The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

Lake St. Lucia, the largest estuarine system in Africa (325 km²), was chosen as the field study area for a 3.5-yr ((1980)–83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded.The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and Zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution of juveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns.Turbidity and food type influences were difficult to separate but exceptions were the anchovyThryssa vitrirostris (Gilchrist and Thompson) and the soleSolea bleekeri Boulenger which occurred only in turbid water despite the widespread occurrence of their prey, andGenes acinaces Bleeker,G. rappi (Barnard), andG. fllamentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparidsRhabdosargus holubi (Steindachner) andR. sarba (Forsskal) were distributed according to turbidity and not their preferred foods.Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g.Liza dumerilii (Steindachner) andL. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g.Valamugil cunnesius (Valenciennes) andLeiognathus equula (Forsskal)) in 10–80 NTU, turbid-water species (e.g.Elops machnata (Forsskal) andThryssa vitrirostris) in > 50 NTU, and species indifferent to turbidity (e.g.Acanthopagrus berda (Forsskal) andTeraponjarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant roˆle, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries.     

The influence of turbidity on juvenile marine fishes in estuaries. Part 2. Laboratory studies,comparisons with field data and conclusions

The turbidity preferences of juveniles of 10 marine species common in estuaries of southeastern Africa were investigated in an experimental turbidity gradient. The tests allowed the elimination of all environmental factors except turbidity.Liza dumerilii (Steindachner), was found to be a clear-water species (< 10 NTU);L. macrolepis (Smith),Rhabdosargus sarba (Forsskal),Gerres filamentosus Cuvier, andValamugil buchanani (Bleeker), preferred “clear to partially turbid” water (< 50 NTU);Monodactylus argenteus (L.) preferred intermediate turbidities (10–80 NTU); and the remaining four,Rhabdosargus holubi (Steindachner),Acanthopagrus berda (Forsskal),Pomadasys commersonni (Lacépède) andTerapon jarbua (Forsskal), were indifferent to turbidity. Statistical analysis of laboratory and field data revealed significant correlations. The results of these are discussed in relation to fish distribution patterns, ecology, and the importance of turbidity to juvenile marine fishes in estuaries.     

Factors affecting the distribution of juvenile estuarine and inshore fish

The differential distributions of juveniles and adults of 25 species of teleost were investigated and compared from four habitat types in sub-tropical Moreton Bay, Queensland. The aim of the study was to identify factors influencing the distribution of juveniles, particularly the species which enter estuaries. The following habitats were sampled: a shallow, sheltered tidal estuary (Caboolture); a shallow, exposed bay with muddy substrates (Deception Bay); an exposed area of sandy substrates and seagrass (Toorbol Point) and a sheltered oceanic site with sandy substrates and seagrass (Kooringal). Data on diet, spawning seasons and recruitment periods of fry are presented together with measurements of salinity, temperature and turbidity. Species entering estuaries recruited mainly in summer (rainy season). The possible preference of juveniles for calm water, the roles of food and predation pressure, the effects of salinity, temperature and turbidity are discussed in relation to the biology and distribution of the fish. Salinity and temperature were probably not important to most juvenile fish. The effects of calm water, suitable food and predators vary according to species. Although all juveniles studied preferred shallow water, in the case of those entering estuaries, turbidity was the single most important factor. Juveniles of the same species occurred in both the estuary and Deception Bay where abiotic and biotic factors other than turbidity were different. During summer, turbidity gradients extended from east to west in Moreton Bay with highest turbidities in Caboolture estuary and Deception Bay. In winter, turbidities throughout Moreton Bay were low and relatively uniform. At this time many of the ‘clear water’ species occurred in Deception Bay. The influence of high turbidity on fish may be linked to reduced predation pressure and perhaps food supply in shallow water. Turbidity gradients in summer may aid fry in locating estuarine nursery grounds. It is apparent however, that juveniles of many species are probably not attracted to estuaries per se but to shallow turbid areas.     

