mesozooplankton. In the nutrient-limited post-bloom scenario, an efficient microbial loop, fuelled by dissolved organic carbon from abundant mesozooplankton feeding on phytoplankton and protozooplankton, facilitated maximum integrated primary production rates. A tight microbial food web consisting of heterotrophic bacteria and phototrophic and heterotrophic picoplankton and nanoflagellates was found in the stratified water masses encountered in July and September. Microbial stocks and rates were low but persistent under winter conditions. Seasonal comparisons of microbial biomass and production revealed that structure and function of the microbial food web were fundamentally different during the spring bloom when compared with other seasons. While the microbial food web was in a regenerative mode most of the time, during the spring bloom, a microbial transfer mode represented a trophic link for organic carbon in time and space. The microbial food web’s ability to fill different functional roles in periods dominated by new and regenerated production may enhance the ecological flexibility of pelagic ecosystems in the present era of climate change.     

Composition and distribution of ciliates inner and outer of Daya Bay, China

Daya Bay was undergoing eutrophication process by increased nutrient loading, and the changes in nutrients have strongly influenced the phytoplankton community structure. Ciliates are common component of planktonic community, what role do ciliates play in Daya Bay was still unknown. In this study, ciliates were enumerated and identified from the inner and outer Daya Bay during three seasons. Thirty-one species belong to 16 genera of ciliates were recorded, and Cyclotrichida, Strombidiida, Tintinnida were most common. In spring, ciliates abundance of D2 was lower than D1 and D3, abundance in the surface of D1 and D3 were higher than in the bottom, while D2 showed the opposite character. The lowest ciliate abundance was found in the E1 in summer. In fall, Ciliates abundance of D3 was lower than in fall, and abundance in the surface was lower than in the bottom, which was different compared to in spring. Different hydrographic character was shown between the surface (high temperature, low salinity and nutrients) and bottom layer (low temperature, high salinity and nutrients) at E1 in summer. Ciliate abundance had no significant variation between the surface and bottom at E1 in summer, but ciliates community structure changed a lot.     

Composition and distribution of ciliates inner and outer of Daya Bay, China

Daya Bay was undergoing eutrophication process by increased nutrient loading, and the changes in nutrients have strongly influenced the phytoplankton community structure. Ciliates are common component of planktonic community, what role do ciliates play in Daya Bay was still unknown. In this study, ciliates were enumerated and identified from the inner and outer Daya Bay during three seasons. Thirty-one species belong to 16 genera of ciliates were recorded, and Cyclotrichida, Strombidiida, Tintinnida were most common. In spring, ciliates abundance of D2 was lower than D1 and D3, abundance in the surface of D1 and D3 were higher than in the bottom, while D2 showed the opposite character. The lowest ciliate abundance was found in the E1 in summer. In fall, Ciliates abundance of D3 was lower than in fall, and abundance in the surface was lower than in the bottom, which was different compared to in spring. Different hydrographic character was shown between the surface (high temperature, low salinity and nutrients) and bottom layer (low temperature, high salinity and nutrients) at E1 in summer. Ciliate abundance had no significant variation between the surface and bottom at E1 in summer, but ciliates community structure changed a lot.     

Spring weather determines the relative importance of ciliates, rotifers and crustaceans for the initiation of the clear-water phase in a large, deep lake

Clear-water phase (CWP) is an important event in seasonal planktonsuccession. We examined the influence of all herbivorous zooplanktonon its initiation under different weather and climatic conditionsusing up to 19 years of observations from the large, deep LakeConstance (Europe) and estimates of relative clearance rates.A CWP occurred regularly, even if daphnid biomass was stillvery low. CWP was attributed to strong grazing either by a daphnid-dominatedzooplankton community or by a diverse assemblage consistingof micro- and meso-zooplankton. Both types of zooplankton communitiesoccurred with approximately the same frequency. The timing ofthe CWP was unrelated to the North Atlantic Oscillation (NAO)but correlated with the wind-dependent intensity of deep verticalmixing 3 months earlier, during early spring. Less mixing enabledearly growth of phytoplankton, ciliates and rotifers despitelow temperatures, which prevented daphnid development at thistime. This resulted in enhanced grazing of ciliates and rotifers,which increased the importance of phytoplankton less ediblefor most ciliates, rotifers and daphnids. Ciliates clearly dominatedthe grazing pressure on phytoplankton throughout spring, maintaininghigh biomasses together with the phytoplankton for up to 2 months.A CWP was observed when herbivores grazing on larger phytoplanktondeveloped in addition to ciliates.     

Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.     

