Low-nutrient Water Research Articles

Heterologous expression of genes from a cyanobacterial endosymbiont highlights substrate exchanges with its diatom host.

A few genera of diatoms are widespread and thrive in low-nutrient waters of the open ocean due to their close association with N2-fixing, filamentous heterocyst-forming cyanobacteria. In one of these symbioses, the symbiont, Richelia euintracellularis, has penetrated the cell envelope of the host, Hemiaulus hauckii, and lives inside the host cytoplasm. How the partners interact, including how the symbiont sustains high rates of N2 fixation, is unstudied. Since R. euintracellularis has evaded isolation, heterologous expression of genes in model laboratory organisms was performed to identify the function of proteins from the endosymbiont. Gene complementation of a cyanobacterial invertase mutant and expression of the protein in Escherichia coli showed that R. euintracellularis HH01 possesses a neutral invertase that splits sucrose producing glucose and fructose. Several solute-binding proteins (SBPs) of ABC transporters encoded in the genome of R. euintracellularis HH01 were expressed in E. coli, and their substrates were characterized. The selected SBPs directly linked the host as the source of several substrates, e.g. sugars (sucrose and galactose), amino acids (glutamate and phenylalanine), and a polyamine (spermidine), to support the cyanobacterial symbiont. Finally, transcripts of genes encoding the invertase and SBPs were consistently detected in wild populations of H. hauckii collected from multiple stations and depths in the western tropical North Atlantic. Our results support the idea that the diatom host provides the endosymbiotic cyanobacterium with organic carbon to fuel N2 fixation. This knowledge is key to understanding the physiology of the globally significant H. hauckii-R. euintracellularis symbiosis.

Effect of food amount and temperature on growth rate and aerobic scope of juvenile chum salmon

Offshore migration of Pacific salmon Oncorhynchus spp. is partly triggered by increasing body size and high motility in the early stages of life. The survival of juvenile salmon may depend on their growth rate during the first few months in the sea, and this factor partly regulates the dynamics of adult populations. Here, we assessed the effects of water temperature and food availability on the growth of juvenile chum salmon O. keta. In addition, by combining the measurements of metabolic performance for growth and activity (Absolute Aerobic Scope: AAS) with a bioenergetics model, we estimated the energy allocation for different activities in the juveniles. Under high temperatures (14 °C), juveniles reared at low food levels (1% body weight) allocated less than half their energy for growth than those reared at high food levels (4% body weight). These findings suggest that high temperature and low food level constrain the growth of juveniles, providing an insight into the effect of the recent increase in warm and low-nutrient water masses on survival of juveniles and catches of adult chum salmon on the Pacific side of Honshu Island, Japan.

Morphological traits, niche-environment interaction and temporal changes in diatoms

Annual phytoplankton succession is a key ecological phenomenon that drives marine species’ life cycles and energy flows within marine ecosystems. Identifying processes that control annual succession is critical to anticipate climate-induced environmental perturbations of this phenomenon and the consequences upon ecosystem functioning. Here, we demonstrate that diatoms in the North Sea undergo strong morphological changes throughout the year and that species with similar phenology possess comparable morphological traits (e.g. cell elongation) and ecological niches. The spring and autumn periods appear to be dominated by oblates (flattened cells) whereas the summer period is dominated by prolates (elongated cells). Elongation of the cell shape enhances buoyancy and confers a selective advantage in stratified low-nutrient waters typical of summer without changing a diatom’s surface area/volume ratio or its ability to absorb nutrients. Diatom shape thus appears to have evolved as a key adaptation to a specific environment and confers upon a species its specific niche and phenology, and therefore its place in the sequence of annual succession. As a result, shape influences temporal changes in the abundance of diatoms and their putative response to environmental forcing. We suggest that biogeochemical and earth-system models should include diatom cell shape as a parameter in order to improve model predictions and help our understanding of the consequences of climate change on marine ecosystems.