Vulnerability of age-0 pallid sturgeon Scaphirhynchus albus to predation; effects of predator type,turbidity, body size,and prey density

Predation can play an important role in the recruitment dynamics of fishes with intensity regulated by behavioral (i.e., prey selectivity) and/or environmental conditions that may be especially important for rare or endangered fishes. We conducted laboratory experiments to quantify prey selection and capture efficiency by three predators employing distinct foraging strategies: pelagic piscivore (walleye Sander vitreus); benthic piscivore (flathead catfish Pylodictis olivaris) and generalist predator (smallmouth bass Micropterus dolomieu) foraging on two size classes of age-0 pallid sturgeon: large (75–100 mm fork length [FL]) and small (40–50 mm FL). Experiments at high (> 70 nephalometric turbidity units [NTU]) and low (< 5 NTU) turbidity for each predator were conducted with high and low densities of pallid sturgeon and contrasting densities of an alternative prey, fathead minnow Pimephales promelas. Predator behaviors (strikes, captures, and consumed prey) were also quantified for each prey type. Walleye and smallmouth bass negatively selected pallid sturgeon (Chesson’s α?=?0.04–0.1) across all treatments, indicating low relative vulnerability to predation. Relative vulnerability to predation by flathead catfish was moderate for small pallid sturgeon (α?=?0.44, neutral selection), but low for large pallid sturgeon (α?=?0.11, negative selection). Turbidity (up to 100 NTU) did not affect pallid sturgeon vulnerability, even at low density of alternative prey. Age-0 pallid sturgeon were easily captured by all predators, but were rarely consumed, suggesting mechanisms other than predator capture efficiency govern sturgeon predation vulnerability.     

Environmental drivers on seasonal abundance of riverine-estuarine V. cholerae in the Indian Sundarban mangrove

Gangetic delta is considered as the homeland of cholera, which is thought to be influenced by changes in populations of estuarine Vibrio cholerae. We aim to identify the environmental, biotic and abiotic driving forces influencing the V. cholerae dynamics in riverine-estuarine environment of southern deltaic Bengal. Cultivable Vibrio count (CVC) ranged between 1 and 10³ colony forming units (CFU)/mL at a salinity gradient of 1.9–30 practical salinity unit (PSU). Increased water temperatrure during summer influences the higher CVC followed by a sudden fall along with the onset of monsoon upto winter. While summer V. cholerae O1 peak (50–100 CFU/mL) can be associated with higher water temperature (P < 0.05) and higher turbidity (P < 0.005); sharp fall during monsoon (15–45 CFU/mL) is attributed to reduced salinity (25–2.5 PSU). Plankton attached V. cholerae O1 varied between 10 and 1000 CFU/mL with a highest peak at winter followed by summer and monsoon. Prevalence of toxigenic V. cholerae O1 in low salinity (2–7.5 PSU) during monsoon identifies that high water temperature (>25 °C), higher turbidity (>100 NTU) and lower salinity plays the pivotal role in toxicity acquisition. Present investigation establishes the role of Sundarban mangrove, where V. cholerae exist in an avirulent condition. During migration towards low saline inland system, V. cholerae pool possibly acquires toxin genes under the influence of environmental factors. Planktonic attachment is possibly a survival strategy at adverse condition, when they do not acquire any toxin gene. Seasonal V. cholerae dynamics has been thoroughly established in environmental settings of high saline mangrove and brackish water flowing to inland low saline condition.     

Development of a quantitative PCR method to differentiate between viable and nonviable bacteria in environmental water samples

Ethidium monoazide bromide (EMA) treatment of pure culture and environmental waters at low concentrations (1.0–7.5 μg/ml) indicated effective enumeration of viable and viable but nonculturable Escherichia coli in pure cultures, creek waters, and secondary activated sludge effluent samples by quantitative polymerase chain reaction (qPCR) amplification of the uidA and fliC gene targets at turbidity values <10 NTU. However, EMA treatment was not effective in primary clarifier and secondary trickling filter effluents where turbidities were ≥10 NTU. In viable pure cultures, rapidly dividing and senescent cells were most affected by increasing EMA concentrations. Amplification of heat-killed pure bacterial cultures decreased 4 to 6 logs depending on EMA concentration and culture age. The greatest difference was observed in 5-h cultures using 7.5 μg/ml EMA. Turbidity (≥100 NTU) in environmental samples inhibited EMA effectiveness on viability discrimination. Enumeration of E. coli in certain wastewaters using EMA-qPCR was similar to culture suggesting that EMA treatment could be incorporated into qPCR assays for the quantification of viable bacteria increasing assay time no more than 30 min. Our results indicate that EMA can be used in routine qPCR assays, but optimum conditions for exposure must be identified for each sample type due to sample matrix effects such as turbidity.     

