Chloropigments a in the Gulf of Gdańsk (Baltic Sea) as markers of the state of this environment

The Gulf of Gdańsk receives the waters of the Wis?a (Vistula), the largest Polish river and the second largest river flowing into the Baltic Sea. Chloropigments a (chlorophyll a and its derivatives) were determined in water and sediments from the Gulf. Samples were collected during cruises of R.V. 'Oceania' at 19 stations located at different water depths and distances from the shore, in May (2003) and October (2004), i.e., at the beginning and at the end of the growing season, respectively. Seawater was also sampled on the beach at Sopot in June, July and August 2004, i.e., during the height of the growing season. All water samples were collected with a bathometer and passed through Whatman GF/F filters. Recent sediments (0-10 cm) were collected with a core sampler and divided into sub-samples (0-1, 1-5 and 5-10 cm). The pigments were determined using HPLC-DAD. The results were first correlated with a set of parameters measured in situ during sample collection (salinity, temperature, oxygen content of seawater) and determined in the same material (organic carbon content, granulometry), then worked up statistically. Although this investigation shows that particular chloropigments a in recent sediments are indicative of the prevailing environmental conditions in the study area and the trends in their variations, they should be treated as a relative and not an absolute measure, since this is the first such study based on a large and coherent set containing both chloropigment a and environmental data. In addition, a chlorophyll a budget is estimated for the Gulf of Gdańsk on the basis of the literature and the authors' own data. Only c. 10% of the chlorophyll a produced annually in the Gulf of Gdańsk is transferred to sediments in the form of chloropigments a; 2.8% is in the form of undecomposed chlorophyll a and 7.2% in the form of chlorophyll a derivatives. About 90% of chlorophyll a is decomposed to colourless products.     

Towards a more ecologically relevant assessment of the impact of heavy metals on the photosynthesis of the seagrass, Zostera capricorni

This in situ study used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of copper, cadmium, lead and zinc on the seagrass Zostera capricorni. Custom-made portable in situ exposure (PIE) chambers were developed so seagrasses could be dosed within the meadow. Z. capricorni was exposed to 0.1 and 1 mg l⁻¹ of metal solutions for 10 h. During this time and for the subsequent four-day recovery period, the effective quantum yield of photosystem II (PS II) (ΔF/Fm^′) was measured. While the results were variable, copper and zinc exposed samples had a depressed ΔF/Fm^′ during the exposure period. Samples exposed to zinc recovered to pre-exposure levels but those exposed to copper did not. Cadmium and lead did not impact on the chlorophyll a fluorescence and the chlorophyll pigment data supported these findings. This study presents an innovative new application of chlorophyll a fluorescence stress assessment.     

Interactions between freshwater input, light, and phytoplankton dynamics on the Louisiana continental shelf

We examined the effects of freshwater flow and light availability on phytoplankton biomass and production along the Louisiana continental shelf in the region characterized by persistent spring–summer stratification and widespread summer hypoxia. Data were collected on 7 cruises from 2005 to 2007, and spatially-averaged estimates of phytoplankton and light variables were calculated for the study area using Voronoi polygon normalization. Shelf-wide phytoplankton production ranged from 0.47 to 1.75 mg C m⁻² d⁻¹ across the 7 cruises. Shelf-wide average light attenuation (k_d) ranged from 0.19–1.01 m⁻¹ and strongly covaried with freshwater discharge from the Mississippi and Atchafalaya Rivers (R²=0.67). Interestingly, we observed that the euphotic zone (as defined by the 1% light depth) extended well below the pycnocline and to the bottom across much of the shelf. Shelf-wide average chlorophyll a (chl a) concentrations ranged from 1.4 to 5.9 mg m⁻³ and, similar to k_d, covaried with river discharge (R²=0.83). Also, chl a concentrations were significantly higher in plume versus non-plume regions of the shelf. When integrated through the water-column, shelf-wide average chl a ranged from 26.3 to 47.6 mg m⁻², but did not covary with river discharge, nor were plume versus non-plume averages statistically different. The high integrated chl a in the non-plume waters resulted from frequent sub-pycnocline chl a maxima. Phytoplankton production rates were highest in the vicinity of the Mississippi River bird's foot delta, but as with integrated chl a were not statistically different in plume versus non-plume waters across the rest of the shelf. Based on the vertical distribution of light and chl a, a substantial fraction of phytoplankton production occurred below the pycnocline, averaging from 25% to 50% among cruises. These results suggest that freshwater and nutrient inputs regulate shelf-wide k_d and, consequently, the vertical distribution of primary production. The substantial below-pycnocline primary production we observed has not been previously quantified for this region, but has important implications about the formation and persistence of hypoxia on the Louisiana continental shelf.     

