Photoprotective acclimation of micro- and nano-size phytoplankton assemblages in the Indian sector of the Southern Ocean

Phytoplankton in the mixed layer is exposed to increasing levels of light when transported to the surface layer of the ocean. The photoprotective response of natural assemblages of phytoplankton can differ among community structures. We investigated photoprotective acclimation and xanthophyll cycle pigments in size-fractionated natural phytoplankton assemblages during the austral summer in the Indian sector of the Southern Ocean. We estimated concentrations of phytoplankton pigments in the micro-size fractions (>20 μm) and nano-size fractions (2–20 μm) by subtracting concentrations in the <20 μm fractions from concentrations in the bulk samples, and by subtracting concentrations in the <2 μm fractions from concentrations in the <20 μm fractions, respectively. Changes in the ratios of the xanthophyll cycle pigments diadinoxanthin (DD) and diatoxanthin (DT) were determined at three optical depths in the mixed layer and during 48 h deck incubations under solar photosynthetically available radiation and ultraviolet radiation. Large variations in (DD + DT)/Chl a in the mixed layer (percent coefficient of variation >67 %) and in deck incubation bottles under variable light conditions (>75 % of the temporal variation) for the micro-size fractions suggest a higher potential for photoprotective acclimation than for the nano-size fractions. Decreases in DT/(DD + DT) with increases in the optical depth of the mixed layer (ζ _MLD) suggest that larger variations in light availability in the mixed layer might predict lower values of DT/(DD + DT) at the surface, regardless of cell size.     

Carbon fixation and carbon availability in marine phytoplankton

It is widely believed that inorganic C does not limit the rate of short-term photosynthesis, the net productivity, or the maximum biomass, of marine phytoplankton. This lack of inorganic C restriction is less widely believed to hold for phytoplankton in many low alkalinity freshwaters or for seaweed in nutrient-enriched rock pools. These views are examined in the context of the physical chemistry of the inorganic C system in natural waters and of the ways in which various taxa of phytoplankton deal with inorganic C and discriminate between ¹²C and ¹³C. Using this information to interpret data obtained in the ocean or in freshwater suggests that short-term photosynthesis, production rate, and achieved biomass, of phytoplankton are rarely limited by inorganic C supply but, rather, that the widely suggested factors of limited light, nitrogen or phosphorus supply are the resource inputs which restrict productivity. Global change, by increasing atmospheric CO₂ partial pressure and global mean temperatures, is likely to increase the mean CO₂ concentration in the atmosphere, but the corresponding change in the oceans will be much less. There are, however, genotypic differences in the handling of inorganic C among the diversity of marine phytoplankton, and in impact on use of limiting nutrients, so increases in the mean CO₂ and HCO₃ ^- concentrations in surface ocean waters could cause changes in species composition. However, the rarity of inorganic C limitation of marine phytoplankton short-term photosynthesis, net productivity, or the maximum biomass, in today's ocean means that global change is unlikely to increase these three values in the ocean.     

Ingestion of phytoplankton by the micro- and mesozooplankton communities in a productive subtropical estuary

Grazing on phytoplankton by the micro- and mesozooplankton communitieswas measured during four cruises in a shallow (1.5 m) productive(up to 6 g C m^–2 day^–1 estuary in the northern Gulfof Mexico. Grazing-induced mortality on phytoplankton was alwayshigh and >95% of the grazing was by the microzooplanktoncommunity The grazing contribution from the mesozooplanktoncommunity, comprised primarily of Acartia tonsa, is believedto be small because populations were kept low by predation andadvective losses. A simple model is developed to describe phytoplankton-zooplanktoninteractions in this estuary. Attempts to understand the distributionand abundance of phytoplankton in estuaries must include estimatesof grazer-induced mortality on the phytoplankton.     

Silicate availability, vertical mixing and grazing control of phytoplankton blooms in mesocosms

