Phytoplankton Biomass Production Research Articles

Phytoplankton community composition as a driver of annual autochthonous organic carbon dynamics in the northern coastal Baltic Sea

Phytoplankton are the major primary producers in the pelagic system. They greatly influence biogeochemical cycles but little is known about the importance of shifting phytoplankton community composition for carbon dynamics. This study investigates the impact of seasonal changes in coastal phytoplankton communities on pelagic carbon fluxes. A field sampling campaign, covering an annual cycle in primary production, was conducted to assess the seasonal changes of phytoplankton communities and relevant organic carbon parameters in the coastal Baltic Sea. The monitoring frequency ranged from 1 to 3 wk, adapted to match the seasonal phytoplankton blooms. In addition, sediment traps were deployed to determine the particulate carbon and nutrient export to the seafloor in every season. We found that the phytoplankton biomass during the spring bloom was as high as 550 µg C l-1 and was dominated by diatom species (88% of total phytoplankton biomass). In comparison, the more species-rich summer bloom reached a combined maximum biomass of 236 µg C l-1. However, the highest export flux of particulate organic carbon was found in the middle of August (561 mmol C m-2 d-1) and, not as expected, around the spring bloom in May (226 mmol C m-2 d-1), suggesting a high potential for carbon recycling within the pelagic food web rather than being exported to the seafloor or advected laterally. Our study emphasizes the importance of keystone species and diversity for carbon transport processes in marine coastal ecosystems and highlights complex relationships between phytoplankton biomass production, community composition and carbon dynamics.

Physical control on the inter-annual variability of summer dissolved nutrient concentration and phytoplankton biomass in the Indian sector of the Southern Ocean

To understand the role of physical processes and their interannual variability on the dissolved nutrient concentration and phytoplankton biomass distribution, field data collected in the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2009–2011 are used. In the subtropical zone, macronutrients were limited (N:P < 1, N:Si < 1, Si:P ≈ 1) and the phytoplankton biomass variability was mainly governed by the mesoscale eddy activity associated with the Agulhas Return Current. High nutrient low chlorophyll condition prevailed in the sub-Antarctic zone and further south. A South-North gradient of the upper layer dissolved SiO2 was higher than that of NO3. The sub-Antarctic zone was characterized by the highest N:Si ratio (>4) and it was associated with the enhanced draw down of silicate due to the winter/spring diatom blooms in the region. The chlorophyll-a (CHL) concentration in the Polar Frontal Zone was low (∼0.2 mg m–3) in 2009 and 2010 but it was high (0.5 mg m–3) in 2011. This increase in CHL in 2011 was due to the supply of dissolved iron from the strong winds and subsequent mixing during the winter of 2010. Further, the increased CHL values in the Antarctic zone (0.5 mg m–3) in 2011 compared to 2009 and 2010 could be due to the increased sea ice melting associated with positive Southern Annular Mode. The increased phytoplankton biomass in the summer of 2011 coincides with an increase in nitrate utilization (N:P≈13) compared to 2009 and 2010. Observations showed that ISSO frontal zones are characterized by inter-annual variability in terms of nutrient utilization and phytoplankton biomass production.

Impacts of sediment resuspension on phytoplankton biomass production and trophic transfer: Implications for shallow lake restoration

Wind-induced sediment resuspension in shallow lakes may enhance nutrient availability while reducing light availability for phytoplankton growth, thereby affecting the entire food-web. Lake restoration projects that reduce wind-induced resuspension are expected to enhance trophic transfer efficiencies, thereby improving food-web structure and functioning. Yet, reduced resuspension may also lead to lower nutrient concentrations in the water column, promote benthic algae development, reduce phytoplankton biomass production and thereby reduce secondary production by zooplankton. Lake Markermeer is a shallow delta lake in The Netherlands subject to wind-induced sediment resuspension. Restoration project Marker Wadden consists of newly built islands aiming to reduce sediment resuspension and promote higher trophic levels. Here, we tested the effects of reduced sediment resuspension on phytoplankton biomass build-up, benthic algae development, and zooplankton abundances at different temperatures in a 14-day indoor microcosm experiment. We used Marker Wadden sediment with three resuspension intensities combined with three temperatures, to also test effects of higher temperatures in shallow sheltered waters. Reduced sediment resuspension decreased nutrient concentrations and phytoplankton biomass build-up, while increasing light availability and enhancing benthic algae biomass development. Reduced sediment resuspension furthermore increased zooplankton biomass. Enhanced sediment resuspension and higher temperatures synergistically interacted, maintaining a high level of inorganic suspended solids. Our experimental results are in line with long-term seasonal observations from Lake Markermeer. Our findings demonstrate that for shallow lakes suffering from wind effects, measures such as Marker Wadden aimed at reducing sediment resuspension can be effective in restoring secondary production and supporting higher trophic levels.