Phytoplankton absorption spectra along the water column in deep North Patagonian Andean lakes (Argentina)

During a summer period we studied the vertical variation of in vivo and chlorophyll a specific phytoplankton absorption spectra in relation to the underwater light climate of ten deep North Patagonian Andean lakes of Argentina. The lakes were thermally stratified, and the underwater light climate was characterized by extended euphotic zones which included highly illuminated epilimnetic layers (both UVR and PAR) and metalimnia exposed to dim blue-green light. Most of the lakes presented the development of Deep Chlorophyll Maxima (DCM) at the metalimnetic layers, near 1% of surface PAR irradiance. Analyzing the fourth-derivative plots of in vivo phytoplankton absorption spectra [d^IVa_ph(λ)], we were able to identify several maxima absorption values attributed to different pigments. Considering lakes with DCM, a significant positive linear relationship was found between d^IVa_ph (495–500 nm) normalized by chlorophyll a and downward irradiance. Indeed, a negative significant relationship was found between d^IVa_ph (495–500 nm) normalized by chlorophyll a and diffuse PAR attenuation coefficients. These results point out an increase in the relative concentration of different carotenoids at surface layers indicating the role of photoprotection of these pigments. On the other hand, significant negative linear relationships were found between fourth-derivative spectra normalized by chlorophyll a at 650, 590–595, 560–565 and 520–525 nm and downward irradiance. These results indicated an increase in the relative concentration of photosynthetic accessory pigments at deep layers of the euphotic zone. Furthermore, we found a decrease in depth of specific absorption spectra at 440, 670 nm and in the ratio a_ph* (440 nm) to a_ph* (670 nm). This pattern was associated with the package effect concept. The increase in relative photosynthetic accessory pigment concentrations and the decrease in values of specific absorption spectra at the bottom of the euphotic zone were attributed to changes in phytoplankton communities between surface and deep layers. These outcomes pointed out that the underwater light climate and temperature water structure are, like in marine systems, very important factors governing the distribution of phytoplanktonic organisms. In addition, the possession of specific photosynthetic accessory pigments suggests that dominant species in the DCM are well adapted to these dim blue-green light scenarios.     

Spatial distribution of phosphorus forms in sediments in the Gulf of Gdańsk (southern Baltic Sea)

The paper analyses the concentrations of total phosphorus and its forms in sediments from the Gulf of Gdańsk on the basis of studies conducted at 25 sampling sites in 2001–05. The phosphorus speciation analysis was performed by sequential extraction. The extensive spatial variability of P_tot concentrations and speciation was found to be dependent on the physicochemical properties of the sediments, the oxygen conditions in the water and sediments, and the depth of the water column above the sediment surface. In the coastal zone, the sedimentation of riverine suspended matter and the sorption and chemisorption processes exert a considerable influence on P speciation. Over 70% of variation of total phosphorus concentration in sediments in the Gulf of Gdańsk could be explained by changes of proportion of fine fraction of sediments (grain size <0.0625 mm). Maximum P_tot concentrations were recorded in clays and silts in the deep water, stratified part of the Gulf of Gdańsk. In the coastal zone, where sandy sediments are dominant, phosphorus concentrations were much lower; this was due to the considerable dynamics of the bottom water and intensive sea floor transport. P_tot concentrations in the Gulf of Gdańsk sediments ranged from 1.75 to 957.17 μmol g⁻¹ d.w. Of all the forms of phosphorus, the highest concentrations were found for organic phosphorus (Org-P). Of its inorganic forms, the highest concentrations were of phosphorus bound to clay minerals and aluminium oxides (NaOH-P), the lowest ones were of loosely bound phosphorus (NaCl-P). On the basis of determinations of total phosphorus concentrations in sediments of a given type and the available data on the seabed areas covered by particular sediment types in the Gulf of Gdańsk, the mass of total phosphorus in the surficial sediment layer (0–2 cm) was estimated at ca. 15.6×10³ tonnes.     