Lake Sempach, located in the central part of Switzerland, has a surface area of 14 km², a maximum depth of 87 m and a water residence time of 15 years. Restoration measures to correct historic eutrophication, including artificial mixing and oxygenation of the hypolimnion, were implemented in 1984. By means of the combination of external and internal load reductions, total phosphorus concentrations decreased in the period 1984–2000 from 160 to 42 mg P m^–3. Starting from 1997, hypolimnion oxygenation with pure oxygen was replaced by aeration with fine air bubbles. The reaction of the plankton has been investigated as part of a long-term monitoring program. Taxa numbers, evenness and biodiversity of phytoplankton increased significantly during the last 15 years, concomitant with a marked decline of phosphorus concentration in the lake. Seasonal development of phytoplankton seems to be strongly influenced by the artificial mixing during winter and spring and by changes of the trophic state. Dominance of nitrogen fixing cyanobacteria (Aphanizomenon sp.), causing a severe fish kill in 1984, has been correlated with lower N/P-ratio in the epilimnion. Buoyant algae such as Planktothrix rubescens (syn. Oscillatoria rubescens) increased in abundance due to enlargement of the trophogenic layer and extended mixing depth during winter. The interactions between zoo- and phytoplankton seemed to be depressed as a result of restoration measures. Zooplankton composition changed to more carnivorous and less herbivorous species. Oxygenation of the hypolimnion induced bioturbation of sediments, mainly by oligochaetae worms, and stimulated germination of spores and cysts and hatching of resting eggs.     

Impact of phosphorus availability on modelling phytoplankton dynamics

Regulation of phosphorus loading is considered to be the primary method of eutrophication control for many lake systems. It is therefore necessary to have accurate estimates of the forms and bioavailability of all phosphorus sources in order to develop the most cost effective load control measures. Research at Clarkson University, aimed at improving the accuracy of estimates of the form and reactivity of phosphorus loadings to Lake Erie, has revealed a significant difference between the algal-availability of allochthonous and autochthonous particulate phosphorus. This paper presents the results of modifying an existing multi-nutrient phytoplankton model by separating allochthonous phosphorus into three forms: soluble reactive phosphorus (SRP) — immediately available for algal uptake; external ultimately-available phosphorus—not immediately available but converted to an available form at a specific rate; an external refractory phosphorus (ERP)—not available while in the water column. Comparisons between the original and modified models showed that the modified phosphorus dynamics proved to be a viable alternative to the concept of invoking an unexplained soluble phosphorus water column loss term, employed in the original model. The work also demonstrates that the distinction is significant for lakes receiving a significant portion of their external phosphorus load in a particulate (not immediately available) form and having a morphometry and hydrology such that this particulate phosphorus remains in the water column for longer than about two weeks.     

Calibrating Ellenberg indicator values for moisture,acidity, nutrient availability and salinity in the Netherlands

A general calibration of Ellenberg indicator values for moisture, acidity, nutrient availability and salinity was carried out on a large database of relevées and environmental variables from a variety of ecosystems in the Netherlands.Satisfying relationships with Ellenberg indicator values for moisture, acidity and salinity were found for mean groundwater level in spring time, soil pH and chloride concentration in groundwater. For mean groundwater level in spring and chloride concentration in groundwater subdivision of the database led to clearer relationships with indicator values. For the Ellenberg indicator value for nutrient availability satisfying calibration results were only achieved with data on standing crops and N stock in standing crop. The relationship with soil chemical variables was less clear.Although the correlation between indicator and measured values is obvious, the variation around the regression lines is considerable. However, because of the size and composition of the database, it is unlikely that our calibration results can be much improved by adding more (Dutch) data.The calibration results will be applied in the multi-stress model SMART-MOVE, developed to predict changes in species composition due to acidification, eutrophication and the effects of lowering groundwater.     

Quantifying the impact of periphytic algae on nutrient availability for phytoplankton

SUMMARY.

• 1 Recent laboratory studies demonstrate that periphytic algae growing on the sediment surface reduce nutrient availability in the overlying water. Consequently, periphytic algae may competitively reduce growth of phytoplankton.
• 2 The aim of this study was to quantify the competitive impact of sediment-attached periphytic algae on phytoplankton in the presence of all other factors simultaneously affecting nutrient dynamics in natural systems.
• 3 In enclosure experiments, performed in three lakes of different productivity, the periphytic algal biomass was manipulated. When compared to enclosures with high biomass of periphytic algae, those with reduced biomass showed an increase in total phosphorus concentration in the water of 32–44%. Extrapolation of the experimental results to whole lakes predicts an increase in original total phosphorus concentration of between 1.5% and 8.0%. According to existing regressions between total phosphorus and phytoplankton chlorophyll, the potential increase in original phytoplankton biomass will be between 2.5% and 12.6%.
• 4 With respect to the shallow parts of lakes, my results support the conclusions revealed from laboratory studies that periphytic algae have a significant impact on the phosphorus concentration in the overlying water. However, when considering whole-lake dynamics, the competitive impact of periphytic algae on phytoplankton biomass development is probably of minor importance.
• 5 Rather, the main competitive advantage of growing on the sediment surface, compared to in the water, may be the exclusive access to nutrients in the sediment.