Effects of different N- and P-loading on primary and secondary production in an experimental marine ecosystem

The effects of nutrient loading on phytoplankton, zooplankton and macrozoobenthos in experimental ecosystems was studied in a 7-month experiment. The mesocosms were designed to mimic the major physical characteristics (irradiance, temperature, mixing) of the Dutch coastal zone in the river Rhine plume. Three different nutrient loading scenarios were used, representing present and future conditions. The level of the spring phytoplankton bloom was determined by phosphorus loading, whereas during summer the nitrogen loading determined phytoplankton biomass. The differences in nutrient loading did not result in shifts in phytoplankton species composition. With exception of the early phase of the spring bloom, diatoms dominated phytoplankton biomass in all nutrient treatments. This was ascribed to microzooplankton grazing on smaller algal species. Microzooplankton biomass showed a positive correlation with primary production, and also significant differences between nutrient treatments. Copepod development was limited, probably due to competition with microzooplankton and predation by benthic fauna. Macrobenthos biomass correlated with primary production, and was lower in the lowest nutrient treatment.     

Phytoplankton blooms: a 'loophole' in microzooplankton grazing impact?

Phytoplankton size and relations between phytoplankton and microzooplankton(ciliates and heterotrophic dinoflagellates) biomass are analysedin 12 globally distributed areas. In view of the results, ahypothesis is posed where blooming species are those able toescape control by microzooplankton through a combination ofpredation avoidance mechanisms (e.g. larger size, colonies,spines, and toxic compounds) at the beginning of the bloom.Factors that help to enhance subsequent bloom development includepositive feedback from the poor nutritional status of the phototrophicprey which adversely affects predation, inter-microzooplanktongrazing and top–down grazing by mesozooplankton on microzooplankton.Blooming conditions are interpreted as physical or chemicalperturbations disrupting the predator–prey controls thatnormally operate at the level of the microbial loop, opening‘loopholes’ into which some phytoplankton speciespopulations can explode.     

Annual Changes of Abundance and Biomass of Planktonic Ciliates in the Gdańsk Basin,Southern Baltic

Seasonal changes in the species composition, abundance and biomass of planktonic ciliates were determined every 2–3 weeks at two sites of 30 m depth and one location of 105 m depth in the southwestern Gdańsk Basin between January 1987 and January 1988. A total of 40 ciliate taxa were observed during this period. Autotrophic Mesodinium rubrum dominated ciliate abundance and biomass: maximal values of 50 · 10⁻¹ ind. ^1-1 and 65 μg C 1⁻¹ were recorded. The annual mean biomass of M. rubrum comprised 6 to 9% of the annual mean phytoplankton biomass. The highest abundances and biomasses of heterotrophic ciliates were noted at all stations in the spring and summer in the euphotic zone with maximum values of 28 · 10³ ind. 1⁻¹ and 23 μg C 1⁻¹. Three ciliates assemblages were distinguished in the epipelagic layer: large and medium-size non-predatory ciliates, achieving peak abundance in spring and autumn; small-size microphagous ciliates and epibiotic ciliates which were abundant in summer, and large-size predacious ciliates dominating in spring. Below 60 m, a separate deep-water ciliate community composed of Prorodon-like ciliates and Metacystis spp. was found. The ciliate biomass in the 60–105 m layer was similar to the ciliate biomass in the euphotic zone. The heterotrophic ciliate community contributed 10 to 13% to the annual mean zooplankton biomass. The potential annual production of M. rubrum comprised 6 to 9% of the total primary production. Carbon demand of non-predatory ciliates, calculated on the basis of their potential production, was estimated to be equivalent to 12–15% of the gross primary production.     

Annual microplankton cycles in Villefranche Bay, Ligurian Sea, NW Mediterranean

Abundance and composition of microplankton were studied overa period of 2 years at two depths in Villefranche Bay (LigurianSea, NW Mediterranean Sea). Diatoms dominated the microplanktonin late spring and autumn, whereas dinoflagellates composedthe major part of the microplankton in summer. The silicoflagellateDictyocha fibula and the diatom Thalassionema frauenfeldii dominatedin winter. Ciliates showed low variability throughout the yearwith the lowest abundance in February and an increase whichcoincided with the diatom maxima during autumn in both years.In 1998, the spring bloom (in May) was mainly composed of dinoflagellatesnear the surface and of diatoms in deeper layers. Subsurfacediatom maxima were observed in August–September and November.In 1999, diatoms peaked in May both at the surface and at thedepth of 50 m. They showed a strong maximum in October. Dinoflagellatesand tintinnids showed maxima in early November. Comparisonswith previous studies reveal that (i) changes in species compositionhave not been significant, (ii) the silicoflagellate’sabundance is lower during the present study, (iii) the sequentialspring bloom is composed of a pico-nanoplankton bloom in Marchand microphytoplankton in May, whereas in other western Mediterraneanareas the spring microphytoplankton bloom is reported in Februaryand March, (iv) high water transport through the Corsica channelcoinciding with low or negative winter values of North AtlanticOscillation (NAO) index are associated with the anomalous strongdevelopment of the spring diatom blooms in the Bay of Villefranche,whereas the usual trend is the lack of or weak development ofthe spring diatom bloom. This feature may determine the natureand the fate of primary production and the interannual variabilityin the relative importance of the microbial food web versusthe microbial loop.     