Bio-dependent bed parameters as a proxy tool for sediment stability in mixed habitat intertidal areas

The stability of cohesive and non-cohesive sediments in a mixed intertidal habitat within the Ria Formosa tidal lagoon, Portugal, was examined during two field campaigns as part of the EU F-ECTS project. The cohesive strength meter Mk III was used to determine critical erosion shear stress (τ_c) within a variety of different intertidal habitats and substrata, including Spartina maritima fields and Zostera noltii beds. The best predictor(s) for τ_c were derived from a range of properties measured for the surface sediments (chlorophyll a, colloidal carbohydrate, water, organic content, % fraction <63 μm, and seabed elevation). Pigment biomarkers were used to identify the dominant algal groups within the surface phytobenthic assemblage.Strong, seasonally dependent relationships were found between τ_c and habitat type, chl a, colloidal carbohydrate and bed elevation. Typically, critical erosion thresholds decreased seawards, reflecting a change from biostabilisation by cyanobacteria in the upper intertidal areas, to biostabilisation by diatoms on the bare substrata of the channel edges. In the late summer/early autumn, cyanobacteria were the main sediment stabilisers, and colloidal carbohydrate was the best bio-dependent predictor of τ_c across the entire field area. In the late winter/early spring, cyanobacterial activity was lower, and sediment stabilisation by Enteromorpha clathrata was important; the best predictor of τ_c was bed elevation. The implications and use of proxies for sediment stability are discussed in terms of feedback and sedimentation processes operating across the intertidal area.     

Distribution of dissolved free amino acids, dissolved inorganic nitrogen and chlorophyll a in the surface microlayer and subsurface water of the Yellow Sea, China

Concentrations of 14 different dissolved free amino acids (DFAA), dissolved inorganic nitrogen (DIN), and chlorophyll a were determined in the surface microlayer and subsurface waters at 41 stations in the Yellow Sea, China in April 2006. The concentrations of DFAA in the subsurface water ranged from 0.13 to 1.62 μM, with an average of 0.57±0.05 μM, while those in the surface microlayer varied between 0.22 and 2.6 μM, with an average of 0.94±0.08 μM. Major constituents of DFAA present in the study area were glycine, alanine, glutamic acid, serine and histidine. One-way analysis of variance (ANOVA) showed no significant difference in average mol fractions of DFAA between microlayer and subsurface water (F=0.0440, P=0.8355). Hierarchical cluster analysis of the station similarity based on the DFAA composition in both the surface microlayer and subsurface water separated three clusters of stations at the 70% Bray–Curtis similarity level. The average concentrations of chlorophyll a and DIN were 1.18 (0.34–4.44) μg L⁻¹ and 16.57 (3.98–49.59) μM in the subsurface water, and those in the surface microlayer were 1.30 (0.24–3.97) μg L⁻¹ and 18.56 (5.77–48.93) μM, respectively. Our results showed that concentrations of chlorophyll a (r²=0.7940, n=41, p<0.0001), DIN (r²=0.6939, n=41, p<0.0001) and DFAA (r²=0.2416, n=41, P=0.0011) in the microlayer were, respectively, correlated with their subsurface water concentrations, implying that there was a strong exchange effect between the microlayer and subsurface water. The enrichment factor of DFAA in the microlayer ranged from 0.47 to 2.24 with a mean of 1.88±0.16.     

Small-scale,patchy distributions of infauna in hydrodynamically mobile continental shelf sands: Do ripple crests and troughs support different communities?