     

The influence of copepod and krill grazing on the species composition of phytoplankton communities from the Scotia Weddell sea

The influence of copepods (mainly Oithona sim-ilis) and krill (Euphausia superba) grazing on the species composition of plankton communities in ship board con tainers was investigated during the spring and post spring period in the Scotia Weddell Sea in the Antarctic ocean. Numbers of grazers were experimentally manipulated in containers with natural phytoplankton assemblages. With ratural levels of copepods but no krill a high (700–950 g C·l¹, ca 30 g chl a·.l¹) phytoplankton biomass developed. In these cultures large diatoms, e.g. Corethron criophilum and chains of Thalassiosira sp., made up 80% of total phytoplankton cell carbon at the end of the experiment. In cultures with elevated numbers of copepods (5X or 10X the natural level) phytoplankton biomass was somewhat reduced (ca 23 g chl a · l¹) compared to cultures with natural copepod abundance, but still high. Phytoplankton species composition was on the other hand greatly influenced. Instead of large diatoms these cultures were dominated by Phaeocystis pouchetii (70%) together with small Nitszchia sp. and Chaetoceros neogracile (20%). In containers with krill (both juveniles and adults), but without elevated numbers of copepods, phytoplankton biomass rapidly approached zero. With 10X the in situ level of copepods, krill first preyed on these before Corethron criophilum and Thalassiosira sp. were grazed. When krill were removed a plankton community dominated by flagellates (60–90%), e.g. Pyramimonas sp. and a Cryptophycean species, grazed by an unidentified droplet-shaped heterothrophic flagellate, developed. These flagellates were the same as those which dominated the plankton community in the Weddell Sea after the spring bloom. A similar succession was observed in situ when a krill swarm grazed down a phytoplankton bloom in a few hours. Our experiments show that copepods cannot control phytoplankton biomass in shipboard cultures even at artificially elevated numbers. Krill at concentrations similar to those in natural swarms have a great impact on both phytoplankton biomass and species composition in shipboard cultures. Both copepods and krill may have an impact on phytoplankton species composition and biomass in situ since the rates of phytoplankton cell division were probably artificially increased in shipboard cultures compared to natural conditions, where lower growth rates make phytoplankton more vulnerable to grazing. A similarity between phytoplankton successions in containers and in situ, especially with respect to krill grazing, supports the conclusion that grazing may structure phytoplankton communities in the Scotia-Weddell Sea.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

An algal pigment ratio as an indicator of the nitrogen supply to phytoplankton in three Norfolk broads

SUMMARY. A pigment ratio representing the ratio of carotenoid pigments to chlorophyll- a was calculated by dividing the optical density at 480 nm by that at 664 nm for a 90% acetone extract of phytoplankton. Its seasonal variation in three Norfolk broads was examined and, except for nitrogen-fixing species, it was found to vary independently of the algal species composition.
A good inverse correlation with the particulate nitrogen to carbon ratio was found. The seasonal variation in the pigment ratio was considerably affected by measurable lake concentrations of inorganic nitrogen supplied from ttie catchment. Other sources of nitrogen such as nitrogen fixation and release from the sediment were also indicated by changes in the ratio.
By comparison with the distinctions of extreme, moderate and no nitrogen deficiency drawn by Healey (1975) for the nitrogen/carbon ratio, values for the pigment ratio of < 1.4, 1.4–2.4 and > 2.4 were found to indicate no, moderate and extreme nitrogen deficiency respectively in the phytoplankton. The relation ship is attributed to the measurement of secondary carotenoids in response to nitrogen deficiency.     

Relationships between phytoplankton dynamics and the availability of light and nutrients in a shallow sub-tropical lake

The role of light and nutrient availability in controlling theabundance and structure of phytoplankton populations was studiedin Lake Okeechobee, a large eutrophic lake in south florida,USA. Measurement of selected environmental parameters at samplingsites within four ecologically distinct regions of the lakewere combined with direct experimental determinations of limitinglevels of light and nutrients for phytoplankton growth to determinespatial and temporal variations in the relative roles of theselimiting factors. Estimated mean light availability in the mixedlayer, I_m, was significantly lower in the turbid central regionof the lake than in other regions. Correlations between I_m andphytoplankton standing crops led to the conclusion that lowlight availability in the central region of the lake, and toa lesser extent in other areas, restricts phytoplankton standingcrops to levels below the potential provided by the nutrientsavailable. The results of the irradiance-growth experimentsconfirmed the conclusions of the correlation analyses that phytoplanktongrowth is restricted by the levels of light availability experiencedduring the winter and spring in the central region of the lake.Bioassays indicated that nitrogen was the most frequently limitingnutrient for phytoplankton growth. High rates of nitrogen fixationwere frequently observed in the lake, along with correspondinglyhigh abundances of nitrogen-fixing cyanobacteria and nitrogenfixation activity. Elevated concentrations of soluble inorganicnitrogen appeared to suppress both nitrogen fixation and therelative abundance of nitrogen-fixing cyanobacteria.     