Spatial hydrochemical structure in surface waters of the Southern ocean between Africa and Antarctica

Some hydrochemical characteristics and, first of all, the main nutrients (phosphorus, nitrogen, silicon) can be used as markers for distinguishing different types of water masses and positions of the main fronts of the Southern Ocean. The seasonal and interannual variability of these characteristics also reflects the character of biological processes in the surface layer of the ocean, which is important for assessing biological productivity. The aim of this study was to analyze the main features of the spatial distribution of hydrochemical characteristics in the surface layer in the Atlantic and Indian Ocean sectors of the Southern Ocean between the Subantarctic Front and the shores of Antarctica and assess their seasonal (spring–autumn) and interannual variability for the observation period from 2008 to 2020. We describe the surface nutrient concentrations between Africa and Antarctica along the transects that cross the Subantarctic Front (SAF) in the north, the Polar Frontal Zone (PFS), Polar Front (PF) and Antarctic Zone water in the south. The findings revealed an increase in dissolved oxygen and nutrients towards the south. Nitrates changed values within the SAF from 15 μM to 24 μM, whereas values from 1.2 μM to 1.7 μM were observed for phosphates. Silicate increased considerably within the Polar Front, from 6.6 μM to 20.8 μM. An analysis was carried out of the seasonal and interannual variability of the hydrochemical conditions in the surface layer of the Southern Ocean. The interannual variability of the nutrients was determined by the spatial variability of the main fronts of the Antarctic Circumpolar Current (ACC) and the intensity of the largescale Weddell Gyre (WG). Since 2017, there has been an increase in the meridional transfer of waters: in the Antarctic Summer 2017–2018, there was a spreading of high-nutrient WG waters toward the north, and in the Summer 2019–2020, the low-nutrient waters anomaly was transferred far to the south (up to 60°S).According to the data obtained, the seasonal dynamics of the nutrients in the surface layer of the Southern Ocean was rather weakly expressed. An exception is the high-latitude waters of the Cooperation and Davis Seas, where maximum seasonal variability of the hydrochemical characteristics was observed. The highest rate of nutrient consumption was observed in the coastal area of the Cooperation Sea near the fast ice edge from mid–December to early January and reached 3.2 μM per day for silicate, 1.8 μM per day for nitrates, and 0.12 μM per day for mineral phosphorus. The results of the long-term monitoring of the hydrochemical conditions in the Cooperation Sea made it possible to distinguish conditionally “warm” years with early vegetation (at the end of December) and intensive consumption of nutrients by phytoplankton, and “cold” years, when the formation of high-latitude “oases” in December–January was not observed.

Arsenolipids in Plankton from High- and Low-Nutrient Oceanic Waters Along a Transect in the North Atlantic.

Although the natural occurrence of arsenic-containing lipids (arsenolipids) in marine organisms is now well established, the possible role of these unusual compounds in organisms and in the cycling of arsenic in marine systems remains largely unexplored. We report the finding of arsenolipids in 61 plankton samples collected from surface marine waters of high- and low-nutrient content along a transect spanning the Gulf Stream in the North Atlantic Ocean. Using high-performance liquid chromatography (HPLC) coupled to both elemental and molecular mass spectrometry, we show that all 61 plankton samples contained six identifiable arsenolipids, namely, three arsenosugar phospholipids (AsPL958, 10-13%; AsPL978, 13-25%; and AsPL1006, 7-10% of total arsenolipids), two arsenic-containing hydrocarbons (AsHC332, 4-10% and AsHC360, 1-2%), and a methoxy-sugar arsenolipid that contained phytol (AsSugPhytol, 1-3%). The relative amounts of the six arsenolipids showed clear dependence on the nutrient status of the ambient water with plankton collected from high-nutrient waters having less of the arsenosugar phospholipids and more of the three non-P containing arsenolipids compared to low-nutrient waters. By combining these first field data of arsenolipids in plankton with reported global phytoplankton productivity, we estimate that the oceans' phytoplankton transform per year 50 000-100 000 tons of arsenic into arsenolipids.

Effect of addition of lanthanum-modified bentonite(Phoslock<sup>®</sup>) in sediments on growth of <i>Hydrilla verticillata</i> under different water nutrient concentration