Spatial variability of infauna with respect to distribution of topographic habitat features was examined in hydrodynamically mobile sandy sediments on the inner continental shelf off New Jersey, USA (39° 27.69′ N, 74° 15.81′W). Sediment cores for infauna were taken by SCUBA divers at multiple spatial scales over time at 12-m depth in the LEO-15 research area on Beach Haven Ridge. Crests, troughs and less consistently flanks of sand ripples 5–15-cm in height, were characterized by different infaunal community patterns at spatial scales of centimeters to kilometers on several sampling dates. Overall, infaunal community differences among ripple crests, troughs, and/or flanks within areas <1-m² were greater than those found for each of these habitats (i.e., either crests, troughs, or flanks) that were separated by distances of 2 m–4 km. Infaunal density and species richness were consistently higher in troughs compared to crests. Indirect measures of food resources such as particulate organic carbon, chl a, and pheophytin were associated with ripple crests and troughs. Troughs contained significantly higher levels of particulate organic carbon (~1.2 times higher) associated with finer sediments, compared with crests and flanks. Various combinations of taxa had higher densities in either crests or troughs of sand ripples depending on date, and the relative abundances of three taxa, the deposit-feeding polychaete Polygordius jouinae, the suspension-feeding surfclam Spisula solidissima, and predatory nemerteans were important in distinguishing between crests and troughs on most dates. Thus, a priori knowledge of whether a benthic sample comes from a crest or trough helped to explain small-scale infaunal patchiness in relatively homogeneous, subtidal sandy sediments. Consideration of such topographic features in sampling designs can help in explaining variation in species’ distributions at several spatial and temporal scales.     

Distribution and cycling of dimethylsulfide (DMS) and dimethylsulfoniopropionate (DMSP) in the sea-surface microlayer of the Yellow Sea, China, in spring

The distributions of dimethylsulfide (DMS) and its precursor, dimethylsulfoniopropionate (DMSP), were examined in the surface microlayer and corresponding subsurface water of the Yellow Sea, China, in April 2006. The average concentrations of DMS and DMSP of dissolved (DMSPd) and particulate (DMSPp) forms were 5.42 (1.78–12.75), 9.22 (2.85–19.73) and 17.50 (4.33–36.09) nmol L⁻¹ in the subsurface water, and those in the surface microlayer were 4.92 (1.69–10.66), 17.08 (3.13–38.82) and 22.54 (4.85–47.24) nmol L⁻¹, respectively. The enrichment factor (EF) of DMS in the microlayer ranged from 0.47 to 2.24 with a mean of 0.98. In contrast, DMSPd and DMSPp appeared to be enriched in the microlayer with average EFs of 1.98 and 1.39, respectively. A close correlation of integrated DMS, DMSPp and chlorophyll a concentrations for compiled data from all stations in the microlayer and the subsurface water indicated that phytoplankton biomass might play an important role in controlling the distributions of biogenic sulfurs in the study area. Moreover, a statistically significant relationship was found between the microlayer concentrations of DMS, DMSP and chlorophyll a and their subsurface water concentrations, suggesting a close linkage between these two water compartments. Interestingly, we observed higher biological production rates and consumption rates of DMS in the microlayer relative to the subsurface water. Furthermore, the DMS production rates were closely correlated both with DMSPd and chlorophyll a concentrations. Our study showed that the major sink of DMS in microlayer was escape into the atmosphere, which greatly exceeded its bacterial consumption. A preliminary estimate for average flux of DMS from the Yellow Sea to the atmosphere was 6.41 μmol m⁻² d⁻¹ during spring.     

The cysts ofCeratium hirundinella: Their dynamics and role within a eutrophic (Lake Sempach,Switzerland)

The dynamics of theCeratium hirundinella population and the abundance of dinocysts in the plankton and sediments were studied in Lake Sempach in 1988. In 1987, a rich population ofCeratium (380 cells ml^–1) accompanied byPeridinium spp. developed in the lake. The dinocysts were found entrapped in a kind of flocs, in the deepest part of the lake, in the upper flocculent layer. The number of viable cysts ofCeratium in the sediments decreased gradually from April to July 1988. TheCeratium population increased slowly starting in April, and reached a maximum number in August (31 cells ml^–1).Peridinium willei reached 100 cells ml^–1. Newly formed cysts ofCeratium were recorded in the plankton and sediments at the end of July — beginning of August. They appear in the sediments as separate cells. Their number increased gradually, reaching a maximum of 600 cysts l^–1 at the end of October.Ceratium formed more cysts than didPeridinium, but the rate of survival of theCeratium cysts appears to be lower than that ofPeridinium cysts. In addition to their biological functions, the cysts also have an impact on the ecosystem as carriers of nutrients from down to up and from up to down.     

