Chlorophyll a and turbidity patterns over coral reefs systems of La Parguera Natural Reserve, Puerto Rico

Studies of temporal and spatial changes in phytoplankton biomass and turbidity provide essential information on coral reef ecosystem function and health. Fluctuation of phytoplankton biomass responds to several factors including nutrient inputs, both anthropogenic and natural, while turbidity is mostly affected by sediment resuspension or transport from terrestrial systems. These parameters can be used as sentinels of significant environmental factors "modifying" coral reef systems. A chlorophyll a concentration (Chl a) and turbidity (Turb) in situ logger was installed at 10 stations from June 4 to July 7, 2003 in La Parguera Natural Reserve (Southwestern Puerto Rico) to assess short-term temporal and geographic variation in patterns of phytoplankton biomass and turbidity at pre-selected sites as part of an interdisciplinary long-term study. Average station Ch1 a variation was 0.17-1.12 microg 1(-1) and 0.2-23.4 NTU for Turb. Results indicate that the western near-coastal stations had higher levels of Turb and Ch1 a. The easternmost mid shelf station, Romero reef, was similar to coastal stations probably due to nutrient and suspended sediment inputs from a source external to our study area to the east, Guánica Bay. Comparisons between different sampling days indicate significant differences between days for most stations suggesting that one-time discrete sampling may not be representative of average water column conditions and illustrate the dynamic nature of coral reef systems. Further work is warranted to assess seasonal changes that integrate short-term (daily) variability in both Turb and Ch1 a.     

Summer fluctuations of microbial planktonic communities in a eutrophic lake--Cierva Point, Antarctica

A eutrophic lake at Cierva Point, Antarctic Peninsula was surveyedduring the summers of 1997 and 1998. Phytoplankton size fractions(micro-, nano- and picoplankton) were analysed, as well as theabundance of bacterioplankton and planktonic ciliates. No permanentvertical stratification was found owing to the shallowness ofthe lake. Both nutrient concentrations and chlorophyll a valuesindicated highly eutrophic conditions, which are a consequenceof a natural enrichment by seabirds. Significant differencesin temperature between the 1997 and 1998 seasons strongly influencedmost of the biological features. The phytoplankton communityshowed a high algal species-richness, with important contributionsof epilithic, cryobiontic and soil algae. The dominant algalgroup was Chlorophyta, mainly represented by Chlamydomonas aff.celerrima, followed by Chl. aff. braunii. Some replacement ofphytoplanktonic species took place in summer and was more evidentin 1998. Picophytoplankton reached high densities, similar tothose reported from other Antarctic lakes. Bacterioplanktonabundances were typical of eutrophic and hypereutrophic lakes.There was a positive correlation between bacterial and totalphytoplankton abundance. Ciliates reached some very high peaks,with higher figures than those reported for other Antarcticsystems with similar trophic status.     

Adaptations of tropical marine microphytobenthic assemblages along a gradient of light and nutrient availability in Suva Lagoon,Fiji