Microbial plankton responses to multiple environmental drivers in marine ecosystems with different phosphorus limitation degrees

Multiple drivers are threatening the functioning of the microbial food webs and trophic interactions. Our understanding about how temperature, CO2, nutrient inputs, and solar ultraviolet radiation (UVR) availability interact to alter ecosystem functioning is scarce because research has focused on single and double interactions. Moreover, the role that the degree of in situ nutrient limitation could play in the outcome of these interactions has been largely neglected, despite it is predominant in marine ecosystems. We address these uncertainties by combining remote-sensing analyses, and a collapsed experimental design with natural microbial communities from Mediterranean Sea and Atlantic Ocean exposed to temperature, nutrients, CO2, and UVR interactions. At the decade scale, we found that more intense and frequent (and longer lasting) Saharan dust inputs (and marine heatwaves) were only coupled with reduced phytoplankton biomass production. When microbial communities were concurrently exposed to future temperature, CO2, nutrient, and UVR conditions (i.e. the drivers studied over long-term scales), we found shifts from net autotrophy [primary production:respiration (PP:R) ratio > 1] towards a metabolic equilibrium (PP:R ratio ~ 1) or even a net heterotrophy (PP:R ratio < 1), as P-limitation degree was higher (i.e. Atlantic Ocean). These changes in the metabolic balance were coupled with a weakened phytoplankton-bacteria interaction (i.e. bacterial carbon demand exceeded phytoplankton carbon supply. Our work reveals that an accentuated in situ P limitation may promote reductions both in carbon uptake and fluxes between trophic levels in microbial plankton communities under global-change conditions. We show that considering long-term series can aid in identifying major local environmental drivers (i.e. temperature and nutrients in our case), easing the design of future global-change studies, but also that the abiotic environment to which microbial plankton communities are acclimated should be taken into account to avoid biased predictions concerning the effects of multiple interacting global-change drivers on marine ecosystems.

Changes in spectral quality of underwater light alter phytoplankton community composition

AbstractLight is a fundamental resource for phytoplankton. To utilize the available light, most phytoplankton species possess pigments in taxon‐specific combinations and quantities, which in turn result in a specific use of certain wavelengths. This optimizes the light use efficiency, allows for a complementary use of light, and may be an additional driver for community structure. While the effects of light intensity on phytoplankton biomass production and community composition have been intensively studied, here we focused on the effects of specific light spectrum quality (thus light color) on a natural phytoplankton community. In a controlled mesocosm experiment we reduced the supplied wavelength range to its blue, green, or red part of the light spectrum and compared the responses of each treatment to a full spectrum control over 28 d. Highest community growth rates were observed under blue, lowest under red light. Light absorption by the communities showed adaptation toward the supplied wavelength range. Community composition was significantly affected by light quality treatments, driven by Bacillariophyta and Chlorophyta, whereas pigment composition was not. Furthermore, lower species richness but higher evenness occurred when communities were exposed to red light compared to the full spectrum. We expected the response of phytoplankton communities to changes in the light spectrum to be driven by a combination of species sorting and pigment acclimation; however, the effect of species sorting turned out to be stronger. Our study showed that, even if species might acclimate, changes in the available light spectrum affect primary production and phytoplankton community composition.