A hydrobiological study to interpret the presence of desmids in Lake Zirahuén, México

Monthly collections of phytoplankton were supported by physical–chemical data and measures of chlorophyll a concentrations in the search for particular environmental factors that could explain the constant presence of desmids in a Mexican tropical lake, a characteristic not common among the phytoplanktic communities of Mexican lakes. Samplings were taken from the water column in the deepest part of the lake (40 m) and intensified in the metalimnetic zone, whose establishment was monitored by observations of temperature and oxygen profiles. The general behavior of Lake Zirahuén was typical of warm monomictic tropical lakes at high elevation: a short mixing phase during the hemispheric winter. The depth of Z_MIX and Z_eu revealed a well-illuminated epilimnion, suggesting that phytoplankton communities are not likely to be light-limited. The oligotrophic nature of the lake is indicated by discrete concentrations of inorganic nutrients, P_TOT in the interval of 0.01–0.03 mg l⁻¹ and chlorophyll a between 0.23 and 3.98 μg l⁻¹. These characteristics together with a low concentration of calcium, define a lacustrine environment different from other Mexican lakes, and one that could be suitable for desmids communities.     

Phytoplankton biomass size structure and its regulation in the Southern Yellow Sea (China): Seasonal variability

Phytoplankton size structure plays a significant role in controlling the carbon flux of marine pelagic ecosystems. The mesoscale distribution and seasonal variation of total and size-fractionated phytoplankton biomass in surface waters, as measured by chlorophyll a (Chl a), was studied in the Southern Yellow Sea using data from four cruises during 2006–2007. The distribution of Chl a showed a high degree of spatial and temporal variation in the study area. Chl a concentrations were relatively high in the summer and autumn, with a mean of 1.42 and 1.27 mg m⁻³, respectively. Conversely, in the winter and spring, the average Chl a levels were only 0.98 and 0.99 mg m⁻³. Total Chl a showed a clear decreasing gradient from coastal areas to the open sea in the summer, autumn and winter cruises. Patches of high Chl a were observed in the central part of the Southern Yellow Sea in the spring due to the onset of the phytoplankton bloom. The eutrophic coastal waters contributed at least 68% of the total phytoplankton biomass in the surface layer. Picophytoplankton showed a consistent and absolute dominance in the central region of the Southern Yellow Sea (>40%) in all of the cruises, while the proportion of microphytoplankton was the highest in coastal waters. The relative proportions of pico- and nanophytoplankton decreased with total biomass, whereas the proportion of the micro-fraction increased with total biomass. Relationships between phytoplankton biomass and environmental factors were also analysed. The results showed that the onset of the spring bloom was highly dependent on water column stability. Phytoplankton growth was limited by nutrient availability in the summer due to the strong thermocline. The combined effects of P-limitation and vertical mixing in the autumn restrained the further increase of phytoplankton biomass in the surface layer. The low phytoplankton biomass in winter was caused by vertical dispersion due to intense mixing. Compared with the availability of nutrients, temperature did not seem to cause direct effects on phytoplankton biomass and its size structure. Although interactions of many different environmental factors affected phytoplankton distributions, hydrodynamic conditions seemed to be the dominant factor. Phytoplankton size structure was determined mainly by the size-differential capacity in acquiring resource. Short time scale events, such as the spring bloom and the extension of Yangtze River plume, can have substantial influences, both on the total Chl a concentration and on the size structure of the phytoplankton.     

Does chlorophyllidea reduce reliability of chlorophylla measurements in marine coastal sediments?