How are microphytobenthic biofilms adapted to the high incident irradiances and temperatures, low inorganic nutrient concentrations and high desiccation stresses on intertidal flats present in tropical environments? This study investigated biofilms subject to different environmental conditions in a range of tropical sites in Suva lagoon, Fiji. PAM fluorescence was used to measure photophysiological responses to the light climate. Biofilm colloidal carbohydrate, extracellular polymeric substances (EPS) and low molecular weight (MW) carbohydrate concentrations and diel carbohydrate production patterns were measured. Average biomass (Chl a) ranged from 15 to 36?mg?m^?2, and was highest in seagrass bed sediments, but biomass was not correlated with water column or sediment porewater nutrient concentrations. Biofilm photophysiology differed significantly along a combined gradient of light and nutrient availability, with F _v/F _m, relative ETR_max and E _k of biofilms highest in mangrove and intertidal main island sites and lowest in subtidal coral reef flats. Subtidal biofilms showed photoinhibition at irradiances > 1000?µmol?m^?2. Significant correlations between Chl a and colloidal carbohydrate concentrations were present (except on intertidal sandflats), and tropical biofilms had higher ratios of colloidal carbohydrate and EPS to Chl a than temperate estuarine biofilms, probably due to a combination of high irradiance and low nutrient availability leading to the production of excess photoassimilates. The percentage of EPS present in the colloidal fraction was highest in coral sand biofilms (42%), which had the lowest nutrient concentrations, compared with other sites (25–32%). Intertidal biofilms predominantly consisted of large motile taxa and showed strong rhythms of vertical migration. During tidal emersion, high sediment temperatures (41?°C), irradiance (>2300?µmol?m^?2?s^?1) and salinity (49‰) stimulated downward migration. In silty sediments, migration resulted in a reduction in photosynthetic activity during the midday period but, in sands with high light penetration (to a depth of > 1700?µm), high production rates of EPS (18.2?µg carbo. µg Chl a^?1 h^?1) and low MW carbohydrate exudates (40.2?µg carbo. µg Chl a^?1 h^?1) occurred. Vertical migration, high E _k and high rates of photoassimilate dumping are all adaptations to living in the tropical intertidal zone. Seagrass and reef flat biofilms consisted of a diverse non-migratory flora of motile and non-motile taxa that were not subject to such extreme temperature and irradiance conditions. Low values of photosynthetic parameters and high colloidal and EPS content indicated that these biofilms were nutrient-limited.     

Size-dependent changes in phytoplankton C and N uptake in the dynamic mixed layer of Lake Biwa

1. Short-term (days) hydrodynamic effects of wind-induced mixing on phytoplankton size structure, and C and N uptake characteristics, were studied in the surface mixed layer (epilimnion) of Lake Biwa (North Basin), before and during a period of high winds (typhoons). 2. The latter period was characterized by two major typhoon events associated with deepening of the seasonal thermocline, reduced water column stability, decreased underwater irradiance and increased dissolved reactive N and particulate P. 3. Nutrient concentrations, seston C/N ratios, and uptake rates indicated that phytoplankton biomass and production were limited by P and not N throughout the study. Higher C- and N-based productivity during the typhoon period than before reflected the increased phytoplankton biomass and higher specific uptake rates due to increased nutrient supply. 4. Changes in the size-structure of phytoplankton (< 2 and > 2 μm) were associated with variations in the stratification and mixing regime. When vertical stability was high (before the typhoons) concentrations of > 2 μm biomass (chlorophyll a, particulate organic C and N) were higher at the bottom of the mixed layer than at the surface whereas, when stability of the mixed layer was low (the typhoon period), the contribution of picoplankton (< 2 μm) to total Chl a increased at the surface and decreased at the bottom following the first high winds. 5. Photoadaptive adjustments of the phytoplankton provided further evidence of hydrodynamic control. The lower intracellular Chl a concentrations and C and N uptake efficiencies in the < 2 μm fraction suggest that they experienced, on average, higher irradiance than the larger cells because of their lower sinking rates. During the stability period, picoplankton exhibited higher photosynthetic efficiencies at the bottom of the mixed layer than at the surface. Such differences disappeared during the typhoon period indicating that the mixing rate was then probably higher than the photoacclimation rate in the small size fraction. 6. The present results stress the highly transient nature of biological homogeneity in the surface mixed layer of the lake.     