Phytoplankton RNA/DNA and 18S rRNA/rDNA ratios in a coastal marine ecosystem

AbstractThe RNA/DNA ratio is used as indicator of growth in various marine organisms and to assess physiological status at species or community level. To evaluate the utility of the RNA/DNA ratio as a proxy of phytoplankton primary production, the relationships between phytoplankton RNA/DNA, taxon-specific diatom and dinoflagellate 18S rRNA/rDNA ratios and autotrophic phytoplankton biomass were investigated as a first step. Significant correlations between all phytoplankton ratios and total phytoplankton, diatom and dinoflagellate biomass as chlorophyll a (chl a) and carbon content were found. Diatoms showed higher correlation than dinoflagellates (18S rRNA/rDNA vs. chl a, rs = 0.74 and 0.64, P &amp;lt; 0.001; 18S rRNA/rDNA vs. carbon, rs = 0.66 and 0.53, P &amp;lt; 0.001, respectively), because they represented the most abundant and frequent group within sampled assemblages. Further, phytoplankton biomass production is known to be linked to protein biosynthesis and significant relationships between RNA/DNA ratios and protein content of phytoplankton assemblage were found (rs = 0.62 and 0.52, P &amp;lt; 0.001 for diatom and dinoflagellates, respectively). As taxon-specific RNA/DNA ratios were correlated with biomass and protein content, our results can be regarded as the first step toward further studies on the applicability of RNA/DNA ratios as indicators of growth rate and primary production in phytoplankton assemblages.

Phytoplankton response to long-term wind dynamics at large shallow Lake Taihu (shallow lake phytoplankton response to long-term wind dynamics)

Shallow lakes are highly sensitive to respond wind dynamics. The phytoplankton biomass production and distribution scenarios vary annually and seasonally in shallow lakes. The present study investigated the impact of long-term wind dynamics to phytoplankton biomass at Lake Taihu. The response of phytoplankton taxonomic groups to annual and seasonal wind dynamics using generalized additive model showed significant correlation. The critical annual and seasonal wind speed was 2.2 m/s showing the highest correlation with phytoplankton biomass whereas, wind speed declined by 0.02 m/s annually from Jan 2000 to Dec 2016 at Lake Taihu. The lower to medium wind speeds with frequent directional switching toward shore areas developed higher mixing scenarios which helped in reduction of water temperatures, distribution and colony formation of phytoplankton near the Meiliang Bay. The onshore winds were most frequent winds with low wind speeds whereas, offshore winds have been observed with medium to higher wind speeds. The results of generalized additive model well described the wind impacts on phytoplankton biomass and successfully modeled the response of phytoplankton to wind speed. Therefore, it has been concluded that long-term wind dynamic control the movement, mixing and colony formation processes of phytoplankton at Lake Taihu. The fundamental mechanism found in the present study was the long-term higher variations (annual and seasonal) of the wind-induced phytoplankton biomass changes which has been considered as the major physical factor regulating the phytoplankton blooms formation in the large shallow lake Taihu.

Mixed layer deepening due to wind-induced shear-driven turbulence and scaling of the deepening rate in the stratified ocean

Wind-induced mixing forms the surface mixed layer (ML) above the stratified interior oceans. The ML depth (MLD), a key quantity for several upper ocean processes such as the air-sea interaction and primary production of phytoplankton biomass, is often scaled as $U_{*}/\sqrt {Nf}$, where U∗ is the friction velocity, N is the Brunt-Vaisala frequency, and f is the Coriolis parameter. Here, we performed large-eddy simulations (LESs) to evaluate this scaling. It was found that the ML deepens rapidly until one-half inertial period (0.5Tf) by which the MLD becomes $1.6 - 1.7 U_{*}/\sqrt {Nf}$. Thereafter, the ML deepening slows down but never stops, and the MLD keeps increasing gradually. The MLDs at Tf, 1.5Tf, and 5Tf become greater than those at 0.5Tf by 6.2 %, 16 %, and 40 %, respectively. Therefore, time-dependent scaling of the MLD is necessary for more quantitative estimates than the classical theory. LESs performed with several U∗, N, and f showed that the deepening rate of the ML depends on the Rossby number and the Froude number. The present study proposes time-dependent scalings of the ML deepening rate and the MLD as a function of the Rossby number and the Froude number, which cover the classical scaling but can be extended even after 0.5Tf.

Inorganic carbon promotes photosynthesis, growth, and maximum biomass of phytoplankton in eutrophic water bodies

Abstract The traditional perception in limnology has been that phytoplankton biomass in lakes is limited by phosphorus, nitrogen, and light, but not by dissolved inorganic carbon (DIC) because CO2 can be supplied from the atmosphere. We tested the possibility of carbon limitation of photosynthesis, growth, and biomass accumulation of phytoplankton communities across an alkalinity and DIC gradient (0.15–3.26 mM) in nutrient‐rich freshwater. During 47‐day long experiments, we measured phytoplankton biomass, organic carbon, calcium, DIC, pH, and oxygen in indoor, constantly mixed mesocosms with either no removal or a 70% weekly removal of the biomass. Photosynthesis was measured in the morning and in the afternoon at high biomass. Maximum biomass and organic carbon production increased two‐ to four‐fold with DIC, which supported 7% of organic carbon production at low DIC and 53% at high DIC concentration, while atmospheric CO2 uptake supplied the remainder. Weekly biomass removal increased growth rates through improved light conditions leading to enhanced total phytoplankton biomass production at high DIC. Photosynthesis was significantly higher in the morning compared to afternoon due to daily DIC depletion. We conclude that phytoplankton photosynthesis, growth rate, maximum biomass, and organic carbon production can be markedly carbon limited in eutrophic lake waters. Consequently, lakes of high DIC and pH can support a faster primary production by greater DIC use and chemically enhanced atmospheric CO2 uptake.