Notothenioid fish,krill and phytoplankton from Antarctica contain a vitamin E constituent (alpha-tocomonoenol) functionally associated with cold-water adaptation

The vitamin E (VE) content of tissues from the Antarctic notothenioid fish, Chaenocephalus aceratus, Champsocephalus gunnari and Gobionotothen gibberifrons, and extracts of Antarctic krill Euphausia superba and phytoplankton collected from the Antarctic Peninsula was examined. Included in the VE composition was a newly described 'marine-derived' tocopherol (MDT), an unsaturated-isoprenoid derivative of alpha-tocopherol, that is attributed to enhancing antioxidant protection of cellular lipids at low temperature. MDT was found to co-exist with alpha-tocopherol in all Antarctic samples, ranging from 2.8 to 22.3% of the total VE composition. The highest level of VE was found in the liver of G. gibberifrons (VE=416.7 pmol/mg wet tissue) although this tissue had a low MDT composition (7.7%), whereas the greatest MDT composition was measured in the liver of C. gunnari (MDT=22.3%). In notothenioids, the pectoral adductor muscle, which has a high density of mitochondria, contained higher levels of VE than white myotomal muscle, but differences in MDT composition were small. Phytoplankton and krill also contained MDT, which supports the contention that MDT is obtained directly from the primary food chain. Our finding of MDT in Antarctic organisms is consistent with its putatively adaptive function to enhance antioxidant protection in coldwater metabolism.     

Ten years after: krill as indicator of changes in the macro-zooplankton communities of two Arctic fjords

A macro-zooplankton study from 1996 was repeated in 2006 and focused on euphausiid species as indicators of advection and warming effects in Kongsfjorden, West Spitsbergen, Svalbard. The influence of warmer Atlantic water in Kongsfjorden was indicated by the findings of three additional euphausiid species of typically Atlantic origin, relative to the previous study 10 years ago. The predominant presence of Thysanoessa inermis in Hornsund suggested persisting cold conditions in this more southerly, but more Arctic influenced fjord. In this species, moult stage analysis showed that trophic effects can override temperature forcing. Histology and lipid analysis suggest that reproductive activity should be monitored as an indication of warming and possibly a shift in food web composition.     

P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

Potential phytoplankton indicator species for monitoring Baltic coastal waters in the summer period

There are very few time series documenting clear trends of change in the biomass of total phytoplankton or single taxa that coincide with trends of increasing nutrient concentrations. Weekly or biweekly monitoring since 1997 on a cross section of the central Gulf of Finland (NE Baltic Sea) with similar climatic and hydrographic conditions, but different nutrient levels, provided a uniform dataset. In order to evaluate seasonal (June–September) patterns of phytoplankton succession, more than 1,200 samples were statistically analyzed by selecting 12 dominant taxa using wet weight biomass values. In addition, the continuously measured hydrographic parameters on board the ships of opportunity, and simultaneous nutrient analyses gave high frequency information on the water masses. The objective of this study was to identify the taxa that may prove indicative in the assessment of eutrophication in the appropriate monitoring time periods. None of the most common bloom-forming species (Aphanizomenon sp., Nodularia spumigena, and Heterocapsa triquetra) showed reliable correlations with enhanced nutrient concentrations. The species we suggest as reliable eutrophication indicators—oscillatorialean cyanobacteria and the diatoms Cyclotella choctawhatcheeana and Cylindrotheca closterium—showed the best relationships with total phosphorus concentrations. Their maxima appear toward the end of July or in August–September when phytoplankton community structure is more stable, and less frequent observations may give adequate results. Another diatom, Skeletonema costatum, exhibited stronger correlations with dissolved inorganic and total nitrogen in June, during the period of the summer phytoplankton minimum. Guest editors: J. H. Andersen & D. J. Conley Eutrophication in Coastal Ecosystems: Selected papers from the Second International Symposium on Research and Management of Eutrophication in Coastal Ecosystems, 20–23 June 2006, Nyborg, Denmark     

Using carapace measurements to determine the sex of Antarctic krill, Euphausia superba