Identity and diversity of coral endosymbionts (zooxanthellae) from three Palauan reefs with contrasting bleaching, temperature and shading histories

The potential of corals to associate with more temperature-tolerant strains of algae (zooxanthellae, Symbiodinium) can have important implications for the future of coral reefs in an era of global climate change. In this study, the genetic identity and diversity of zooxanthellae was investigated at three reefs with contrasting histories of bleaching mortality, water temperature and shading, in the Republic of Palau (Micronesia). Single-stranded conformation polymorphism and sequence analysis of the ribosomal DNA internal transcribed spacer (ITS)1 region was used for genotyping. A chronically warm but partly shaded coral reef in a marine lake that is hydrographically well connected to the surrounding waters harboured only two single-stranded conformation polymorphism profiles (i.e. zooxanthella communities). It consisted only of Symbiodinium D in all 13 nonporitid species and two Porites species investigated, with the remaining five Porites harbouring C*. Despite the high temperature in this lake (> 0.5 degrees above ambient), this reef did not suffer coral mortality during the (1998) bleaching event, however, no bleaching-sensitive coral families and genera occur in the coral community. This setting contrasts strongly with two other reefs with generally lower temperatures, in which 10 and 12 zooxanthella communities with moderate to low proportions of clade D zooxanthellae were found. The data indicate that whole coral assemblages, when growing in elevated seawater temperatures and at reduced irradiance, can be composed of colonies associated with the more thermo-tolerant clade D zooxanthellae. Future increases in seawater temperature might, therefore, result in an increasing prevalence of Symbiodinium phylotype D in scleractinian corals, possibly associated with a loss of diversity in both zooxanthellae and corals.     

Distribution of dissolved organic carbon and nitrogen in a coral reef

Dissolved organic matter (DOM) concentrations in a fringing coral reef were measured for both carbon and nitrogen with the analytical technique of high-temperature catalytic oxidation. Because of high precision of the analytical system, not only the concentrations of dissolved organic carbon and nitrogen (DOC and DON, respectively) but the C:N ratio was also determined from the distribution of DOC and DON concentrations. The observed concentrations of DOC and DON ranged 57–76 and 3.8–5.6 μmol l⁻¹, respectively. The C:N ratios of the DOM that was produced on the reef flat were very similar between seagrass- and coral-dominated areas; the C:N ratio was 10 on average. The C:N ratio of DOM was significantly higher than that of particulate organic matter (POM) that was produced on the reef flat. Production rates of DOC were measured on the reef flat during stagnant periods and accounted for 3–7% of the net primary production, depending on the sampling site. The production rate of DON was estimated to be 10–30% of the net uptake of dissolved inorganic N in the reef community. Considering that the DOM and POM concentrations were not correlated with each other, a major source of the reef-derived DOM may be the benthic community and not POM such as phytoplankton. It was concluded that a widely distributed benthic community in the coral reef released C-rich DOM to the overlying seawater, conserving N in the community.     

Extreme climatic events reduce ocean productivity and larval supply in a tropical reef ecosystem

Increasing ocean temperatures due to global warming are predicted to have negative effects on coral reef fishes. El Niño events are associated with elevated water temperatures at large spatial (1000s of km) and temporal (annual) scales, providing environmental conditions that enable temperature effects on reef fishes to be tested directly. We compared remote sensing data of sea surface temperature (SST) anomalies, surface current flow and chlorophyll‐a (Chl‐a) concentration with monthly patterns in larval supply of coral reef fishes in nearshore waters around Rangiroa Atoll (French Polynesia) from January 1996 to March 2000. This time included an intense El Niño (April 1997–May 1998) event between two periods of La Niña (January–March 1996 and August 1998–March 2000) conditions. There was a strong relationship between the timing of the El Niño event, current flow, ocean productivity (as measured by Chl‐a) and larval supply. In the warm conditions of the event, there was an increase in the SST anomaly index up to 3.5 °C above mean values and a decrease in the strength of the westward surface current toward the reef. These conditions coincided with low concentrations of Chl‐a (mean: 0.06 mg m^?3, SE ± 0.004) and a 51% decline in larval supply from mean values. Conversely, during strong La Niña conditions when SST anomalies were almost 2 °C below mean values and there was a strong westward surface current, Chl‐a concentration was 150% greater than mean values and larval supply was 249% greater. A lag in larval supply suggested that productivity maybe affecting both the production of larvae by adults and larval survival. Our results suggest that warming temperatures in the world's oceans will have negative effects on the reproduction of reef fishes and survival of their larvae within the plankton, ultimately impacting on the replenishment of benthic populations.     