Relationship among physicochemical conditions, chlorophyll-a concentration, and water level in a tropical river–floodplain system

The free-flowing Usumacinta River maintains an average annual water-level fluctuation of 6.7 m. This study evaluated relationships between 14 physicochemical and biological variables and key factors in four water-level conditions in a river–floodplain system. The analysis incorporated intra-annual variation in all variables, with each selected in accordance with multiple statistical and physicochemical criteria. Possible correlations were examined as a function of various physicochemical and biological factors at each water level. Within the study area, defined by a temporal gradient in principal component 1, the yearly river overflow above the bankfull stage is characterized by water with a low level of total suspended solids. Data on riverine wetlands suggest that seasonal changes are key to determining intra- and inter-annual chlorophyll-a levels and water clarity. However, opposite trends are observed for high and low water-level conditions. Nutrient enrichment cannot be taken as the key physicochemical factor of water level, under either water-level conditions, due to the lack of a temporal gradient in principal component 2 and the high biochemical variability of nitrate and orthophosphate levels. In conclusion, the hypothesis was accepted for the production of phytoplankton biomass and light attenuation at low and high water levels, as both were dependent on intra-annual changes. The increases in chlorophyll-a related to the minimal variability at the lowest water level open the opportunity to gauge this relationship as a possible environmental predictor for river–floodplain systems.

Enlightening the dark side of Arthrospira maxima cultivation: evaluation of carbon supply modes and performance at optimal growth conditions

Night biomass loss due to respiratory activity can significantly impact the phytoplankton biomass productivity and the yield of specific biomolecules. A decrease in biomass loss can be achieved by cyclic two-stage cultivation (CTSC) employing an organic carbon source during the dark phase of the photoperiod. Thus, sugarcane vinasse was used as carbon supply in CTSC to influence the Arthrospira maxima growth. The impact of carbon supply on CTSC was assessed and the results show that splitting the supplementation volume of 60 mL supplemented vinasse into three doses of 20 mL during the dark phase increased biomass productivity by 35.4%, although the maximum exponential growth rate was unaffected. The cyanobacterium also takes up CO2 (supplied in the light phase) as $$ {\mathrm{HCO}}_3^{-} $$ , high concentrations of which increase biomass loss. Thus, carbon assimilation in CTSC is higher when the concentration of dissolved inorganic carbon is reduced. Optimization of organic and inorganic carbon supply in CTSC with the response surface methodology resulted in an increased growth rate (from 0.73 to 1.18–1.62 day−1) and a reduction of cultivation time (from 7 to 5 days), thus producing a synergistic effect of reduced night biomass loss and vinasse treatment. After cultivating A. maxima for 5 days under optimal conditions (culture medium with 6.81 g L−1 of NaHCO3 and adding 1.5% (v/v) vinasse every 4 h during the dark phase), the reduction of chemical oxygen demand and electrical conductivity demonstrates that removal of organic matter present in the vinasse was due to cell growth during CTSC.

Niveles nutricionales asociados con umbrales de impacto ecológico: Alternativa para estimar los criterios numéricos de nutrientes para los embalses en Puerto Rico