Krill (Euphausia superba) carapace measurements (length and width; mm) collected from plankton tows in the South Shetland Islands (SSI), Antarctica are used to test the generality of a common discriminant function developed to reconstruct krill length frequencies in Antarctic fur seal diets for the area surrounding South Georgia (SG). Total length and sex ratio of krill in the SSI were overestimated by 5.6 and 154%, respectively, when the SG allometric equations were applied to 3 years (2003–2005) of data. These errors arise and increase as a result of krill population dynamics, specifically recruitment that contributes large proportions of immature krill, misclassified as males by the SG discriminant function. We develop sex-specific regression models based on separate discriminant functions that provide significantly better discriminatory power. However, our analysis indicates that reconstructions of krill sex ratio and length composition in the ocean environment are less reliable in years when the ratio of immature to mature krill is high. For the SSI area, five out of 14 years (35.7%) surveyed (1992–2005) had proportions of immature to mature adult krill ≥ 0.50.     

Interactions between nutrient availability and climatic fluctuations as determinants of the long-term phytoplankton community changes in Lake Garda,Northern Italy

Major focus in interpreting phytoplankton changes in specific typologies of waterbodies or in single lakes is directed towards nutrients and climatic dynamics. During the last 35 years, Lake Garda (Northern Italy; A = 368 km², z _max = 350 m, V = 49 km³) underwent a significant increase of phosphorus in the water column, from ca. 10 μg P l⁻¹ to 18–22 μg P l⁻¹. At the multi-decadal scale, the increase of the trophic status had a positive impact on the growth of Cyanobacteria (mainly Oscillatoriales) and, partly, diatoms, as demonstrated by the long-term ecological research carried out since the beginning of the 1990s in the deepest zone of the lake. Conversely, the increase of Peridiniales (mostly Ceratium hirundinella) in the recent years appeared also associated with the interannual variations of lake temperature. At the seasonal and annual scale, the development of the large diatoms and Oscillatoriales during the periods of their maximum growth (early spring, and summer and autumn, respectively) was strongly controlled by the extent of spring vertical water mixing and nutrient fertilization of surface waters, which, in turn, were negatively dependent on the air and water temperatures in winter and early spring. Therefore, contrary to the positive impact of milder winters on phytoplankton growth in many lakes of high latitudes, warmer winter temperatures in deep oligomictic lakes of lower latitudes can determine periodic shifts towards more oligotrophic conditions and a minor development of diatoms and specific harmful cyanobacterial groups (Oscillatoriales). The complex relationships between the explanatory and response variables were tested by applying Path Analysis (Structural Equation Modeling). This multiequational technique has great potential for studying causal relationships in temporally ordered variables. The results highlight the necessity to study the consequences of climatic fluctuations on the phytoplankton communities at different temporal scales and complexity, also including the indirect effects of climatic dynamics mediated by the morphometric, morphological and hydrological characteristics of lakes, and the possible synergic or opposite effects with other forcing variables, including nutrients.     

Temporal concentrations of sunscreen compounds (Mycosporine-like Amino Acids) in phytoplankton and in the New Zealand krill, Nyctiphanes australis G.O. Sars

This study investigated the relationship between seasonal changesin ambient UV-R, and sunscreen concentrations in phytoplanktonand krill. Concentrations of mycosprine-like amino acid (MAA)sunscreens were quantified in phytoplankton communities andin the krill Nyctiphanes australis over a 1-year period offthe Otago Coast, New Zealand. Ambient UV-B and UV-A ranged froma minimum mean daily dose of 2.19 x 10⁴ kJ day^–1 and 0.73x 10⁶ kJ day^–1 in June, to a maximum in January of 20.19x 10⁴ kJ day^–1 and 4.88 x 10⁶ kJ day^–1, respectively.Concentrations of MAAs (consisting almost entirely of Mycosporine-glycine)in the phytoplankton community were lowest in August (5.6 nmolµg^–1 Chl) when UV-R irradiances were minimal andhighest in January (41.4 nmol µg^–1 Chl) when UV-Rirradiances were maximal. Nyctiphanes australis was found tocontain five identified MAAs (mycosporine-glycine, shinorine,Porphyra-334, palythine and palythinol) and several unknownUV-R absorbing compounds. Concentrations ranged from 4.73 to15.51 nmol mg^–1 dw, with little indication of a seasonalcycle that could be correlated with changes in either phytoplanktonMAA concentrations or ambient UV-R irradiances. The findingssuggest that krill are neither accumulating MAAs in responseto changes in MAA concentrations in their phytoplankton food,or that MAA concentrations in krill are increased in responseto higher ambient UV-R irradiances. Concentrations of MAAs inkrill body parts (carapace, legs, eyes, antennae, muscle) weresimilar (4.89–5.98 nmol mg^–1 dw), with the exceptionof the carapace (2.03 nmol mg^–1 dw).