Linkages between bacterioplankton community composition, heterotrophic carbon cycling and environmental conditions in a highly dynamic coastal ecosystem

We used mesocosm experiments to study the bacterioplankton community in a highly dynamic coastal ecosystem during four contrasting periods of the seasonal cycle: winter mixing, spring phytoplankton bloom, summer stratification and autumn upwelling. A correlation approach was used in order to measure the degree of coupling between the dynamics of major bacterial groups, heterotrophic carbon cycling and environmental factors. We used catalysed reporter deposition-fluorescence in situ hybridization to follow changes in the relative abundance of the most abundant groups of bacteria (Alphaproteobacteria, Gammaproteobacteria and Bacteroidetes). Bacterial carbon flux-related variables included bacterial standing stock, bacterial production and microbial respiration. The environmental factors included both, biotic variables such as chlorophyll-a concentration, primary production, phytoplankton extracellular release, and abiotic variables such as the concentration of dissolved inorganic and organic nutrients. Rapid shifts in the dominant bacterial groups occurred associated to environmental changes and bacterial bulk functions. An alternation between Alphaproteobacteria and Bacteroidetes was observed associated to different phytoplankton growth phases. The dominance of the group Bacteroidetes was related to high bacterial biomass and production. We found a significant, non-spurious, linkage between the relative abundances of major bacterial groups and bacterial carbon cycling. Our results suggest that bacteria belonging to these major groups could actually share a function in planktonic ecosystems.     

Phytoplankton biomass and production in shelf waters off NW Spain: spatial and seasonal variability in relation to upwelling

Summary Chlorophyll-a and primary production on the euphotic zone of the N-NW Spanish shelf were studied at 125 stations between 1984 and 1992. Three geographic areas (Cantabrian Sea, Rías Altas and Was Baixas), three bathymetric ranges (20 to 60 m, 60 to 150 m and stations deeper than 200 m), and four oceanographic stages (spring and autumn blooms, summer upwelling, summer stratification and winter mixing) were considered. One of the major sources of variability of chlorophyll and production data was season. Bloom and summer upwelling stages have equivalent mean and maximum values. Average chlorophyll-a concentrations approximately doubled in every step of the increasing productivity sequence: winter mixing — summer stratification — high productivity (upwelling and bloom) stages. Average primary production rates increased only 60% in the described sequence. Mean (± sd) values of chlorophyll-a and primary production rates during the high productivity stages were 59.7 ± 39.5 mg Chl-a m^–2 and 86.9 ± 44.0 mg C m^–2 h^–1, respectively. Significant differences in both chlorophyll and primary production resulted between geographic areas in most stages. Only 27 stations showed the effects of the summer upwelling that affected coastal areas in the Cantabrian Sea and Rías Baixas shelf, but also shelf-break stations in the Rías Altas area. The Rías Baixas area had lower chlorophyll than both the Rías Altas and the Cantabrian Sea areas during spring and autumn blooms, but higher during summer upwelling events. On the contrary, primary production rates were higher in the Rías Baixas area during blooms in spring and autumn. Mid-shelf areas showed the highest chlorophyll concentrations during high productivity stages, probably due to the existence of frontal zones in all geographic areas considered. The estimated phytoplankton growth rates were comparable to those of other coastal upwelling systems, with average values lower than the maximum potential growth rates. Doubling rates for upwelling and stratification stages in the northern and Rías Altas shelf areas were equivalent, despite larger biomass accumulations during upwelling events. Low turnover rates of the existing biomass in the Rías Baixas shelf in upwelling stages suggests that the accumulation of phytoplankton was due mainly to the export from the highly productive rías, while the contribution of in situ production to these accumulations was relatively lower.     

Nutrients,light and primary production by phytoplankton and microphytobenthos in the eutrophic,turbid Westerschelde estuary (The Netherlands)

Abiotic factors and primary production by phytoplankton and microphytobenthos was studied in the turbid Westeschelde estuary. Because of the high turbidity and high nutrient concentrations primary production by phytoplankton is light-limited. In the inner and central parts of the estuary maximum rates of primary production were therefore measured during the summer, whereas in the more marine part spring and autumn bloom were observed. Organic loading is high, causing near anaerobic conditions upstream in the river Schelde. Because of this there were no important phytoplankton grazers in this part of the estuary and hence the grazing pressure on phytoplankton was minimal. As this reduced losses, biomass is maximal in the river Schelde, despite the very low growth rates.On a number of occasions, primary production by benthic micro-algae on intertidal flats was studied. Comparison of their rates of primary production to phytoplankton production in the same period led to the conclusion that the contribution to total primary production by benthic algae was small. The main reason for this is that the photosynthetic activity declines rapidly after the flats emerged from the water. It is argued that CO₂-limitation could only be partially responsible for the noticed decrease in activity.