Seis embalses de Puerto Rico se monitorearon durante un periodo de 32 meses con el propósito de establecer relaciones entre el estado nutrlclonal de sus aguas y diferentes umbrales de impacto ecológico. Los embalses seleccionados representan el espectro de productividad prevaleciente en los embalses en la Isla. Las concentraciones medianas de fósforo total (TP) en el epilimnio (1 m) fueron las siguientes: Cerrillos, 10 µg/L; Cidra, 33 µg/L; Guajataca, 10 µg/L; La Plata, 49 µg/L; Patillas, 6 µg/L; y Toa Vaca, 22 µg/L. Igualmente, las concentraciones medianas de nitrógeno total (TN) fueron: Cerrillos, 0.22 mg/L; Cidra, 0.74 mg/L; Guajataca, 0.28 mg/L; La Plata, 0.55 mg/L; Patillas, 0.23 mg/L; y Toa Vaca, 0.34 mg/L. Se observaron fuertes correlaciones positivas entre nutrientes y clorofila a (Chl-a). Se obtuvo un coeficiente de correlación de 0.74 para la relación entre TP y Chl-a, y de 0.66 para la relación entre TN y Chl-a. Aumentos en la biomasa algal (determinada por Chl-a) disminuyeron la penetración de luz en la columna de agua (r= -0.70). Se estableció como umbral biológico indicativo de impacto a los usos designados, una concentración de clorofila a de 24 µg/L, basado en la reducción en penetración de luz a ese nivel de Chl-a y su efecto en la capacidad de los embalses de cumplir con el criterio de vida acuática. Las concentraciones de TP y TN asociadas al umbral de impacto de Chl-a (i.e., 24 µg/L) se establecieron basado en un análisis de punto de cambio de los datos. Los valores resultantes, 0.035 mg/L para TP y 0.43 para TN, se pueden considerar como umbrales nutricionales asociados con impacto. Se incorporó un margen de seguridad utilizando la 5, a percentila menor de la distribución de valores "bootstrap" asociados umbral de impacto de Chl-a (i.e., 24 µg/L) (i.e., p&lt;0.05- prueba unilateral). Los criterios resultantes, 0.026 mg/L para TP y 0.41 mg/L para TN, representan la base propuesta para el establecimiento de estándares nutricionales en embalses de Puerto Rico. Diferencias significativas en la profundidad máxima de cumplimiento con el criterio de vida acuática de USEPA (i.e., 5.0 mg/L) se observaron entre los datos ubicados por encima y por debajo de los criterios numéricos propuestos. Esta diferencia, que puede ser crítica para la sustentabilidad de la población de peces en la isla, evidencia la importancia de mantener el estado nutricional de las aguas en nuestros embalses a niveles bajo los umbrales propuestos. La comunidad zooplanctónica se caracterizó por una mezcla de especies previamente identificadas bajo condiciones ambientales similares en la región Neotropical. Se identificaron sesenta y cinco taxones de zooplancton. Los rotíferos fueron el grupo más diverso con 37 taxones, seguido de los cladóceros con 20 taxones y los copépodos con ocho taxones. La abundancia absoluta de los rotíferos evidenció un incremento con aumentos en diferentes índices de eutroficación. Se utilizó un análisis de curva por segmentos para describir la relación entre los rotíferos del género Brachionus (#ind/L) y el fósforo (lnTP). La relación produjo un cambio de punto a una concentración de 26.58 µg/L de fósforo total, lo cual coincide con el criterio numérico propuesto. Además, se obtuvo un cambio de punto de 1.16 m para la relación entre los rotíferos del género Brachionus (#ind/L) y la profundidad Secchi (m), lo cual se puede considerar como un indicador de impacto adicional en los embalses de la isla.

The effect of a flood pulse on the water column of western Lake Superior, USA

On June 19 and 20, 2012, western Lake Superior was impacted by a “mega-rain event” that raised lake levels by 8 to 10cm. Within the flood plume on June 21, 2012, total suspended solids, total phosphorus, and soluble reactive phosphorus concentrations were elevated, with measurements of 87mg/L, >100μg/L, and 5.8μg/L, respectively. Despite the initially high phosphorus loadings, little impact was seen on water column particulate chlorophyll content, which remained in the range 0.7–1.9μg/L, in the weeks to months following the flood. Both total phosphorus and soluble reactive phosphorus levels tracked those of total suspended solids, returning to background levels within two weeks. However, the availability of photosynthetically available radiation (PAR) was impacted for a month after the flood event, due mainly to colored dissolved organic matter that remained in the surface layer of the stratified lake water column. It appears that the mismatch in timing of nutrient and light availability acted as a check on phytoplankton biomass production in the flood-impacted portion of the lake.