Growth rhythms recorded in stable isotopes and density bands in the reef coral Porites lobata (Cebu,Philippines)

Growth rhythms in the reef coral Porites lobata are revealed by X-radiography and stable carbon and oxygen isotopic analysis. High density increments were deposited during warm temperatures in summer and low density increments during winter. The seasonal temperature variations are reflected in the oxygen isotope ratios. The coral carbonate shows a constant depletion in ¹⁸O of –2.7%₀ relative to calcite in equilibrium with the ambient seawater. The mean annual growth rate of the specimen studied was 1.3±0.3 cm/year.     

Detriments to post-bleaching recovery of corals

Predicting the response of coral reefs to large-scale mortality induced by climate change will depend greatly on the factors that influence recovery after bleaching events. We experimentally transplanted hard corals from a shallow reef with highly variable seawater temperature (23–36°C) to three unfished marine parks and three fished reefs with variable coral predator abundance and benthic cover. The transplanted corals were fragmented colonies collected from a reef that was relatively undisturbed by the 1997–1998 warm-water temperature anomaly, one of the most extreme thermal events of the past century, and it was assumed that they would represent corals likely to succeed in the future temperature environment. We examined the effects of four taxa, two fragment sizes, an acclimation period, benthic cover components, predators and tourists on the survival of the coral fragments. We found the lowest survival of transplants occurred in the unfished marine parks and this could be attributed to predation and not tourist damage. The density of small coral recruits approximately 6 months after the spawning season was generally moderate (~40–60/m²), and not different on fished and unfished reefs. Coral recovery between 1998 and 2002 was variable (0–25%), low (mean of 6.5%), and not different between fished and unfished reefs. There was high variability in coral mortality among the three unfished areas despite low variation in estimates of predator biomass, with the highest predation occurring in the Malindi MNP, a site with high coralline algal cover. Stepwise multiple regression analysis with 14 variables of coral predators and substratum showed that coralline algae was positively, and turf algae negatively associated with mortality of the transplants, with all other variables being statistically insignificant. This suggests that alternate food resources and predator choices are more important than predator biomass in determining coral survival. Nonetheless, large predatory fish in areas dominated by coralline algae may considerably retard recovery of eurythermal corals. This will not necessarily retard total hard coral recovery, as other more predator-tolerant taxa can recover. Based on the results, global climate change will not necessarily favor eurythermal over stenothermal coral taxa in remote or unfished reefs, where predation is a major cause of coral mortality.     

Marine pollution and coral reefs

Coral reefs are exposed to many anthropogenic stresses increasing in impact and range, both on local and regional scales. The main ones discussed here are nutrient enrichment, sewage disposal, sedimentation, oil-related pollution, metals and thermal pollution. The stress comprising the main topic of this article, eutrophication, is examined from the point of view of its physiological and ecological mechanisms of action, on a number of levels. Nutrient enrichment can introduce an imbalance in the exchange of nutrients between the zooxanthellae and the host coral, it reduces light penetration to the reef due to nutrient- stimulated phytoplankton growth, and, most harmful of all, may bring about proliferation of seaweeds. The latter rapidly outgrow, smother and eventually replace, the slow-growing coral reef, adapted to cope with the low nutrient concentrations typical in tropical seas.
Eutrophication seldom takes place by itself. Sewage disposal invariably results in nutrient enrichment, but it also enriches the water with organic matter which stimulates proliferation of oxygen-consuming microbes. These may kill corals and other reef organisms, either directly by anoxia, or by related hydrogen sulfide production. Increased sediment deposition is in many cases associated with other human activities leading to eutrophication, such as deforestation and topsoil erosion.
Realistically achievable goals to ensure conservation, and in some instances, rehabilitation of coral reefs are listed.     