Pigment concentrations (chlorophylla, chlorophyllidea and pheopigmentsa) were measured by HPLC and spectrophotometry with acidification on 57 samples collected in different marine coastal sediments, containing autochthonous microphytes, and with various organic matter contents (plant detritus, biodeposits or hydrocarbons). Statistical analysis shows that the spectrophotometry with acidification, as compared to HPLC, gives reliable values for chlorophylla. Chlorophyllidea concentrations may be considered as negligible. Though spectrophotometric methods are sometimes questioned when applied to sediments they appear to give easy, quick and good estimates of Chla contents in benthic microphytes for hydrobiological studies in coastal areas.     

The short term response to eutrophication abatement

We compare results of a new model for predicting the short term inter annual changes in chlorophyll-a (chl-a) in lakes after reductions in total phosphorus (TP) to predictions made by least squares regression models. In the new method, slopes of chl-a/TP graphs (both axes in mg · m^–3) are depicted in frequency diagrams and used to extract information on the expected, short term chl-a/TP response. The short term response for nine shallow (< 10 m deep) and nutrient rich lakes to changes in TP was found to be: Chl-a = 0.49 · TP + 17.3, and for nine deep, P-limited lakes: Chl-a = 0.08 · TP + 3.5. If the TP-reduction is known to be greater than 10 mg · m^–3, the expected slope increases to 0.58 for shallow lakes and to 0.26 for deep lakes. The slope, 0.58, is 8% lower than the slope for the long term response calculated by regression for the shallow lakes. For deep lakes the slope, 0.26, is 2 to 3 times higher than that calculated by regression, indicating that reductions in TP for deep lakes give greater effects than least squares regression equations suggest. We have also calculated the reduction in TP which will give about 80% probability that a reduction in chl-a will be observed next year. For shallow, P-limited lakes this reduction is about 30 mg · m^–3 (5% of average initial in-lake TP concentration), and for deep lakes about 14 mg · m^–3 (35% of average initial in-lake TP concentration).     

Transport ofBe andBe in the ocean

Beryllium isotopes (¹⁰Be and⁹Be) have been measured in suspended particles of < 1 mm size collected by mid-water sediment traps deployed in the eastern Pacific at MANOP sites H (6°32′N, 92°50′W, water depth 3600 m) and M (8°50′N, 104°00′W, 3100 m). For comparison, surface sediments from box cores taken from the two sites were also studied. The concentrations of¹⁰Be and⁹Be in sediment-trap particles are about an order of magnitude smaller than those in the bottom sediments which contain about 8 × 10⁹ and 6 × 10¹⁶ atoms g⁻¹ of¹⁰Be and⁹Be, respectively. The sediment trap samples collected from 50 m off the bottom showed significant (26–63%) contributions from resuspended bottom sediments. The¹⁰Be/⁹Be ratio in trap samples varies from 3 to 20 × 10⁻⁸. The variation may partly result from varied proportion of authigenic/detrital material. The fluxes of both isotopes exhibit a very strong seasonality. The fluxes of¹⁰Be into the traps at about 1500 m are estimated as 9 × 10⁵ and 4 × 10⁵ atoms cm⁻² a⁻¹ at sites H and M respectively. These values are to be compared with the fluxes into the sediments of 4–5 × 10⁵ atoms cm⁻² a⁻¹ at both locations. Good correlations exist between¹⁰Be,⁹Be and²⁷Al indicating that the primary carrier phase(s) for the beryllium isotopes in the water column may be aluminosilicates.     

Seasonal dynamics of zooplankton in Lake Huetzalin, Xochimilco (Mexico City, Mexico)