Composition of inorganic and organic nutrient sources influences phytoplankton community structure in the New River Estuary, North Carolina

The New River Estuary, NC, is a nutrient-sensitive, eutrophic water body that is prone to harmful algal blooms. High annual loading from the watershed of varying nutrient forms, including inorganic phosphorus and inorganic and organic nitrogen, may be linked to the persistence of algal blooms in the estuary. In order to evaluate phytoplankton response to nutrient inputs, a series of in situ nutrient addition experiments were carried out during June 2010 to July 2011 on water from an estuarine site known to support algal blooms. Estuarine water was enriched with nutrients consisting of individual and combined sources of dissolved inorganic nitrogen, orthophosphate, urea, and a natural dissolved organic nitrogen (DON) addition derived from upstream New River water. The combined inorganic N and P addition most frequently stimulated phytoplankton biomass production as total chlorophyll a. The responses of diagnostic (of major algal groups) photopigments were also evaluated. Significant increases in peridinin (dinoflagellates), chlorophyll b (chlorophytes), and myxoxanthophyll (cyanobacteria) were most frequently promoted by additions containing riverine DON. Significant increases in zeaxanthin (cyanobacteria) were more frequently promoted by inorganic nitrogen additions, while increases in fucoxanthin (diatoms) and alloxanthin (cryptophytes) were not promoted consistently by any one nutrient treatment. Evaluating the impact of varying nutrient forms on phytoplankton community dynamics is necessary in order to develop strategies to avoid long-term changes in community structure and larger-scale changes in ecosystem condition.

Nutrients in precipitation and the phytoplankton responses to enrichment in surface waters of the Albemarle Peninsula, NC, USA after the establishment of a large-scale chicken egg farm

Nitrogen (N) and phosphorus (P) released in waste from animal feeding operations can stimulate phytoplankton biomass production in local receiving waters. Changes in weekly wet atmospheric N and P were measured from 2005 to 2008 at monitoring stations located 0.8, 7.9, and 10.3 km downwind from a new chicken egg production facility on the Albemarle Peninsula, North Carolina. After this farm began operating, there was a significant doubling in mean wet NH4 + concentrations (465–1,022 μg l−1) at 0.8 km with no change at the other sites. To measure the phytoplankton responses to nutrient enrichment, we conducted seasonal N and P enrichment bioassays from 2006 to 2008 on nearby Phelps Lake and Alligator River. These low-nutrient waters responded to nutrient additions, with the highest increases in phytoplankton primary productivity (14C uptake) and biomass (chlorophyll a) occurring in the combined N and P treatments suggesting co-limitation of N and P. Although we did not find an increased nutrient signal in precipitation at sites >0.8 km from the farm, there is the potential for atmospheric deposition of N to these and other waters located N/NE of the farm given prevailing winds and distance that NH4 + aerosols can travel. Furthermore, surface runoff from the farm may impact receiving waters downstream (e.g., Pungo and Pamlico Rivers). In order to prevent excessive phytoplankton productivity and biomass both N and P inputs should be carefully assessed and potentially controlled in these nutrient-sensitive waters.

Productivity, herbivory, and species traits rather than diversity influence invasibility of experimental phytoplankton communities

Biological invasions are a major threat to natural biodiversity; hence, understanding the mechanisms underlying invasibility (i.e., the susceptibility of a community to invasions by new species) is crucial. Invasibility of a resident community may be affected by a complex but hitherto hardly understood interplay of (1) productivity of the habitat, (2) diversity, (3) herbivory, and (4) the characteristics of both invasive and resident species. Using experimental phytoplankton microcosms, we investigated the effect of nutrient supply and species diversity on the invasibility of resident communities for two functionally different invaders in the presence or absence of an herbivore. With increasing nutrient supply, increased herbivore abundance indicated enhanced phytoplankton biomass production, and the invasion success of both invaders showed a unimodal pattern. At low nutrient supply (i.e., low influence of herbivory), the invasibility depended mainly on the competitive abilities of the invaders, whereas at high nutrient supply, the susceptibility to herbivory dominated. This resulted in different optimum nutrient levels for invasion success of the two species due to their individual functional traits. To test the effect of diversity on invasibility, a species richness gradient was generated by random selection from a resident species pool at an intermediate nutrient level. Invasibility was not affected by species richness; instead, it was driven by the functional traits of the resident and/or invasive species mediated by herbivore density. Overall, herbivory was the driving factor for invasibility of phytoplankton communities, which implies that other factors affecting the intensity of herbivory (e.g., productivity or edibility of primary producers) indirectly influence invasions.