轮叶黑藻（Hydrilla verticillata）是我国浅水湖泊常见的沉水植物，经常用于湖泊生态修复工程.然而在生态修复中后期，轮叶黑藻又往往会过度繁殖，影响其他湖泊功能的发挥.镧改性膨润土（锁磷剂）可以有效地降低沉积物中生物可利用磷含量，近年来被用于控制湖泊沉积物磷释放，其对轮叶黑藻生长和形态的影响还有待阐明.本研究在2种不同上覆水营养盐浓度条件下，设置添加和不添加锁磷剂2种处理，探究沉积物添加锁磷剂对轮叶黑藻生长及形态特征的影响.结果表明：1）沉积物添加锁磷剂之后，铁锰结合态磷、有机磷、铝结合态磷等生物可利用磷浓度显著降低，而生物不可用磷，如钙结合态磷浓度显著升高；2）沉积物添加锁磷剂显著降低了轮叶黑藻的生物量和生长率.添加锁磷剂的处理组植物的株高、地上部分生物量均明显低于无锁磷剂组；相反，添加锁磷剂显著增加了植物的根长和地下部分生物量，进而提高了植物的根冠比；3）锁磷剂对轮叶黑藻生长的影响程度与水体营养盐浓度有关，在低营养盐浓度水体中锁磷剂对植物生长和形态特征的影响更为显著.本研究表明，锁磷剂的添加能降低沉积物中生物可利用磷浓度，抑制轮叶黑藻的生长，同时这种影响程度与在水体营养盐浓度有关.;Hydrilla verticillata is a common submerged plant in shallow lakes in China, and is often used as a pioneer species in lake ecological restoration projects. However, the Hydrilla verticillata tends to overgrowth, forming a canopy which affects other functions of lakes. Lanthanum-modified bentonite (Phoslock^®) can effectively reduce the bioavailable phosphorus content in sediments. In recent years, it has been used to control phosphorus release from lake sediments. Its impact on the growth and morphology of Hydrilla verticillata remains to be clarified. In this study, two treatments with and without Phoslock^® in the sediments were set up to investigate the effects of Phoslock^® on the growth and morphological characteristics of Hydrilla verticillata under two different water nutrient concentrations. The results show that:1) After adding Phoslock^® to the sediments, the bioavailable phosphorus concentration, such as Fe/Mn combined phosphorus (BD-P), organic phosphorus (Org-P), Aluminum-bound phosphorus (Al-P), is significantly reduced, while the non-bioavailable phosphorus concentration, such as Calcium-bound phosphorus (Ca-P), is significantly increased. 2) The addition of Phoslock^® in the sediments significantly reduced the biomass and growth rate of the Hydrilla verticillata. The plant height and aboveground biomass of Phoslock^® group were significantly lower than those without Phoslock^®; on the contrary, the addition of Phoslock^® significantly increased the root length of the plant and the underground biomass, and increased the plant's root-shoot ratio. 3) The effect of Phoslock^® on the growth of Hydrilla verticillata is related to the concentration of nutrients in the water, and the effect of Phoslock^® on plant growth and morphological characteristics is more significant in low-nutrient water. Overall, this study shows that the addition of Phoslock^® can effectively reduce the concentration of bioavailable phosphorus in sediments and inhibit the growth of Hydrilla verticillata, and this effect is more significant when the concentration of nutrients in the water is reduced.

STRUCTURAL AND FUNCTIONAL INDICATORS OF BACTERIOPLANKTON OF THE SOUTH STRETCH OF LAKE MUNOZERO (KARELIA)

In article are presented the results of the state of the pelagic part of the south stretch of Lake Munozero (Karelia) according to microbiological indicators for the growing season of 2018. In the furtherance of this goal, 27 water samples were chosen in May, July and September at different strata. Munozero Lake is one of the unique lakes of Karelia due to its high salinity (100 mg/l), low-nutrient water sand organic substances. The total number, cell size dimensional structure and biomass of bacteria were determination by method of luminescence microscopy, by means of filtration through black nucleopore track membranes, after coloration bacteria by acridine orange. The total abundance of bacteria varied from 0,66 to 1,85 million cells/ml, and the biomass from 0,13 to 0,66 g/m3. The average cell volume varied in the range 0,18-0,34 μm3. Coccus form of bacteria prevailed in numbers. The doubling time and bacterial production were determined by the method of isolated samples. Bacterial production varied between 0,01 and 0,07 g/(m3•day-1). Specific growth rates (day-1) ranged around 0,05-0,2, giving doubling times of 70-364 h. The average daily production for the growing season was 0,65 g/m3•day-1. The physiological activity of bacteria was greatest at the end of the growing season. During the research period, the abundance of saprophytic bacteria growing on fish peptone agar was in the range of 19-550 CFU/ml. the abundance of oligotrophic bacteria growing on from starvation agar variable 200 to 850 CFU/ml. During the study period Water of the south stretch of the Lake Munozero was estimated as clean - moderately polluted with an intermediate quality class of 2-3. The south stretch of the Munozero is characterized as a mesotrophic water body.