We quantified the seasonal changes in the zooplankton abundances collected from the Huetzalin Lake (Mexico City, Mexico) for two years (February 2003–January 2004 and then March 2005–February 2006). Selected physicochemical variables (Secchi depth, temperature, pH, conductivity, dissolved oxygen, phosphorus, nitrogen, carbon and chlorophyll a concentration) were also measured at the time of zooplankton collection. The data on zooplankton abundances and the physicochemical variables were subjected to multiple correlation analysis and we also derived Shannon–Wiener species diversity index. Secchi depth ranged from 9 to 65 cm. Generally the lake was alkaline (pH 7–12). The conductivity ranged from 500–1000 mS cm⁻¹, while the mean water temperature was 20.5 °C. Dissolved oxygen levels were generally >3 mg L⁻¹ and were higher in the winter than warmer months. Nitrates (90–95 μg L⁻¹) and phosphates (.2–.5 mg L⁻¹) indicated that the water was eutrophic. Chlorophyll a levels ranged from 143 to 696 μg L⁻¹ during the study period. The zooplankton community was dominated by rotifers (46 species), followed by cladocerans (9 species) and there were only two copepod species. The dominant rotifer species were Brachionus angularis, Brachionus calyciflorus, Brachionus havanaensis, Brachionus quadridentatus, Lecane bulla and Polyarthra vulgaris. Rare rotifer species in Lake Huetzalin were Lecane ohioensis, Dicranophorus forcipatus, Lecane pyriformis, Lindia torulosa, Pleurotrocha petromyzon and Brachionus durgae. Highest densities (occasional peaks of 400 ind L⁻¹) of B. quadridentatus occurred between April and December, while B. havanaensis reached peak densities, during June to October. B. calyciflorus reached densities higher than 1240 ind L⁻¹ during May–September. Cladoceran and copepod densities in Lake Huetzalin were much lower than that of the rotifers. This study confirmed the earlier findings that Xochimilco system of canals is dominated by rotifers and the crustacean zooplankton have much lower abundances possibly due to predation from fish.     

Nitrogen cycling across the sediment-water interface in an eutrophic,artificially oxygenated lake

Processes controlling the nitrogen (N) exchange between water and sediment in eutrophic Lake Sempach were studied using three different independent methods: benthic flux chambers, interstitial water data and hypolimnetic mass balances. The sediments released NH ₄ ⁺ (1.1–16.1 mmoles m^–2 d^–1), NO ₂ ^- (0.1–0.4 mmoles m^–2 d^–1) and dissolved organic N (<0.25 mmoles m^–2 d^–1). A net NO ₃ ^- consumption (2.4–11.1 mmoles m^–2 d^–1) related to the NO ₃ ^- concentrations in the overlying water was observed in all benthic chamber experiments. The flux of the reactive species NO ₃ ^- and NH ₄ ⁺ was found to depend on hydrodynamic conditions in the water overlying the sediment. For this reason, benthic chambers overestimated the fluxes of inorganic N compared to the other methods. Thus, in short-term flux chamber experiments the sediment may either become a sink or a source for inorganic N depending on the O₂ concentration in the water overlying the sediment and the stirring rate. As demonstrated with a¹⁵NO ₃ ^- experiment, nitrate-ammonification accounted for less than 12% of the total NO ₃ ^- consumption. After six years of artificial oxygenation in Lake Sempach, a decrease in hypolimnetic total inorganic nitrogen (TIN) was observed in the last two years. The occurrence of dense mats of H₂S-oxidizingBeggiatoa sp. indicated micro-aerobic conditions at the sediment surface. Under these conditions, a shorter distance between the ecological niches of nitrifying and denitrifying bacteria, and therefore a faster NO ₃ ^- -transport, can possibly explain the lowering of TIN by enhanced net denitrification.     

Consequences of coastal upwelling events on physical and chemical patterns in the central Gulf of Finland (Baltic Sea)