Seasonal Dynamics and Biomass of Mixotrophic Flagellate Dinobryon sertularia Ehrenberg (Chrysophyceae) in Derbent Reservoir (Samsun, Turkey)

Mixotrophic protists, combining both heterotrophy and phototrophy, are found abundantly in eutrophic waters. Dinobryon sertularia Ehr. from Chrysophyceae (golden algae) are mixotrophic organisms often make up blooms and colony in pools, lakes and dam reservoirs. This study was carried out in Derbent Dam Lake in the Middle Black Sea Region. Seasonal dynamics and biomass of D. sertularia were investigated at four stations between February 2001 and July 2002. D. sertularia consisted of 47-60% in spring, 61-79% in summer, and 82-88% in autumn of all phytoplankton population. In August 2001, D. sertularia was calculated in the highest amounts numbers (18740-13140 cells ml-1). This species provided important contributions to phytoplankton biomass. In the present study, it was found that there is an inverse correlation between water depth and phytoplankton biomass production. According to Bray-Curties Similarity Indice, 88% similarity was determined among Station 3, Station 4 and 4-6m of Station 1. D. sertularia was dominant and common organism in all seasons. The reservoir water was defined as oligomesotrophic and alkaline characters (pH 7.98), and average temperature was 15±56○C..

The Puck Bay as an example of deep dredging unfavorably affecting the aquatic environment

Abstract The presented study is a review of available geochemical data concerning a dredge area in the Puck Bay. It is a small, semi-closed and degraded water body on the Polish Baltic coast. In this region very deep dredging was conducted in the years 1989-95. Five dredge pits were created in the bottom of the bay. Only two of them have been geochemically investigated so far. In the deepwater parts of both the dredge pits organic matter accumulation resulted in sulfate reduction and subsequent hydrogen sulfide production in the sediment, as well as elevated nutrient concentrations in the interstitial waters. Moreover, temporal oxygen deficits in the near bottom waters were observed. Such conditions favor nitrogen and phosphorus release to the water column and potentially enhance the eutrophication. The impact of deep dredging on the chemical conditions in the dredged area seems to be controlled by a complex interplay of the season, phytoplankton biomass production and hydrodynamic conditions. Areas with high biomass production and low water dynamic are probably more vulnerable to environmental degradation due to deep dredging. This implies that some of the dredge pits require different methods of recultivation than others. However the decision if, and with which methods, to remediate dredge pits in the Puck Bay requires successive research including all the pits in all four seasons.

Linking allochthonous dissolved organic matter and boreal lake sediment carbon sequestration: The role of light‐mediated flocculation

We measured flocculation of dissolved organic carbon (DOC) in the water from a humic lake (DOC = 14.9 mg C L‐1) and from an adjacent mire (DOC = 25.7 mg C L‐1), in in situ enclosure experiments with different light regimes. Light stimulated the formation of organic particles in both waters, and organic particle formation was observed at all incubation depths, even in the dark controls. Production of phytoplankton biomass was negligible, and allochthonous DOC was the most important precursor of the sinking particles. 8‐22% and 25‐ 60% of the loss of DOC in lake and mire water, respectively, could be accounted for by flocculation. Depthintegrated flocculation based on the enclosure experiments was 14.7 mg C m‐2 d‐1. Lake‐water DOC concentration and water color has been increasing during the last decade, and sediment trap studies show that gross sedimentation of organic carbon also increased. Thus flocculation of allochthonous DOC, stimulated by light, constitutes a pathway for the sequestration of carbon in lake sediments.

Nitrous oxide measurements during EIFEX, the European Iron Fertilization Experiment in the subpolar South Atlantic Ocean

We measured the vertical water column distribution of nitrous oxide (N2O) during the European Iron Fertilization Experiment (EIFEX) in the subpolar South Atlantic Ocean during February/March 2004 (R/V Polarstern cruise ANT XXI/3). Despite a huge build‐up and sedimentation of a phytoplankton bloom, a comparison of the N2O concentrations within the fertilized patch with concentrations measured outside the fertilized patch revealed no N2O accumulation within 33 days. This is in contrast to a previous study in the Southern Ocean, where enhanced N2O accumulation occurred in the pycnocline. Thus, we conclude that Fe fertilization does not necessarily trigger additional N2O formation and we caution that a predicted radiative offset due to a Fe‐induced additional release of oceanic N2O might be overestimated. Rapid sedimentation events during EIFEX might have hindered the build‐up of N2O and suggest, that not only the production of phytoplankton biomass but also its pathway in the water column needs to be considered if N2O radiative offset is modeled.