Response of microplankton size structure to summer stratification, freshwater influx and coastal upwelling in the Southeastern Arabian Sea

A year-round FlowCAM-based study of micro-plankton (20–200 μm) size structure was conducted in several locations along a cross-shore transect off Kochi in the near-shore waters of the southeastern Arabian Sea (SEAS; along the southwest coast of India). The study area was characterized by a warmer and thermally stratified water column during the Pre-Southwest Monsoon (PSWM; March to May), which was replaced by the cool and hypoxic upwelled waters at the sub-surface and low salinity estuarine waters at the surface during the Southwest Monsoon (SWM; June to September). Irrespective of seasons, micro-autotrophs displayed higher abundance and biomass closer to the coast as compared to the offshore locations. The seasonal trend in the abundance and biomass of micro-autotrophs mismatched in the study region, the former was the highest (av. 8430 ± 2927 ind/L and av. 10.3 ± 2 μgC/L) during the PSWM and the latter during the SWM (av. 7703 ± 1237 and av. 16.8 ± 3.5 μgC/L). Micro-autotroph biomass was found to be higher in the surface waters, with an exception during the SWM, when larger micro-autotrophs (av. 137184 ± 10196 μm³/ind.) were observed in the subsurface. The highest seasonal abundance of micro-autotrophs (av. 8430 ± 2927 ind/L) during the PSWM could be attributed to the presence of smaller individuals (biovolume av. 59801 ± 6906 μm³/ind.). The overall spatial distribution of micro-heterotrophs was similar to that of the micro-autotrophs, with their high abundance and biomass in locations closer to the coast. Also, their abundance was found to be higher throughout the water column during PSWM, while it decreased noticeably in the deeper waters during the rest of the period. During the SWM, low PAR, high nutrients and high salinity in the sub-surface waters facilitated the dominance of larger micro-autotrophs (diatoms) such as Fragilaria (av. 211093 ± 8431 μm³/ind.) and Thalassiosira (av.80027 ± 5031 μm³/ind), whereas low salinity and high nutrient water mass from the adjacent Cochin (Kochi) backwaters favored high abundance of smaller micro-autotrophs such as Thalassionema (av. 17220 ± 756 μm³/ind.) and Skeletonema (av. 11831 ± 533 μm³/ind.) in the surface water. This study evidences for the first time that due to temporal changes in water masses in the near-shore waters in the SEAS, there are considerable seasonal and spatial variations in the size structure and composition even within the micro-plankton community itself.

Effects of Eutrophication and Different Water Levels on Overwintering of Eichhornia crassipes at the Northern Margin of Its Distribution in China.

When exotic species are introduced into new areas, establishment is a vital step in their invasion process. Therefore, overwintering strategies determine whether an exotic species from low latitudes can successfully invade middle- and high-latitude areas. In this study, we investigated the effects of nutrient and water drawdown on overwintering in an exotic aquatic plant from the tropical zone, Eichhornia crassipes, at the northern margin of its distribution in China. The population density, size of individuals, and the size and nitrogen concentration of overwintering organs (stem base) of E. crassipes that grew in high-nutrition water were greater than those that grew in low-nutrient water before winter. The overwinter survival rate of E. crassipes was significantly affected by the water level and nutrient. The thick and dense floating mat of E. crassipes can increase the temperature of water bodies; therefore, the overwinter survival rate of E. crassipes was higher in constant-water-level and high-nutrient treatment. In contrast, due to the loss of heat preservation provided by the floating mats and the low nitrogen concentration in the stem base, all individuals of E. crassipes died in constant-water-level and low-nutrient treatment. In the water-drawdown treatments, the stem base of E. crassipes was directly exposed to low-temperature air; therefore, the overwinter survival rate of E. crassipes was lower. Our results reveal that eutrophication can not only improve the competitiveness of E. crassipes but can also improve the survival rate of overwintering plants in temperate regions. Our study also suggests that removing nutrients from the water and regulating the water level can limit the invasion of E. crassipes in temperate and subtropical regions.