The Seasonal Dynamics and Composition of Photosynthetic Picoplankton Communities in Temperate Lakes in Ontario,Canada

The seasonal abundance and composition of photosynthetic picoplankton (0.2-2 μm) was compared among five oligotrophic to mesotrophic lakes in Ontario. Epilimnetic picocyanobacteria abundance followed a similar pattern in all lakes; maximum abundance (2-4 × 10⁵ cells · ml⁻¹) occurred in late summer following a period of rapid, often exponential increase after epilimnetic temperatures reached 20 °C. In half of the lakes picocyanobacteria abundance was significantly correlated with temperature, while in other lakes the presence of a small spring peak resulted in a poor correlation with temperature. In all lakes there was a significant correlation between epilimnetic abundance and day of the year. Correlations with water chemistry parameters (soluble reactive phosphorus, total phosphorus, particulate C: P and C: N) were generally weaker or insignificant. However, in the three lakes with the highest spring nitrate concentrations, a significant negative correlation with nitrate was observed. During summer stratification, picocyanobacteria abundance reached a maximum within the metalimnion and at or above the euphotic zone (1% of incident light) in all lakes. These peaks were not related to nutrient gradients. The average total phytoplankton biomass ranged from 0.5 g m⁻³ (wet weight) in the most oligotrophic lake to 1.4 g m⁻³ for the most mesotrophic with picoplankton biomass ranging from 0.01 g m⁻³ to 0.3 g m⁻³. Picocyanobacteria biomass comprised 1 to 9 % of total phytoplankton biomass in late summer, but in one year for one lake represented a maximum of 56%. Other photosynthetic picoplankton (unidentified eukaryotes, Chlorella spp. Nannochloris spp.), although less abundant (10³ cells · ml⁻¹) than picocyanobacteria, represented biomass equal or greater than that of the picocyanobacteria in spring and early summer. On average, half of the photosynthetic picoplankton biomass was eukaryotic in the more coloured lakes, while in the clear lakes less than 20% was eukaryotic. Among the lakes there was a significant positive correlation between the average light extinction coefficient and the proportion of eukaryotic biomass of the picoplankton. In mesotrophic Jack's Lake, the contribution of picoplankton to the maximum photosynthetic rate ranged from 10 to 47% with the highest values in the spring (47%) and late summer (33%), as a result of eukaryotic picoplankton and picocyanobacteria respectively. Picocyanobacteria cell specific growth rates were high during July (0.6-0.8 day⁻¹) and losses were close to 80% of the growth rate. Thus, despite low biomass, photosynthetic picoplankton populations appeared to turn over rapidly and potentially contributed significantly to planktonic food webs in early spring and late summer.     

Microbial signatures of protected and impacted Northern Caribbean reefs: changes from Cuba to the Florida Keys

There are a few baseline reef-systems available for understanding the microbiology of healthy coral reefs and their surrounding seawater. Here, we examined the seawater microbial ecology of 25 Northern Caribbean reefs varying in human impact and protection in Cuba and the Florida Keys, USA, by measuring nutrient concentrations, microbial abundances, and respiration rates as well as sequencing bacterial and archaeal amplicons and community functional genes. Overall, seawater microbial composition and biogeochemistry were influenced by reef location and hydrogeography. Seawater from the highly protected ‘crown jewel’ offshore reefs in Jardines de la Reina, Cuba had low concentrations of nutrients and organic carbon, abundant Prochlorococcus, and high microbial community alpha diversity. Seawater from the less protected system of Los Canarreos, Cuba had elevated microbial community beta-diversity whereas waters from the most impacted nearshore reefs in the Florida Keys contained high organic carbon and nitrogen concentrations and potential microbial functions characteristic of microbialized reefs. Each reef system had distinct microbial signatures and within this context, we propose that the protection and offshore nature of Jardines de la Reina may preserve the oligotrophic paradigm and the metabolic dependence of the community on primary production by picocyanobacteria.     

Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Global- and local-scale anthropogenic stressors have been the main drivers of coral reef decline, causing shifts in coral reef community composition and ecosystem functioning. Excess nutrient enrichment can make corals more vulnerable to ocean warming by suppressing calcification and reducing photosynthetic performance. However, in some environments, corals can exhibit higher growth rates and thermal performance in response to nutrient enrichment. In this study, we measured how chronic nutrient enrichment at low concentrations affected coral physiology, including endosymbiont and coral host response variables, and holobiont metabolic responses of Pocillopora spp. colonies in Mo'orea, French Polynesia. We experimentally enriched corals with dissolved inorganic nitrogen and phosphate for 15 months on an oligotrophic fore reef in Mo'orea. We first characterized symbiont and coral physiological traits due to enrichment and then used thermal performance curves to quantify the relationship between metabolic rates and temperature for experimentally enriched and control coral colonies. We found that endosymbiont densities and total tissue biomass were 54% and 22% higher in nutrient-enriched corals, respectively, relative to controls. Algal endosymbiont nitrogen content cell⁻¹ was 44% lower in enriched corals relative to the control colonies. In addition, thermal performance metrics indicated that the maximal rate of performance for gross photosynthesis was 29% higher and the rate of oxygen evolution at a reference temperature (26.8 °C) for gross photosynthesis was 33% higher in enriched colonies compared to the control colonies. These differences were not attributed to symbiont community composition between corals in different treatments, as C42, a symbiont type in the Cladocopium genus, was the dominant endosymbiont type found in all corals. Together, our results show that in an oligotrophic fore reef environment, nutrient enrichment can cause changes in coral endosymbiont physiology that increase the performance of the coral holobiont.

     

珠江口及毗邻海域营养盐对浮游植物生长的影响

基于2006年7月(夏季),10月(秋季)和2007年3月(春季)的现场调查数据,对珠江口及毗邻海域中的营养盐和叶绿素a等环境生态因子的时空分布特性进行了对比分析,研究了氮磷比与叶绿素a含量和种群多样性之间的联系,探讨了该海域营养盐对于浮游植物生长的影响。结果表明:(1)研究海域营养盐表现出较强的季节和空间差异性,总氮(TN)和总磷(TP)浓度均值春季(1.545 mg/L、0.056 mg/L)和夏季(1.570 mg/L、0.058 mg/L)均大于秋季(1.442 mg/L、0.034 mg/L),且春夏季浓度空间差异更明显。(2)调查期间海域营养盐含量超标现象突出,夏季尤为明显。无机氮(DIN)总体均值0.99 mg/L,超四类海水标准限值1倍,活性磷酸盐(PO4-P)总体均值0.021 mg/L,DIN∶PO4-P平均值为130;叶绿素a浓度与营养盐、p H、温度有较显著的相关性。(3)叶绿素a浓度较高的站位,具有较高的DIN∶PO4-P值,但浮游植物多样性指数偏低,优势种明显,主要为中肋骨条藻。氮磷比的改变会影响不同生长特性的浮游植物间的竞争和种群结构的改变;今后海洋污染治理中,在控制氮、磷污染时要注意氮磷比的改变可能造成的浮游生态影响。     

Mixing, stratification and the fate of primary production in an oligotrophic multibasin lake system (Quebec, Canada)

