Is inorganic nutrient enrichment a driving force for the formation of red tides?: A case study of the dinoflagellate Scrippsiella trochoidea in an embayment

Red tides (high biomass phytoplankton blooms) have frequently occurred in Hong Kong waters, but most red tides occurred in waters which are not very eutrophic. For example, Port Shelter, a semi-enclosed bay in the northeast of Hong Kong, is one of hot spots for red tides. Concentrations of ambient inorganic nutrients (e.g. N, P), are not high enough to form the high biomass of chlorophyll a (chl a) in a red tide when chl a is converted to its particulate organic nutrient (N) (which should equal the inorganic nutrient, N). When a red tide of the dinoflagellate Scrippsiella trochoidea occurred in the bay, we found that the red tide patch along the shore had a high cell density of 15,000 cells ml⁻¹, and high chl a (56 μg l⁻¹), and pH reached 8.6 at the surface (8.2 at the bottom), indicating active photosynthesis in situ. Ambient inorganic nutrients (NO₃, PO₄, SiO₄, and NH₄) were all low in the waters and deep waters surrounding the red tide patch, suggesting that the nutrients were not high enough to support the high chl a >50 μg l⁻¹ in the red tide. Nutrient addition experiments showed that the addition of all of the inorganic nutrients to a non-red-tide water sample containing low concentrations of Scrippsiella trochoidea did not produce cell density of Scrippsiella trochoidea as high as in the red tide patch, suggesting that nutrients were not an initializing factor for this red tide. During the incubation of the red tide water sample without any nutrient addition, the phytoplankton biomass decreased gradually over 9 days. However, with a N addition, the phytoplankton biomass increased steadily until day 7, which suggested that nitrogen addition was able to sustain the high biomass of the red tide for a week with and without nutrients. In contrast, the red tide in the bay disappeared on the sampling day when the wind direction changed. These results indicated that initiation, maintenance and disappearance of the dinoflagellate Scrippsiella trochoidea red tide in the bay were not directly driven by changes in nutrients. Therefore, how nutrients are linked to the formation of red tides in coastal waters need to be further examined, particularly in relation to dissolved organic nutrients.     

我国近海赤潮多发区域及其生态学特征

根据有关的研究资料,分析我国近海赤潮多发区分布、主要赤潮生物种类、环境状况及赤潮发生生态学特点.结果表明,我国辽东湾、渤海湾、莱州湾、大连湾、长江口、舟山海域、杭州湾、厦门湾、柘林湾、大鹏湾、珠江口等为赤潮多发区.引发赤潮的因素较多,它与气象、水动力、营养盐及生物环境的变化密切相关,人类活动(如海水养殖、陆源污水排放等)影响加剧近海富营养化是引发赤潮的重要因素;但富营养化并非发生赤潮的唯一条件,低营养海区也可能发生赤潮.目前对诱发赤潮的关键因子及赤潮发生机理,应加强定点长期监测和开展深入研究.     

Temporal Occurrence and Spatial Distribution of Red Tide Events in China's Coastal Waters

Between the years 1933–2001, 460 red tide events were found in China's coastal water. The scope of red tide occurrence has extended over all the coastal provinces of China, and the three major areas with high red tide frequency are the Bohai sea, the sea area near the Shengsi Archipelago and Huaniaoshan Island on the outer side of the Changjiang River estuary and the Hangzhou Bay, and the coastal waters on the east side of the Zhujiang River estuary from Hong Kong to East Guangdong. In the past 20 years, the frequency of red tides has been tending upwards. The years of 1988–1990 and 1998–2001 saw the most serious red tides along China's coastal waters, with the latter period as the peak. The average area of a single red tide, i.e., the scale of red tides, is expanding every year, and in 2001 rose to about 500 km². Every year, China's red tides occur from south to north, with those in the South China Sea occurring from March to May, those in the East China Sea from April to August and those in the Bohai and Huanghai Seas from May to September.     