An ensemble modeling framework to study the effects of climate change on the trophic state of shallow reservoirs

Our understanding of the potential impact of climatic change on catchment hydrology and aquatic system dynamics has been advanced over the past decade, but there are still considerable knowledge gaps with respect to its effects on water quality vis-à-vis the increasing demands for drinking water. In this study, we developed an integrated hydrological-water quality (SWAT-YRWQM) model to elucidate the effects of a changing climate on the trophic state of the shallow Yuqiao Reservoir. Using a two-step downscaling process, we reproduced the prevailing meteorological conditions, as well as the streamflows in three major tributaries of the study area. A sensitivity analysis exercise showed that the nature of the calibration dataset used, namely the range of flows (i.e., dry versus wet years) included, can profoundly influence the predictive power of our modeling framework. Our climatic scenarios projected a minor change of the streamflow rates, but a variant degree of increase of the riverine total phosphorus (TP) concentrations and associated loading rates into the reservoir. Consequently, a significant rise of in-lake TP concentrations is projected for the near (2016–2030) and distant (2031–2050) future compared to the reference (2006–2015) conditions. Interestingly, the ambient TP levels appear to be lower in the distant relative to the near future, owing to changes in the magnitude and relative contribution of both external and internal nutrient loading sources. Our analysis also highlights the importance of reservoir operation practices to regulate water levels as a means for mitigating the climate change impact on the trophic status of the Yuqiao Reservoir, given that the diversion of low-nutrient water from the upstream basin can significantly reduce (30–40%) the TP concentrations. Our findings are highly relevant to the on-going debate about the potential implications of climate change for water availability, highlighting the importance of adaptation strategies to optimize the water resources management.

Effect of chloramine concentration on biofilm maintenance on pipe surfaces exposed to nutrient-limited drinking water

This study addresses the effect of specific monochloramine concentration on biofilm density and bacterial functional potential in nutrient-limited water. The efficacy of monochloramine residual maintenance on biofilm density was studied at a range of 0.5 to 2.0 mg/l, using a 3:1 (w/w) dosing ratio of chlorine to ammonia, with the provision of low-nutrient water (0.18 mg/l as total organic carbon, 0.055 mg/l as biodegradable dissolved organic carbon, and 10.5 μg/l as assimilable organic carbon) using a granular activated carbon (GAC) filter. Biofilm density was monitored using biofilm bacteria counts and analysis of the physiological substrate utilisation profiles in Biolog gram-negative (GN) micro-plates. The monochloramine residuals were maintained stable in the low-nutrient water pipes, which contributed to the inhibition of biofilm density. Increasing the monochloramine residual from 0.5 to 2.0 mg/l suppressed the total cells and heterotrophic plate count (HPC) bacteria in the biofilms by about 1 and 2 log units, respectively. The biofilm HPC densities were more sensitive to monochloramine residual, and the reduction in biofilm HPC densities expressed as log CFU/cm 2 showed an exponential relationship with the increase in monochloramine residual. The Biolog micro-plate-based community-level assay showed that the biofilm communities occurring at 3 levels of chloramination were distinguished by the differences in their substrate utilisation potentials. The functional/metabolic potential of the biofilm community’s ability to utilise specific substrates was much lower at higher monochloramine concentration. Results suggest that the maintenance of a consistently high-level monochloramine residual in the low-nutrient water system led not only to a reduction in biofilm density on pipe surfaces but also depressed potential functional/metabolic ability of the biofilm community. Keywords : biofilm, monochloramine residual, low-nutrient water, HPC, physiological substrate utilisation profile, GAC

A decade of summertime measurements of phytoplankton biomass, productivity and assemblage composition in the Pacific Arctic Region from 2006 to 2016