The consequences of a coastal upwelling event on physical and chemical patterns were studied in the central Gulf of Finland. Weekly mapping of hydrographical and -chemical fields were carried out across the Gulf between Tallinn and Helsinki in July–August 2006. In each survey, vertical profiles of temperature and salinity were recorded at 27 stations and water samples for chemical analyses (PO₄³⁻, NO₂⁻+NO₃⁻) were collected at 14 stations along the transect. An ordinary distribution of hydrophysical and -chemical variables with the seasonal thermocline at the depths of 10–20 m was observed in the beginning of the measurements in July. Nutrient concentrations in the upper mixed layer were below the detection limit and nutriclines were located just below or in the lower part of the thermocline. In the first half of August, a very intense upwelling event occurred near the southern coast of the Gulf when waters with low temperature and high salinity from the intermediate layer surfaced. High nutrient concentrations were measured in the upwelled water – 0.4 μmol l⁻¹ of phosphates and 0.6 μmol l⁻¹ of nitrates+nitrites. We estimated the amount of nutrients transported into the surface layer as 238–290 tons of phosphorus (P)-PO₄³⁻ and 175–255 tons of N-NO_x for a 12 m thick, 20 km wide and 100 km long coastal stretch. Taking into account a characteristic along-shore extension of the upwelling of 200 km, the phosphate-phosphorus amount is approximately equal to the average total monthly riverine load of phosphorus to the Gulf of Finland. It is shown that TS-characteristics of water masses and vertical distribution of nutrients along the study transect experienced drastic changes caused by the upwelling event in the entire studied water column. TS-analysis of profiles obtained before and during the upwelling event suggests that while welled up, the cold intermediate layer water was mixed with the water from the upper mixed layer with a share of 85% and 15%. We suggest that the coastal upwelling events contribute remarkably to the vertical mixing of waters in the Gulf of Finland. Intrusions of nutrient-rich waters along the inclined isopycnal surfaces in the vicinity of upwelling front were revealed. The upwelling event widened the separation of phosphocline and nitracline which in turn prevented surfacing of nitrate+nitrite-nitrogen during the next upwelling event observed a week after the upwelling relaxation. A suggestion is made that such widening of nutricline separation caused by similar upwelling events in early summer could create favourable conditions for late summer cyanobacterial blooms.     

Nanociliates of the order Prostomatida: Their relevance in the microbial food web of a mesotrophic lake

Two nanociliates of the order Prostomatida,Pseudobalanion planctonicum (12–18 µm) andUrotricha furcata (12–21 µm), were found to be the most abundant ciliates in the epilimnion of Piburger See, a small mesotrophic lake. Temperature and food availability were the main factors controlling the sudden increase in abundance, which reached a maximum of 101 cell ml^–1 at the beginning of summer. During their exponential development in numbers and biomass, a strong decrease in chlorophylla and in the abundance of phytoplankton, especiallyRhodomonas, was observed. We assume, therefore, thatPseudobalanion planctonicum andUrotricha furcata together with rotifers, mainlyPolyarthra dolichoptera, are able to reduce the phytoplankton biomass in the upper epilimnion to the same level as found during the clearwater phase. Preliminary results of grazing on bacteria suggest that these nanociliates are omnivorous, although their impact on bacterial assemblages was low (3.1% of the standing stock grazed per day). Feeding on the base of the food web combined with their high abudance at certain times makes them an important link for higher trophic levels. This study constitutes the second report onPseudobalanion as an important component of the microbial food web in lakes.     

Planktonic algae and epiphyton of the littoral in lake Peipsi, Estonia

The littoral plankton of Lake Peipsi (3555 km², mean depth 7.1 m) was studied in summer 1980 and 2000–2002, and the epiphyton was studied on two dominating macrophytes, Phragmites australis (Cav.) Trin. ex Steud. and Potamogeton perfoliatus L., in 2000. The purpose of the study was to estimate to what extent the littoral phytoplankton and epiphytic algae (their biomass, chlorophyll a content and dominant species) can be used as a criterion for the trophic state of the lake. In wind-open areas, phytoplankton biomass in the littoral is commonly more than ten times higher than in open water due to the presence of the macroscopic cyanobacterium Gloeotrichia echinulata (J.S. Smith) P. Richter. This alga is abundant in the moderately eutrophic northern lake's part and rare in the strongly eutrophic southern part; hence also biomass in the southern part is considerably lower. In open water, phytoplankton biomass increases southward with increasing trophy. Algal biomass in the littoral depends on wind direction and can change completely in a few days. Epiphyton biomass and Chla content increased southward with increasing trophic state. They both revealed significant Spearman correlations (P < 0.05) with wind index and transparency (negative), and with abundance of the host plant, both reed and pondweed (positive). The phytoplankton biomass of the littoral of the large and shallow Lake Peipsi can not be used as a criterion of trophic state, however, the species composition of the dominants, particularly cyanobacteria, is well applicable for this purpose. The biomass and Chla content of the epiphyton can be used as a criterion for trophic state.