Mangrove inundation and nutrient dynamics from a GIS perspective

A digital elevation model describing topography, tide elevation and inundation degree and frequency of a mangrove forest in North Brazil is discussed in relation to existing phosphate and physicochemical data in waters of an adjacent tidal creek. Due to smooth topography, an increase of 20 cm in tidal height above average neap tides increases flooded area from about 50 to 80%. Analysis of the relationship between microtopography, tidal height and flooding rate showed that in the upper 60 cm of the mangrove forest, increases of 20 cm in topographical height resulted in a doubling of the inundation frequency. This can be particularly relevant for the analysis of nutrient mobilization and vegetation structure of infrequently inundated wetlands. Throughout the year, low-tide phosphate in creek water was inversely proportional to the maximum area flooded during high tide, this correlation being higher during the dry season. Similarly, the inverse relationship between flooded areas and low-tide/high-tide pH ratios was highly significant during the dry season and the beginning of the rainy season. Although the high correlations obtained are based on data pairs obtained at high and low tide, it has to clarified whether the association between inundation degree and creek water pH is relevant for the stability of P compounds in sediment on the short scale of a tidal cycle.     

Export of Organic Matter from a Coastal Freshwater Wetland to Lake Erie: an Extension of the Outwelling Hypothesis

I tested the hypothesis that seiches in large lakes play a role similar to tides in marine system by exporting detrital carbon from coastal wetlands to adjacent open waters. The study was conducted at a wetland site located along the shore of the Lake Erie. Water samples were collected at the outlet of the marsh, during inflow and outflow events, over a 19-month period. Water isotopes were also measured in the lake and in the marsh to establish the magnitude of the mixing between the two water masses. On average, the concentrations of the outflow water samples was enriched by 7.3 mg DOC l⁻¹ and 3.4 mg POC l⁻¹compared to the inflow water samples, while no difference observed in inorganic carbon fluxes. Organic carbon was exported during fall, winter and early spring which coincide with period of organic matter decomposition. Such a concept of outwelling is not new for marine ecosystem, but is demonstrated in this study for the first time in a large lake setting influenced by seiche events. Ultimately to understand the role that these fluxes may play in maintaining the integrity of the wetland-lake system, it will be necessary to investigate whether the detrital material exported from coastal freshwater wetlands is incorporated into the open lake foodweb.     

Are changes in N:P ratios in coastal waters the key to increased red tide blooms?

There is mounting evidence of a global increase in nutrient levels of coastal waters through riverine and sewage inputs, and in both the numbers and frequency(as well as the species composition) of red tides. However, it is still not possible to conclude the extent to which the increase in red tides in coastal waters can be attributed to the increase in nutrient levels, since so many other factors are involved. Undoubtedly, a relationship exists between red tides and the N and P load of coastal waters, and many nutrient enrichment experiments have shown that marine phytoplankton blooms are often nutrient limited. What is now becoming clear, however, is that although in classical Liebigian terms minimum amounts can be limiting, nutrient ratios (such as N:P and Si:P) are far more important regulators. This paper reviews evidence collected by the authors from Tolo Harbour, Hong Kong together with data collected in Japanese and North European coastal waters by various authors, which indicates that both long term and relatively short term changes in the N:P ratio are accompanied by increased blooms of non-siliceous phytoplankton groups and, furthermore,that the growth of most red tide causative organisms in Hong Kong coastal water is optimized at a low N:P(atomic) ratio of between 6 and 15. This revised version was published online in August 2006 with corrections to the Cover Date.     

The distribution and abundance of planktonic penaeid larvae in Tudor creek,Mombasa, Kenya

The temporal distribution and abundance of penaeid prawn larvae in surface waters of Tudor creek were studied. Only late mysis and early post mysis stages of development were encountered in the creek waters with a descending gradient from the mouth station (station 1) to the innermost station 5. Larval encounter incidence (% incidence) was highest in plankton tows taken during night spring tides (59%) followed by night neap tides (45.4%), day spring tides (34.9%) and day neap tides (21.4%). This encounter incidence was more skewed towards the seaward stations (1 and 2) during the neap tides than during spring tides. Likewise, larval density (mean catch/standard 5 min. tow) decreased less steeply up the creek during spring tides than it did during neap tides. Higher catch rates were in general obtained in night samples than in day samples at each station and in each neap/spring phase. The diel cycle had a greater effect on both incidence and abundance of larvae than did the tidal cycle.Intensified larval incursion into the creek was observed between March and June.     

Larval life history strategies of sub-tropical southern African estuarine brachyuran crabs and implications for tidal inlet management

Soon after hatching the larvae of many estuarine crabs migrate from estuaries to adjacent coastal waters soon, where larval development is completed before the post larval stages recruit to estuaries to settle. This study investigated the larval flux of several brachyuran crabs resident within a subtropical estuary on the east coast of South Africa. Plankton sampleswere collected over two intensive sampling periods during the spring of1997 (19 days) and the summer of 1998 (26 days). Larvae were releasedmaximally when high tide within the estuary was crepuscular, whilerecruitment occurred during nocturnal flood tides, with peak abundanceson the maximum amplitude nocturnal flood tides. The data are discussedin relation to freshwater inflow problems facing South African estuaries andthe active management of the tidal inlets of affected systems.     