We present phytoplankton and nutrient observations from a period of ten years within five biological ‘hotspots’ in the Bering and Chukchi Seas, as identified by the Distributed Biological Observatory (DBO). Nitrate (NO3-) and total and size-fractionated (< and >5 µm) chlorophyll a (Chl a) concentrations, and rates of carbon (ρC, ‘primary productivity’) and NO3- utilization (ρNO3) were measured throughout the euphotic zone during eight cruises in July 2006, 2008 and yearly from 2011 to 2016. Samples were collected at one station within each of these five hotspots, which were located south of St. Lawrence Island (DBO1), south of the Bering Strait in the Chirikov Basin (DBO2), in the southeastern (DBO3) and northeastern (DBO4) Chukchi Sea, and in Barrow Canyon (DBO5). Nitrate concentrations averaged over the 10 years increased with depth and euphotic-zone integrated values were highest in the Chirikov Basin. Subsurface maxima in Chl a were present at about 30 m depth at most locations during every cruise, although the maximum ρC and ρNO3 rates were shallower, within the top 10 m of the water column. The f-ratio (calculated as ρNO3/ρC) averaged for all DBO regions and for the 10-year study-period was 0.41 (± 0.24). Similarly, phytoplankton > 5 µm in size accounted for 65 (± 23) % of total Chl a for all regions over the 10 years. Taxonomic analysis done in 2013 showed that diatoms were the dominant taxa throughout all of the DBO regions, with the exception of areas influenced by low-nutrient waters on the eastern side of the Chukchi shelf near the Alaska coast. These coastal waters were dominated by coccolithophores and small (< 7 μm) flagellates and had much lower Chl a concentrations, ρC and ρNO3 than farther west. In addition, the proportion of pennate diatoms to total diatom abundance was found to be elevated relative to centric diatoms when sea-ice was present. Our measurements of phytoplankton biomass and ρC indicated that the higher abundance of pennate diatoms in the euphotic zone was the result of phytoplankton blooms happening below the ice, rather than pennate diatoms being supplied by a sea-ice diatom bloom. The dynamic nature of the Pacific Arctic Region (PAR) resulted in strong interannual variability within each DBO region for all parameters, with no clear increasing or decreasing trends from 2006 to 2016. Spatial variations were more consistent, with the highest rates of ρC and ρNO3 occurring in the nutrient-rich waters of the southeastern Chukchi Sea (away from shore), and decreasing in regions further north as NO3- concentrations were lower. An east-west gradient in phytoplankton biomass and productivity was also observed in the southeastern Chukchi Sea, which can be attributed to differences in the nutrient content of the water masses along the gradient. This study shows that the observed strong interannual variability in phytoplankton biomass and productivity cannot be attributed to differences in methodology or sampling time. It also highlights the need for better temporal and spatial sampling resolution such that the long-term effects of climate-induced changes can be identified against the backdrop of the naturally-strong interannual variability in the PAR.

When Cleaning Too Much Pollution Can Be a Bad Thing: A Field Experiment of Consumer Demand for Oysters

Oysters are a special kind of green product. They filter phytoplankton from water and thereby reduce nutrients, the primary driver of eutrophication of water that can consequently harm human health. Yet, where they can provide the most ecosystem benefit is in highly eutrophic waters and being raised in these ‘polluted’ waters may be an unattractive attribute for consumers. In this research, we use revealed-preference dichotomous-choice field experiments to test if and under what pollution mitigation circumstances oyster consumers will pay price premiums for oysters. The results from 290 adult participants in the Mid-Atlantic of the US suggest that providing information about eutrophication and oysters' ability to filter nutrients increases participants' WTP price premiums for oysters from low-nutrient waters and decreases their WTP price premiums for oysters from eutrophic waters with more nutrients. These results illustrate an important tension in how best to market green products like oysters, as the situations where they provide the most ecosystem benefits (in eutrophic waters) are also situations, which appear to raise the highest level of concerns among consumers. These results have implications on whether oysters should be actively marketed as a green product.

Dispersion/dilution enhances phytoplankton blooms in low-nutrient waters

Spatial characteristics of phytoplankton blooms often reflect the horizontal transport properties of the oceanic turbulent flow in which they are embedded. Classically, bloom response to horizontal stirring is regarded in terms of generation of patchiness following large-scale bloom initiation. Here, using satellite observations from the North Pacific Subtropical Gyre and a simple ecosystem model, we show that the opposite scenario of turbulence dispersing and diluting fine-scale (∼1–100 km) nutrient-enriched water patches has the critical effect of regulating the dynamics of nutrients–phytoplankton–zooplankton ecosystems and enhancing accumulation of photosynthetic biomass in low-nutrient oceanic environments. A key factor in determining ecological and biogeochemical consequences of turbulent stirring is the horizontal dilution rate, which depends on the effective eddy diffusivity and surface area of the enriched patches. Implementation of the notion of horizontal dilution rate explains quantitatively plankton response to turbulence and improves our ability to represent ecological and biogeochemical processes in oligotrophic oceans.

