Responses of Phytoplankton Benthic Propagules to Macronutrient Enrichment and Varying Light Intensities: Elucidation from Monsoon-Influenced Mandovi and Zuari Riverine System : Responses of Phytoplankton Benthic Propagules to Macronutrient Enrichment and Varying Light Intensities: Elucidation from Monsoon-Influenced Mandovi and Zuari Riverine System.

The ecological importance of phytoplankton-benthic-propagules (PBP) from coastal sediments, except tropical monsoon-influenced rivers and estuaries, is well documented. PBP in such systems is often transported from upstream (near freshwater) to downstream (estuary) through freshwater discharges during monsoon and thereby experiences higher salinities (>30 PSU) and nutrients with varying light conditions due to reducing discharges during the monsoon-break/withdrawal-phase. However, the PBP responses (germination and subsequent growth) to such changes are unknown and are examined here at ~35 PSU salinity. For the study, macronutrients (nitrate, phosphate, silicate, and nitrate+phosphate+silicate) and light intensities were considered to assess the response of PBP representing estuarine, brackish, and near freshwater locations of monsoon-influenced Mandovi and Zuari rivers (Goa, India). Diatoms dominated the viable PBP community, but the maximum growth and sustained photosynthetic activity were observed when all macronutrients were supplied than individually. Here, variable fluorescence technique utility in PBP resurrection (detection of viability and responses) was also explored. The PBP growth was similar for macronutrients but increased with light intensity indicating a longer growth response during monsoon. For PBP (germination and photosynthetic activity), light intensities drive the rate of improvement/development, whereas the nutrients are essential for maintaining vegetative growth upon germination in the region. The PBP dominance of common planktonic species (Skeletonema and Thalassiosira) along the river also signifies the role of seawater intrusion (up to upstream locations) in distribution. Skeletonema and Thalassiosira, which contribute significantly to the total community, are light-sensitive with a similar response and cause single species blooms during monsoon and non-monsoon, respectively, depending on the species' tolerance to salinity.

Variability in phytoplankton shape dominance in marine pelagic systems: prevalence of different adaptive strategies in the eastern Arabian Sea during the winter monsoon.

Phytoplankton communities from pelagic systems were assessed to explore the potential of using commonly used traits (such as cell geometry and taxa) as ecological function indicators from the data generated during the winter monsoon in the eastern Arabian Sea (AS). Altogether, data from two oceanic, i.e., convective mixing influenced non-oligotrophic northeastern-AS (NEAS-O) and Rossby wave-influenced oligotrophic southeastern-AS (SEAS-O) and one coastal (NEAS-C) cruises were utilized to decipher the ecological inferences. Overall phytoplankton shapes showed a high level of redundancy by selecting only a few dominant shapes (5 of 22 shapes), though taxonomic diversity was rich (164 species). The taxonomic and morphological approach adopted revealed high species and shape diversity in NEAS-O than in high-abundance NEAS-C and low-abundance SEAS-O. Also, the shape diversity and dominant shapes (cylinder, elliptic-prism, and prism-on-parallelogram) remained the same in oceans than NEAS-C where combined (cylinder + 2 half-sphere) and simple (elliptic-prism) shapes dominated. Additionally, the Rossby-wave front and its reminiscence in SEAS-O and sea-surface-temperature fronts in NEAS-C favored simple and combine shaped phytoplankton, respectively. The morphological properties assessment revealed that the dominant shapes adapted the strategy to conserve the optimal surface-to-volume ratio (S:V) irrespective of changes in greatest-axial-linear-dimension (GALD) in NEAS-O and SEAS-O but not in NEAS-C. However, the dominant shapes in the NEAS-O and SEAS-O opted for high S:V with low GALD and low S:V with high GALD, respectively, while high S:V with no relation with GALD in NEAS-C suggests the prevalence of different adaptive strategies to cope with the respective hydrographic conditions, particularly nutrient availability.

Simulation experiments to elucidate variable fluorescence as a potential proxy for bulk microalgal viability from natural water, sediments and biofilms: Implication in ships ballast water management.

The variable fluorescence fluorometry measuring microalgal biomass (initial fluorescence - F0, a chl-a proxy) and photosynthetic efficiency (Fv/Fm) has been suggested as a potential tool in ballast-water assessment. In ballast tank, microalgae can be found in contiguous compartments i.e., in water, sediment, and biofilms. Therefore the utility of F0 and Fv/Fm depends upon proper background corrections, which is straightforward for water samples but not for sediment and biofilms. This study proposes procedures for correcting F0 values from sediment and biofilms. Irrespective of the saturation flash protocol used on any sample types the outcome of the results from viable and non-viable microalgae will remain same. Stress experiments (continuous darkness and biocide treatments) confirm that variable fluorescence (Fv) can be used as a potential proxy for viable cells as the values were negligible for non-viable cells and increased with an increase in abundance. Through this study, the utility of Fv and σPSII (functional-absorption-cross-section of photosystem II) along with F0 and Fv/Fm in providing additional information on cell-viability and algal-size group during assessment is discussed. The findings will have implications not only from the perspective of ballast water but also in testing/assays of specific interest (e.g. toxicity, water treatments, antifouling) and ecological studies involving microalgae.