The climbing behaviour of Cerithidea anticipata (Mollusca: Gastropoda): The roles of physical and biological factors

Abstract Many molluscs in tidal wetlands climb trees as the tide rises, a behaviour usually assumed to be a means of avoiding subtidal predators. Some species are more active during neap tides, when the access of subtidal predators to the forest is limited, but rest on trees during spring tides. Cerithidea anticipata, which inhabits the mangrove forests around Darwin Harbour (Northern Territory, Australia), displayed almost exactly the opposite pattern. This species climbed higher, and was less active, during neap tides that did not flood the forest than during spring tides. In experiments with tethered snails, individuals prevented from climbing died during neap tides, apparently from physiological stress. Further, individuals resting on trees around clearings, usually selected shaded sites. These results suggest that the major reason C. anticipata climbed was to avoid physiological stress during neap tides, not subtidal predators during spring tides. There was some evidence of predation under the canopy, but the rate was relatively low and the species responsible appeared to be resident in the forest.     

Meiofaunal emergence from intertidal sediment measured in the field: significant contribution to nocturnal planktonic biomass in shallow waters

Field studies on the occurrence of meiobenthos in the water column above intertidal sandflats have been performed near the Island of Sylt in the northern Wadden Sea. Swimming meiobenthos was strongly dominated by harpacticoid copepods. Many of them have a semiplanktonic life-style. They rest in superficial sediment layers at low tide and swin in the water column at high tide. Swimming activity correlated negatively with light. The abundance in the water column was one order of magnitude higher during the night. Strong currents caused by storm tides significantly decreased meiobenthic abundance in the water column. Light and flow being constant, no significant changes of meiobenthic abundance per unit area occurred over a tidal cycle. Since holoplankton and meroplankton abundances correlated positively with the height of the water column, semiplanktonic meiobenthos may dominate the mesozooplankton in shallow waters. On an average, emergence of meiobenthos increased the mesozooplanktonic biomass by about 2% during diurnal high tides over the entire tidal cycle, and by about 50% during nocturnal high tides. Because of seasonal cycles of the dominant harpacticoids, this high contribution to planktonic biomass may be a summer phenomenon.     

Contrôle de la dynamique à court terme du microphytobenthos intertidal par le cycle exondation-submersion

We test herein the hypothesis that the succession of low tides — during which biomass increases — and high tides — during which biomass decreases — controls the dynamic behaviour of the microphytobenthic compartment. Results indeed show that during a complete lunar cycle (14 d) biomass exhibits a series of oscillations due to biomass increases during low tides and decreases during high tides. Moreover, the datasets show that the production level is in the range of previously recorded values and they indicate that the major part of the photosynthetically produced biomass during low tide can be exported into the water column during high tide. This clearly suggests that microphytobenthos is the major autochtonous source of organic matter production in the littoral zone and that it directly supplies the associated pelagic trophic web.     

AUTECOLOGICAL STUDIES ON THE MARINE DIATOM RHIZOSOLENIA FRAGILISSIMA BERGON. II. ENRICHMENT AND DARK VIABILITY EXPERIMENTS1

Nutrient enrichment experiments were conducted with a Narragansett Bay clone of Rhizosolenia fragilissima to examine the potential influence of nutrients in regulating its seasonal cycle in this embayment, and their contribution to its coastal tendencies. Growth rates were measured at 18 C and 1000 ft-c continuous illumination in surface waters enriched in 15 different ways. Narragansett Bay was sampled in March, May, September, and December. Six stations on a transect from this Bay to the Sargasso Sea were sampled during late summer. The data and our previous autecological observations are consistent with the idea that the annual cycle of this species in Narragansett Bay is associated with temperature, chemical “water quality,” and unknown factors. Its absence during late fall and winter may reflect low temperatures and a trace metal (Co, Mo) inadequacy. In early spring, low temperature appears to be the limiting factor, whereas in late spring and early summer trace metals again appear to prevent active growth. A late summer-early autumn bloom occurs periodically during optimal temperature conditions; this can terminate independently of grazing pressure, and in spite of seemingly adequate light-temperature-salinity and phosphate conditions.     