Characteristics of reaction field and the reactive distance of a planktivore, Pseudorasbora parva (Cyprinidae), in various environmental conditions

The characteristics of the reaction field and the reactive distance of the Stone moroko (Pseudorasbora parva) were studied under three environmental conditions (structural complexity, light intensity and turbidity) and three prey sizes. In optimal experimental conditions, under no structural complexity, light intensity of 200 lux and turbidity less than 1 NTU (Nephelometric Turbidity Units), the cross-section of the reaction field was found to be elliptic with a bearing angle larger than the elevation angle, but both angles changed slightly depending on environmental conditions. The reactive distance was large, and the fish frequently attacked prey that was located within 15–60 degrees to each side from the frontal direction of a fish (i.e., ± 15 degrees from the axis of the fish body) horizontally. In the light intensity below 50 lux or turbidity above 10 NTU, however, the attack frequency and the reactive distance in the frontal direction of a fish did not differ from other horizontal directions in the reaction field. The average reactive distance increased proportionally with increasing strand distance, but it gradually reached a constant value for strand distances greater than about 3.6 times the fish body length. The average reactive distance increased in the light intensity range of 10–200 lux and decreased negatively with turbidity increasing. The average reactive distance increased with larger prey size, but the rate of increase of the reactive distance gradually decreased.     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

Characterization of phytoplankton assemblages in a tropical coastal environment using Kohonen self‐organizing map

This study was aimed at understanding the main abiotic environmental factors controlling the distribution patterns of abundance and composition of phytoplankton (size less than 10 μm) assemblages in the coastal waters of south‐eastern Côte d'Ivoire. Data were collected during two cruises, in January (low‐water period) and October (high‐water period) of 2014. A total of 67 species were identified and assigned to Bacillariophyceae (49%), Cyanophyceae (21%), Chlorophyceae (13%), Euglenophyceae (10%), Dinophyceae (4%) and Chrysophyceae (3%). Three biotic zones (I, IIA and IIB) were distinguishable on a Kohonen self‐organizing map after an unsupervised learning process. The diatom genera Eunotia sp., Navicula sp. and Actinoptychus senarius are significantly associated with I, IIA and IIB biotic zones, respectively. A clear seasonal cum salinity trend was apparent in phytoplankton distribution patterns. Turbidity and nitrate levels were the main abiotic factors controlling phytoplankton distribution in I, the upland tidal regions of the lagoon. In regions along the lagoon–sea continuum, phosphate and turbidity exert the most control during the low‐water season (IIA), while total dissolved solids control phytoplankton distribution during the high‐water season (IIB). These are climate‐sensitive parameters whose concentrations depend on prevailing hydroclimatic processes. Therefore, seasonality can have important consequences on phytoplankton community and inadvertently the productivity of these systems.     

New distribution record for the Asian tapeworm Bothriocephalus acheilognathi Yamaguti, 1934 in the Eastern Cape province,South Africa

Assuming that the inshore and offshore waters of Lake Victoria are impacted differently by human activities in its catchment, this study investigated the water quality dynamics of the lake. A total of 29 stations were sampled in 2005–2008 for dissolved oxygen (DO), pH, Secchi transparencies, temperature, turbidity, chlorophyll a, NO₃, SRSi, TN and TP. There was a decreasing trend of the measured parameters towards offshore sites, except for Secchi transparency and NO₃, which increased towards the offshore waters. DO concentrations (mean ± SD) varied between 6.97 ± 0.57 mg O₂ l^–1 and 5.80 ± 0.72 mg O₂ l^–1 in the inshore and offshore waters, respectively. Turbidity values were comparatively higher in the inshore (3.73 ± 2.21 NTU) than the offshore waters (2.19 ± 1.81 NTU). Chlorophyll a concentrations varied between 17.36 ± 6.13 µg l^–1 and 8.09 ± 4.38 µg l^–1 in the inshore and offshore waters, respectively. Increases of unsustainable human activities in the lake and its catchments, plus increased degradation of wetlands, are among the causes of the observed water quality changes. In order to be fruitful and sustainable, the management of Lake Victoria and its catchment needs to take an ecosystem approach, and to involve all key stakeholders.     