Interaction between temperature and water nutrient levels on the fitness of Eccritotarsus catarinensis (Hemiptera: Miridae), a biological control agent of water hyacinth

Fitness traits of insect herbivores of aquatic weeds may change with environmental conditions including temperature and water nutrient levels. Both of these factors are likely to increase due to changes in global weather patterns and human activity. We tested the interaction of temperature and nutrient status on the fitness of Eccritotarsus catarinensis, a biocontrol agent against the invasive aquatic weed, water hyacinth Eichhornia crassipes. Several life history traits were evaluated on plants grown at three constant temperatures (20, 25 and 30°C) combined with three nutrient levels: oligotrophic (low), mesotrophic (medium), and eutrophic (high). When the two factors are separated, all fitness traits decreased with increasing temperature and decreasing water nutrient levels. The combination of high temperature and low water nutrient status had a greater and significant negative effect on some fitness traits: lifetime fecundity, realized fecundity and nymphal survival. The relationship between insect fitness traits and water nutrient status followed different patterns depending on temperature: a monotonic, but non-linear response at 20°C, where fitness was highest at medium water nutrient status and reduced at low and high water nutrient status; and a linear monotonic response at 25°C, where fitness increased with increasing water nutrient status. At 30°C fitness traits were equal on plants grown in water with high and medium nutrient status and lower on those grown in water with low nutrient status. We demonstrated that a combination of low nutrient waters with higher temperatures would hamper the efficacy of this agent. Our results suggested that the negative impact of the herbivorous insects on the aquatic weed would be most successful on vigorous plants, applicable for both mesotrophic and eutrophic nutrient levels, particularly in temperate areas.

Dissolved inorganic nitrogen and phosphorus dynamics in seawater following an artificial Saharan dust deposition event

The dynamics of dissolved inorganic nitrogen and phosphorus in seawater after a dust event were followed to better understand the impact of dust deposition in low nutrient waters of the Mediterranean Sea. Three independent abiotic experiments were performed over three seasons (winter, spring, end of summer) characterized by contrasted biogeochemical conditions. Experiments consisted of seeding evapocondensed Saharan dust at the surface of a polyethylene tank filled with filtered surface seawater. Phosphate (PO43-), nitrate (NO3-), size and number of particles and transparent exopolymeric particles production (TEP) were measured over the course of one week following seeding. Dust deposition was followed by a transient increase in [PO43-] during the first three hours with a maximum input of 33, 9 and 39 nM respectively in May, October and February. The removal of almost all the PO43- initially released suggests a scavenging process of PO43- back onto ferric oxide-rich particles leading to concentrations at the end of the experiment close to the initial values (7 nM in May and October, and 6 nM in February). NO3- released from dust was high especially in May and October (maximum input of 23 and 11 µM respectively) and was attributed to nitrogen dissolution from the large amount of small particles (< 1µm) rich in nitrogen in the evapocondensed dust. [NO3-] remained high until the end of the experiment (16 µM in May and 11 µM in October), indicating that NO3- from dust is likely to be bioavailable for a longer period compared to PO43- from dust. The release of PO43- and NO3- was intrinsically linked to particle dynamics, governed by the quality/quantity of dissolved organic matter.

Photosynthetic and nutrient uptake physiology of Padina species in high and low nutrient waters in Bolinao, Pangasinan, Philippines

Abstract The genus Padina (Dictyotales: Dictyotaceae) are ubiquitous tropical macroalgae found in a wide range of environmental conditions, but the physiological mechanisms that allow them to thrive in different light and nutrient conditions remain unclear. This study identified Padina species found at two contrasting sites in Bolinao, Pangasinan – turbid, high-nutrient waters at Rungos, and clear, low-nutrient waters at Patar, and compared their physiological responses. The examined parameters were growth rates, photosynthetic responses to varying irradiances (P-E curves), and NO3 - and NH4 + uptake kinetics. Different species were found at each site, i.e., Padina antillarum in Rungos and Padina minor in Patar. From the P-E curves, both species appeared to have “shade plant” characteristics with high α values, low compensation irradiances, and low saturation irradiances. Though P. antillarum had higher net productivity (Pmax=69.4 mg O2 gdw-1 h-1) than P. minor (Pmax=40.6 mg O2 gdw-1 h-1), their growth rates did not significantly differ. Their NO3 - uptake rates were similar, while the NH4 + uptake rate of P. minor (Vmax=249.60 μmol gdw-1 h-1) was double that of P. antillarum (Vmax=104.50 μmol gdw-1 h-1) during the first 30 min of exposure to NH4 +. These results suggest that each Padina species utilizes different strategies to survive in their respective habitats.