Migratory behaviour of post-smolt Atlantic salmon during initial stages of seaward migration

The movements of 24 hatchery-reared Atlantic salmon Salmo salar smolts, with miniature acoustic transmitters (pingers) implanted surgically, were determined after release in the coastal waters of Passamaquoddy Bay (mean tide range 6 m), New Brunswick, Canada, to describe the first stages of seaward migration. Automated pinger detection at fixed sites, and pinger location and tracking by boat were used. Post-smolts left the release area rapidly, and the majority moved to open waters of the bay within several tidal cycles. Initially, post-smolts moved with a seaward orientation on ebb tides and held positions on flood tides. Their movements into open waters were diurnal, and the timing corresponded with the state of the tide during which they moved through a narrow channel. Post-smolts moved preferentially through this passageway with the aid of the tidal stream. Successful movements out through the channel occurred during ebb tides and any movements back in were during flood tides. Ground speed of fish moving through the channel was 4·2 body lengths s⁻¹ and faster than the tidal stream velocities in the channel. The relative velocity of fish swimming through the channel was 2 body lengths s⁻¹. Post-smolt movement was indicative of active, directed swimming with a reliance on ebb-tide transport for migration through a coastal area with strong tidal currents. Some post-smolts moved seaward directly with no apparent period of acclimation for the transfer to the marine environment, whereas others delayed their departure. These differences in behaviour were probably related to asynchrony in smolting when fish were released.     

Field measurements and modeling of groundwater flow and biogeochemistry at Moses Hammock, a backbarrier island on the Georgia coast

A combination of field measurements, laboratory experiments and model simulations were used to characterize the groundwater biogeochemical dynamics along a shallow monitoring well transect on a coastal hammock. A switch in the redox status of the dissolved inorganic nitrogen (DIN) pool in the well at the upland/saltmarsh interface occurred over the spring-neap tidal transition: the DIN pool was dominated by nitrate during spring tide and by ammonium during neap tide. A density-dependent reaction-transport model was used to investigate the relative importance of individual processes to the observed N redox-switch. The observed N redox-switch was evaluated with regard to the roles of nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), ammonium adsorption, and variations in inflowing water geochemistry between spring and neap tides. Transport was driven by measured pressure heads and process parameterizations were derived from field observations, targeted laboratory experiments, and the literature. Modeling results suggest that the variation in inflow water chemistry was the dominant driver of DIN dynamics and highlight the importance of spring-neap tide variations in the high marsh, which influences groundwater biogeochemistry at the marsh-upland transition.     

Tidal influence upon appendicularian abundance in North Inlet estuary, South Carolina

Tidal influences on appendicularian densities were observedat North Inlet, South Carolina, by sampling along a transectwhich ran from a tidal creek to a station 5 km offshore. Oikopleuradioica was the dominant species in North Inlet, while Oikopleuralongicauda and Appendicularia sicula contributed marginallyto appendicularian numbers during midsummer and fall. A strongtide-dependent density pattern was clear for inshore waters.Low-tide densities of all three species showed a dramatic increasein an offshore direction. At high tide, densities were similarbetween all stations for O. dioica, while O. longicauda andA. sicula showed a less pronounced density gradient than atlow tide. Population densities within the inlet were greateron spring tides than neap tides and tidal influences were generallyconsistent between seasons. Appendicularians enter the estuaryin densities as high as 20 072 animals m^–3, indicatingthat tidal currents may be an important mechanism for exchangeof appendicularian biomass between coastal and estuarine waters. ¹Present address: Allan Hancock Foundation, University of SouthernCalifornia, Los Angeles, CA 90007, USA.     

Carbon flux between an estuary and the ocean: a case for outwelling

The classical outwelling hypothesis states that small coastal embayments (e.g. estuaries, wetlands) export their excess production to inshore marine waters. In line with this notion, the present study tested whether the Swartkops estuary acts as source or sink for carbon. To this end, concentrations of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and particulate organic carbon (POC) were determined hourly during the first monthly spring and neap tides over one year in the tidal waters entering and leaving the estuary. Each sampling session spanned a full tidal cycle, yielding a total of 936 concentration estimates. Carbon fluxes were calculated by integrating concentrations with water flow rates derived from a hydrodynamic model calibrated for each sampling datum. Over the year, exports to marine waters markedly exceeded imports to the estuary for all carbon species: on the basis of total spring tidal drainage area, 1083 g m^–2 of DIC, 103 g m^–2 of DOC, and 123 g m^–2 of POC left the estuary annually. Total carbon export from the estuary to the ocean amounted to 4755 tonnes, of which 83% was in the inorganic form (DIC). Thus, the bulk of carbon moving in the water column is inorganic - yet, DIC seems to be measured only rarely in most flux studies of this nature. Salt marshes cover extensive areas in this estuary and produce some carbon, particularly DOC, but productivity of the local Spartina species is low (P:B=1.1). Consequently, the bulk of carbon exported from the estuary appears to originate from the highly productive macroinvertebrate and the phytoplankton component and not from the salt marsh plants.     