Impacts of mixing and stratification on the fate of primaryproduction were studied in an oligotrophic lake by comparingthe size-distributions of phytoplankton standing stock and productionin two basins, only one of which experiences seasonal thermalstratification. In both basins, the phytoplankton was dominatedby small cells (pico- and nanoplankton). The contribution ofpicoplankton to both biomass and production remained relativelyconstant throughout the season in both basins. Seasonal variationsin the size structure of phytoplankton communities do not agreewith the paradigm of dominance by small cells during summerstratification and dominance of larger cells during spring andfall mixing events. Nutrient control of productivity throughmixing and stratification is unlikely to affect the structureof phytoplankton communities when nutrients (allochthonous)derived from the catchment basin or sediments are in short supply.In such environments, nutrients (autochthonous) are largelyderived in the lake through heterotrophic food web processessuch as grazing, excretion and decomposition. Maximum ratesof production and losses in July and August in both basins areconsistent with increased regeneration and may represent a responseof larger-sized cells to higher nutrient availability resultingfrom enhanced grazing on picoplankton. The high correlationbetween the rates of loss and of potential growth for the phytoplanktoncommunity during all sampling periods, and the relative constancyof the picoplankton biomass, leads us to propose a long-term,steady-state equilibrium in the phytoplankton community underthe control of grazing by herbivores and/or other loss processes.     

Seasonal growth dynamics of Laurencia papillosa and Gracilaria coronopifolia from a highly eutrophic reef in southern Taiwan: temperature limitation and nutrient availability

Both field and laboratory studies were used to investigate the effects of temperature limitation and nutrient availability on seasonal growth dynamics of Laurencia papillosa and Gracilaria coronopifolia from a nearshore coral reef in the southern tip of Taiwan during 1999-2000. L. papillosa was a summer blooming alga abundant in August-November and G. coronopifolia was abundant year round except April-May. L. papillosa blooms in the summer were attributed to its preference for high temperatures and highly sensitivity to low temperatures. A wider temperature range and a significant stimulation of growth by high N inputs can explain the appearance of G. coronopifolia year round and also its maximum growth in November-March. Levels of dissolved inorganic nitrogen (DIN) and soluble reactive phosphorus (SRP) in water column were extremely high, but the growth of these two rhodophytes still suffered nutrient limitation that the type and severity of nutrient limitation were variable over time and also between two species. The growth of L. papillosa was limited by P in the early growth stage (August-September) as indicated by decreased tissue P contents, increased C/P and N/P molar ratios and increased alkaline phosphatase activity (APA) and in the later growth stage, it was subjected to N-limitation, evidenced by decreased tissue N contents and C/P and N/P molar ratios and increased tissue P contents. The growth of G. coronopifolia was also P-limited as indicated by increased tissue N contents and concomitantly decreased tissue P contents, while marked drops in tissue P contents below the subsistence level in mid September and December 1999 reveal severe P limitation, which was supported by increased alkaline phosphatase activity. Higher critical nutrient contents and nutrient thresholds for maximum growth of G. coronopifolia suggest that G. coronopifolia faced more frequent nutrient limitation compared to L. papillosa. In conclusion, the results from these laboratory and field studies provide evidence that the seasonal abundance of L. papillosa and G. coronopifolia from southern Taiwan was determined by seasonal variations in seawater temperatures and nutrient concentrations as well as different physiological growth strategies. Seawater temperature and nutrient availability were important determinants of seasonal abundance of L. papillosa while the seasonal abundance of G. coronopifolia was influenced by nutrient availability.     

Nutrient limitation of phytoplankton growth in Waquoit Bay, Massachusetts, USA: a nutrient enrichment study

We conducted nutrient enrichment experiments and field sampling to address three questions: (1) is there nutrient limitation of phytoplankton accumulation within an estuary whose waters are exposed to relatively high nitrogen loading rates, (2) where in the salinity gradient from fresh to seawater (0 to 32‰) is there a shift from phosphorus to nitrogen limitation of phytoplankton accumulation, and (3) is there a seasonal shift in limiting function of phosphorus and nitrogen anywhere in the estuarine gradient. Nitrogen and phosphorus enrichment experiments in the Childs River, an estuary of Waquoit Bay, Massachusetts, USA, showed that the accumulation of phytoplankton biomass in brackish and saline water was limited by supply of nitrate during warm months. The effects of enrichment were less evident in fresh water, with short-lived responses to phosphate enrichment. There was no specific point along the salinity gradient where there was a shift from phosphorus- to nitrogen-limited phytoplankton accumulation; rather, the relative importance of nitrogen and phosphorus changed along the salinity gradient in the estuary and with season of the year. There was no response to nutrient additions during the colder months, suggesting that some seasonally-varying factor, such as light, temperature or a physiological mechanism, restricted phytoplankton accumulation during months other than May-Aug. There was only slight evidence of a seasonal shift between nitrogen- and phosphorus-limitation of chlorophyll accumulation. Phytoplankton populations in nutrient-rich estuaries with short flushing times grow fast, but at the same time the cells may be advected out of the estuaries while still rapidly dividing, thereby providing an important subsidy to production in nearby deeper waters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nutrient limitation of phytoplankton in solar salt ponds in Shark Bay,Western Australia