Ecology and morphology of the freshwater red alga Chroothece in the British Isles

The ecology of Chroothece at six sites in the UK and one in the Isle of Man is de- scribed, together with morphological observations. Sites were near vertical or steep, wetted by trickles and drips, but subject to marked changes in extent of wetting and with parts of the gen- eral area liable to desiccation. Water chemical data are given for four sites, where the alga oc- curred in base-rich, low nutrient waters and with fi lterable organic phosphorus the principal fraction of total fi lterable orthophosphate. Chroothece was associated with a range of cyanobac- teria, especially species of Schizothrix, but also colonial, heterocystous and presumed nitrogen- fi xing genera, and calcicolous diatoms (e.g. Epithemia and Mastogloia). Morphological meas- urements suggest the Chroothece populations all fi t C. richteriana.

Relative impact of seasonal and oceanographic drivers on surface chlorophyll a along a Western Boundary Current

Strengthening Western Boundary Currents (WBCs) advect warm, low nutrient waters into temperate latitudes, displacing more productive waters. WBCs also influence phytoplankton distribution and growth through current-induced upwelling, mesoscale eddy intrusion and seasonal changes in strength and poleward penetration. Here we examine dynamics of chlorophyll a (Chl. a) in the western Pacific Ocean, a region strongly influenced by the East Australian Current (EAC). We interpreted a spatial and temporal analysis of satellite-derived surface Chl. a, using a hydrodynamic model, a wind-reanalysis product and an altimetry-derived eddy-census. Our analysis revealed regions of persistently elevated surface Chl. a along the continental shelf and showed that different processes have a dominant effect in different locations. In the northern and central zones, upwelling events tend to regulate surface Chl. a patterns, with peaks in phytoplankton biomass corresponding to two known upwelling locations south of Cape Byron (28.5°S) and Smoky Cape (31°S). Within the central EAC separation zone, positive surface Chl. a anomalies occurred 65% of the time when both wind-stress (τw) and bottom-stress (τB) were upwelling-favourable, and only 17% of the time when both were downwelling-favourable. The interaction of wind and the EAC was a critical driver of surface Chl. a dynamics, with upwelling-favourable τW resulting in a 70% increase in surface Chl. a at some locations, when compared to downwelling-favourable τW. In the southern zone, surface Chl. a was driven by a strong seasonal cycle, with phytoplankton biomass increasing up to 152% annually each spring. The Stockton Bight region (32.25–33.25°S) contained ⩾20% of the total shelf Chl. a on 27% of occasions due to its location downstream of upwelling locations, wide shelf area and reduced surface velocities. This region is analogous to productive fisheries regions in the Aghulus Current (Natal Bight) and Kuroshio Current (Enshu-nada Sea). These patterns of phytoplankton biomass show contrasting temporal dynamics north and south of the central EAC separation zone with more episodic upwelling-driven Chl. a anomalies to the north, compared with regular annual spring bloom dynamics to the south. We expect changes in the strength of the EAC to have greater influence on shelf phytoplankton dynamics to the north of the separation zone.

Physical–biological coupling induced aggregation mechanism for the formation of high biomass red tides in low nutrient waters

Port Shelter is a semi-enclosed bay in northeast Hong Kong where high biomass red tides are observed to occur frequently in narrow bands along the local bathymetric isobars. Previous study showed that nutrients in the Bay are not high enough to support high biomass red tides. The hypothesis is that physical aggregation and vertical migration of dinoflagellates appear to be the driving mechanism to promote the formation of red tides in this area. To test this hypothesis, we used a high-resolution estuarine circulation model to simulate the near-shore water dynamics based on in situ measured temperature/salinity profiles, winds and tidal constitutes taken from a well-validated regional tidal model. The model results demonstrated that water convergence occurs in a narrow band along the west shore of Port Shelter under a combined effect of stratified tidal current and easterly or northeasterly wind. Using particles as dinoflagellate cells and giving diel vertical migration, the model results showed that the particles aggregate along the convergent zone. By tracking particles in the model predicted current field, we estimated that the physical–biological coupled processes induced aggregation of the particles could cause 20–45 times enhanced cell density in the convergent zone. This indicated that a high cell density red tide under these processes could be initialized without very high nutrients concentrations. This may explain why Port Shelter, a nutrient-poor Bay, is the hot spot for high biomass red tides in Hong Kong in the past 25 years. Our study explains why red tide occurrences are episodic events and shows the importance of taking the physical–biological aggregation mechanism into consideration in the projection of red tides for coastal management.