Picophytoplankton as Tracers of Environmental Forcing in a Tropical Monsoonal Bay.

In order to better understand the picophytoplankton (PP) dynamics in tropical monsoon influenced coastal regions, samples were collected daily (June-September 2008: monsoon, December 2008: post-monsoon and April 2009: pre-monsoon) from a fixed station in Dona Paula Bay, India. Eight PP abundance peaks comprising Prochlorococcus-like cells, picoeukaryotes, and three groups of Synechococcus occurred. The chlorophyll biomass and PP abundance were negatively influenced by reduced solar radiation, salinity and water transparency due to precipitation and positively influenced by the stabilized waters during precipitation break/non-monsoon periods. Responses to environmental conditions differed with PP groups, wherein the presence of Synechococcus-PEI (phycoerythrin) throughout the year suggested its ability to tolerate salinity and temperature variations and low light conditions. Synechococcus-PEII appearance toward monsoon end and non-monsoon during high water transparency suggests its tidal advection from offshore waters. Dominance of Synechococcus-PC (phycocyanin) at intermediate salinities under low water transparency during MON and high salinities in PrM coinciding with high nitrate concentrations implies a greater influence of light quality or nutrients. Cyanobacteria and not picoeukaryotes were the dominant picophytoplankton in terms of numbers as well as biomass. This study suggests that PP could be used as tracers of environmental forcing driven by tides and freshwater influx and also highlights the importance of high-frequency samplings in dynamic coastal regions through which transient responses can be captured.

Effect of biofilm on fluorescence measurements derived from fast repetition rate fluorometers.

This study evaluates, for the first time, the influence of biofilms on single and double optical window (SOW and DOW, respectively) fast repetition rate fluorometer (FRRF) measurements of microalgal photosystem-II initial fluorescence (F0), maximum fluorescence (Fm), variable fluorescence (Fv = Fm - F0), quantum yield (Fv/Fm) and functional absorption cross section (σPSII)]. Biofilms with chlorophyll > 0.1 µg cm(-2) and > 0.3 µgcm(-2) on SOW and DOW, respectively, produced a substantial increase in fluorescence. However, the relative magnitude of biofouling effects depended on sample chlorophyll concentrations, being more critical at concentrations < 1 mg m(-3). In DOW-FRRF, biofilms affected F0 (increased) and Fv/Fm (decreased) but not Fv and σPSII, whereas in SOW-FRRF, biofilms increased fluorescence and showed a variable effect on Fv/Fm and σPSII, because only biofilms on SOW attained actual Fm. As a result, the biofilm effect was substantial on SOW-FRRF measurements. On the other hand, the neutral-density filters (representing non-chlorophyll containing biofilms) with different transmission levels reduced the fluorescence signal. Correction procedures for the above photosystem-II parameters are proposed here.

Dark survival in biofilm-forming microalgae: potential for colonizing benthic ecosystems.

The biofilm-forming microalgae are known to experience periods of continuous darkness (upto several days), in addition to the natural day-night cycle, especially in the intertidal sediment regions (when transported to deeper sediments) and the ships' ballast water tanks (during voyages). However, the information on community and physiological responses to different periods of darkness is limited. Here, the survival capability of biofilm-forming microalgae to varying periods of darkness (7-35 days) and the growth patterns upon resumption of normal 12 h light:dark photocycle has been addressed through simulation experiments. Diatoms dominated the seawater biofilms, but the dark survival period varied and was species-specific. Of the 25 diatoms, only Amphora, potential toxin producer, followed by Navicula remained viable and photosynthetically healthy under darkness without undergoing asexual reproduction. Both diatoms are essential contributors to fouling and microphytobenthic community. Upon re-exposure of dark-adapted biofilms to 12 h light:dark photocycle, the improvement in photosynthetic efficiency and resumption of growth via asexual-reproduction was observed. However, the lag-phase duration increased with a long dark history. Nevertheless, eurytolerant nature and high dark survival capability (with its quick response to light) of Amphora and Navicula indicated that they have the potential to colonize benthic-ecosystems thus impacting the benthic and fouling community.

Effect of chlorination on the development of marine biofilms dominated by diatoms.

This study addressed the antifouling efficiency of commercially available chlorine at different concentrations (0.5%, 1%, and 2%) and exposure times (0.5 min, 1 min, 5 min, and 15 min). The rapid and non-destructive FIRe (fluorescence induction and relaxation) technique was used to evaluate the effects of the biocide on diatom dominated biofilms. The efficiency of chlorine in removing diatoms from the developed biofilms increased with an increase in concentration and exposure time. The fluorescence measurements revealed low F(v)/F(m) and high σ(PSII) values for chlorine-treated Navicula and Amphora biofilms indicating that chlorination was efficient in damaging the photosystem-II reaction centers. Chlorination also caused mortality of diatom cells by damaging the cell body. In natural biofilms, the biocidal effect of chlorine was species specific; species of Amphiphrora, Navicula, Cylindrotheca, and Coscinodiscus showed an increase in the density of the population, but species of Pleurosigma, Amphora, and Thalassionema did not increase in density after chlorine treatment. It was also demonstrated that diatoms can colonize, grow and photosynthesize on chlorine-treated surfaces. Under pulse chlorination (treatment every 6 h), irrespective of chlorine concentration, the development of biofouling decreased with an increase in exposure time. Differences between exposure times of 1 to 15 min were not significant. Additionally, transmission levels of the control (non-chlorine-treated) fouled coupons reduced significantly (∼20%) compared to the chlorine-treated fouled coupons (<2%). These results suggest that chlorine can be used as a biocide to control the development of diatom biofilms.