The aim of this research was to examine nutrient limitation of phytoplankton in solar salt ponds of varying salinity at Useless Inlet in Western Australia. These ponds use solar energy to evaporate seawater for the purpose of commercial salt production. A combination of techniques involving water column nutrient ratios, comparisons of nutrient concentrations to concentration of magnesium ions and bioassays were used in the investigation. Comparisons of changes in dissolved inorganic nitrogen to phosphorus ratios and concentrations of dissolved inorganic nutrients against changes in concentrations of the conservative cation Mg²⁺ indicated that phytoplankton biomass was potentially nitrogen limited along the entire pond salinity gradient. Nutrient addition bioassays indicated that in low salinity ponds, phytoplankton was nitrogen limited but in high salinity ponds, phosphorus limited. This may be due to isolation of phytoplankton in bioassay bottles from in situ conditions as well as to changes in phytoplankton species composition between ponds, and the variable availability of inorganic and organic nutrient sources. The differences in limiting nutrient between methods indicate that phytoplankton cells may be proximally limited by nutrients that are not theoretically limiting at the pond scale. Dissolved organic nutrients constituted a large proportion of total nutrients, with concentrations increasing through the pond sequence of increasing salinity. From the change in nutrient concentrations in bioassay bottles, sufficient dissolved organic nitrogen may be available for phytoplankton uptake in low salinity ponds, potentially alleviating the dissolved inorganic nitrogen limitation of phytoplankton biomass. Guest Editors: J. John & B. Timms Salt Lake Research: Biodiversity and Conservation—Selected Papers from the 9th Conference of the International Society for Salt Lake Research     

Nitrogen limitation and particulate elemental ratios of seston in hypersaline Mono Lake, California, U.S.A.

Particulate elemental ratios (C:N, N:P and C:Chl a) of seston in hypersaline (70–90 g kg^–1) Mono Lake, California, were examined over an 11-year period (1990–2000) which included the onset and persistence of a 5-year period of persistent chemical stratification. Following the onset of meromixis in mid-1995, phytoplankton and dissolved inorganic nitrogen were substantially reduced with the absence of a winter period of holomixis. C:N, N:P and C:Chl a ratios ranged from 5 to 18 mol mol^–1, 2 to 19 mol mol^–1 and 25 to 150 g g^–1, respectively, and had regular seasonal patterns. Deviations from those expected of nutrient-replete phytoplankton indicated strong nutrient limitation in the summer and roughly balanced growth during the winter prior to the onset of meromixis. Following the onset of meromixis, winter ratios were also indicative of modest nutrient limitation. A 3-year trend in C:N and N:P ratios toward more balanced growth beginning in 1998 suggest the impacts of meromixis weakened due to increased upward fluxes of ammonium associated with weakening stratification and entrainment of ammonium-rich monimolimnetic water. A series of nutrient enrichment experiments with natural assemblages of Mono Lake phytoplankton conducted during the onset of a previous episode of meromixis (1982–1986) confirm the nitrogen will limit phytoplankton before phosphorus or other micronutrients. Particulate ratios of a summer natural assemblage of phytoplankton collected under nitrogen-depleted conditions measured initially, following enrichment, and then after return to a nitrogen-depleted condition followed those expected based on Redfield ratios and laboratory studies.