Transport of microbial biomass through the North Inlet ecosystem

Tidal fluctuations and transports of total microbial biomass (measured as adenosine triphosphate [ATP]) were investigated at three marsh creeks comprising the major transfer points between the North Inlet marsh and the adjoining aquatic ecosystems. Two creeks, Town Creek and North Jones Creek, form the inlet mouth and are the only marsh-ocean exchange points. The third creek, South Jones Creek, connects to a brackish water embayment. The creeks were simultaneously sampled every 1.5 hours for 50 consecutive hours during neap tides (four tidal cycles) and 50 consecutive hours during spring tides of each season. At the inlet, ATP concentration fluctuated in phase with the tide during winter and fall and out of phase with the tide during the spring. Fluctuation patterns at South Jones Creek were irregular. The highest ATP concentrations were during the spring (mean=2.17 mg of ATP per m³) and the lowest concentrations were during the winter (mean=0.65 mg of ATP per m³). Net transports of ATP varied from tidal cycle to tidal cycle with regard to direction of transport (import or export) and magnitude. Net transports were small compared to large instantaneous transports and only 4 of 22 determinations of net transport were different from zero transport.     

The optical qualities of shallow wetland lined bays in Lake Victoria

In tropical lakes, the characteristics and dimension of the coastal wetlands can have a strong influence on the quality of the inshore waters. The interaction between littoral wetlands and the open water environment is complex and requires an understanding of the material and energy exchanges between these compartments. In the present analysis, we examine the impact of wetlands on the optical properties (underwater light environment) of two adjacent bays on the Ugandan side of Lake Victoria. We use both irradiance profiles within the water column as well as measurements of the dissolved and particulate fractions of the water column. By introducing a new term to identify that part of the solar spectrum most affected by wetland released dissolved organic matter, it is possible to determine the area of each bay that is influenced by the wetland. Depending on the period of analysis, wetland released chromophoric dissolved organic matter can play a dominating role in the attenuation of UV and visible radiation in the underwater environment.     

‘Green tides’ are overwhelming the coastline of our blue planet: taking the world’s largest example

A broad spectrum of events that come under the category of green tide are recognized world-wide as a response to elevated levels of seawater nutrients in coastal areas. Green tides involve a wide diversity of sites, macroalgal species, consequences, and possible causes. Here we review the effect of natural and man-induced environmental fluctuations on the frequency and apparent spread of green tides. This article highlights the need for interdisciplinary research aimed at shedding light on the basic mechanisms governing the occurrence and succession of green algae in coastal seas. This will result in more effective management and mitigation of the effects of green tides, thus safeguarding the intrinsic and commercial value of coastal marine ecosystems.     

Linkages between seagrass,mangrove and saltmarsh as fish habitat in the Botany Bay estuary,New South Wales

Shallow-water vegetated estuarine habitats, notably seagrass, mangrove and saltmarsh, are known to be important habitats for many species of small or juvenile fish in temperate Australia. However, the movement of fish between these habitats is poorly understood, and yet critical to the management of the estuarine fisheries resource. We installed a series of buoyant pop nets in adjacent stands of seagrass, mangrove and saltmarsh in order to determine how relative abundance of fishes varied through lunar cycles. Nets were released in all habitats at the peak of the monthly spring tide for 12 months, and in the seagrass habitat at the peak of the neap tide also. The assemblage of fish in each habitat differed during the spring tides. The seagrass assemblage differed between spring and neap tide, with the neap tide assemblage showing greater abundances of fish, particularly those species which visited the adjacent habitats when inundated during spring tides. The result supports the hypothesis that fish move from the seagrass to the adjacent mangrove and saltmarsh during spring tides, taking advantage of high abundances of zooplankton, and use seagrass as a refuge during lower tides. The restoration and preservation of mangrove and saltmarsh utility as fish habitat may in some situations be linked to the proximity of available seagrass.