Variations in phytoplankton community in a monsoon-influenced tropical estuary.

In a monsoon-affected tropical estuary, oscillations in freshwater discharge during monsoon shifted the phytoplankton blooms from those adapted to low salinities to high salinities and vice versa. Salinity stratification during monsoon (onset and restart after an intermittent break) favored diatom (Skeletonema) bloom in low-saline surface waters. In high-saline, nutrient-rich bottom waters, Fragilariopsis (diatom) bloom was observed during onset of monsoon and persisted till the end of monsoon. The break period in monsoon altered the phytoplankton community leading to mixed species bloom of large-sized diatoms and harmful dinoflagellates (Gymnodinium catenatum and Cochlodinium polykrikoides) under high-saline, nutrient-poor, non-stratified, and transparent water column. Such variations in community should be considered for better understanding the biogeochemistry of monsoon-influenced tropical estuaries. The dominance of Skeletonema is determined positively by the extent of low-saline stratified condition whereas most of the observed taxa were favored by high-saline, nutrient-poor, and transparent waters.

Temporal variations in dimethylsulphoniopropionate and dimethyl sulphide in the Zuari estuary, Goa (India).

Despite tropical estuarine systems representing important sites for active biogeochemical processes, studies on dimethyl sulphide (DMS) in these systems are sparse. Here we report on DMS and dimethylsulphoniopropionate (DMSP) variability in relation to physicochemical and biological parameters for a period of 14 months in a tropical estuarine environment. DMS and DMSP showed high temporal variations with maximal concentrations during the southwest monsoon coinciding with a dinoflagellate bloom. Dinoflagellates appear to be the major contributors to the DMSP pool. Average DMS and DMSP concentrations (surface and bottom) suggested that much of the DMSP produced is converted to forms other than DMS. Surface DMS varied between 0.3 and 15.4 nmol dm(-3) while DMSP ranged from 0.8 to 419.5 nmol dm(-3). The DMS flux was 0.03-1.9 microM m(-2) d(-1) (average=0.6 microM m(-2) d(-1)) during the study period, that concurs well with the values reported for temperate estuaries.

Ultraviolet radiation (UV-C): a potential tool for the control of biofouling on marine optical instruments.

In an effort to develop a tool for controlling biofouling of marine optical instruments, the efficiency of ultraviolet radiation (UVR - 254 nm) in preventing biofouling was evaluated by conducting in situ experiments at different intensities (14.7, 9.6, 7.3 Wm(-2)) and exposure times (continuous, on for 30, 15, 5, 1 min h(-1)) using glass as test coupons. Although there was significant seasonal variation in environmental conditions and phytoplankton composition among each experiment, the amount of biofilm relative to the internal control demonstrated consistent trends. The efficiency of UVR in preventing biofouling increased significantly with increase in intensity and exposure time. UVR was effective even in reducing the population of microfoulers from already developed biofilms. UVR exposure for 30 min h(-1) at all intensities as well as for 5 and 15 min h(-1) at the highest intensity was found to be most effective. It was observed that UVR dose is not the sole determinant of UVR effectiveness. The reduction in transmission level of the UVR treated coupons was < 5% irrespective of exposure time except for 1 min h(-1). These results reveal that UV-C radiation can be used as a potential biofouling control tool for optical instruments.

Quantification of diatoms in biofilms: standardisation of methods.

Benthic diatoms, which often dominate marine biofilms are mostly pennate along with a few centric species that have an attached mode of life. Even though the range of diatoms in biofilms is diverse, their ecology is poorly understood because of the difficulty in sampling and enumeration. Scraping or brushing are the traditional methods used for removal of diatoms from biofilms developed on solid substrata. The method of removal is the most critical step in enumerating the biofilm diatom community structure. In this study, a nylon brush and ceramic scraper were used as tools for the removal of diatoms from 1 - 4-day-old biofilms developed on fibreglass coupons and glass microscope slides. Standardisation of methods showed that the sample volume used in the analyses had the least influence on the quantification, whereas the method of removal was critical. The nylon brush was more efficient at recovering diatoms compared to a ceramic scraper. Direct microscopic enumeration of the community in the case of glass slides indicated that scraping resulted in between 30-50% underestimation. Heterogeneity in diatom community structure between replicate samples is one possible reason for such underestimation.

Biofilm diatom community structure: influence of temporal and substratum variability.

Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.