Impact of resuspended sediment on zooplankton feeding in Lake Waihola, New Zealand

1. Wind‐induced sediment resuspension in shallow lakes affects many physical and biological processes, including food gathering by zooplankton. The effects of suspended sediment on clearance rate were determined for a dominant cladoceran, Daphnia carinata, and calanoid copepod, Boeckella hamata, in Lake Waihola, New Zealand. 2. Animals were incubated at multiple densities for 4 days in lake water containing different amounts of suspended lake sediment. Rates of harvest of major food organisms were determined for each sediment level (turbidity) from changes in net growth rate with grazer density. 3. Daphnia cleared all food organisms 7–40 μm in length at similar rates, but was less efficient in its removal of free bacteria, phytoplankton <7 μm, and large cyanobacterial filaments. Elevation of sediment turbidity from 2 to 10 nephelometric turbidity units (NTU) (63 mg DW L^?1 added sediment) reduced Daphnia clearance of phytoplankton, heterotrophic flagellates and ciliates by 72–100%, and of amoebae and attached bacteria by 21–44%. Further inhibition occurred at higher turbidity. 4. Boeckella hamata removed microzooplankton primarily, rather than phytoplankton. The rate at which it cleared rotifers was reduced by 56% when turbidity was increased from 2.5 to 100 NTU. 5. In the absence of macrozooplankton, algal growth increased with sediment turbidity, suggesting that sediment also inhibits rotifer grazing. 6. As mid‐day turbidity in Lake Waihola is ≥10 NTU about 40% of the time, sediment resuspension may play a major role in moderating energy flow and structuring pelagic communities in this lake.     

Structural changes in lecithin-water systems thermal-turbidimetric studies

Turbidity measurements were made of dilute aqueous dispersions of 1,2-dipalmitoyl-l-lecithin as the temperature was varied. In the range from 24 to 33° a decrease in turbidity is associated with the penetration of water between the layers of lipid in the crystalline structure. At the transition to the liquid crystalline form (39–41°) a sharp decrease in turbidity occurs. In solutions of 0.06 M LiCl, 1 mM CaCl₂, MgCl₂ or 0.05 M PO₄³ an increase in turbidity resulted at the transition temperature, whereas in 0.1 M NaCl, or quarternary ammonium salts the turbidity decreased. The presence of small amounts of dicetylphosphoric acid mixed with the lecithin decreased the turbidity, and at elevated pH levels there was no change at the transition temperature.     

冠心病合并糖尿病PCI术后胸痹患者中医证型分布及影响因素调查研究

目的:探讨冠心病合并糖尿病经皮冠状动脉介入(PCI)术后胸痹患者中医证型的分布情况,分析不同证型与影响因素关系。方法:收集2010年3月-2015年3月东直门医院心内科收治的105例冠心病合并糖尿病PCI术后胸痹患者的调查资料,根据中医辨证标准分为气阴两虚、气虚血瘀、气虚痰浊、阳虚气虚、痰浊血瘀、气滞血瘀、阴阳两虚7个证型,分析冠心病合并糖尿病PCI术后胸痹患者的中医证型分布规律及其相关影响因素研究。结果:气阴两虚证有36例(34.29%),其次是气虚血瘀证22例(20.95%)、气滞血瘀证14例(13.33%)、阳虚气虚证10例(9.52%)、痰浊血瘀证9例(8.57%)、气虚痰浊证8例(7.62%)、阴阳两虚证6例(5.71%),气阴两虚证占有比例明显高于其余证型,患者性别、年龄对证型分布无统计学意义(P0.05);气阴两虚证患者吸烟量、饮酒量明显高于其余证型(P0.05);气滞血瘀证患者离异情况明显多于其余各证型(P0.05)。结论:冠心病合并糖尿病PCI术后胸痹患者的主要证候类型为气阴两虚证,且患者饮酒及吸烟量越大,气阴两虚证越明显,而家庭婚姻离异的患者,则多见气滞血瘀证。     

Total ecosystem carbon stocks at the marine‐terrestrial interface: Blue carbon of the Pacific Northwest Coast,United States

The coastal ecosystems of temperate North America provide a variety of ecosystem services including high rates of carbon sequestration. Yet, little data exist for the carbon stocks of major tidal wetland types in the Pacific Northwest, United States. We quantified the total ecosystem carbon stocks (TECS) in seagrass, emergent marshes, and forested tidal wetlands, occurring along increasing elevation and decreasing salinity gradients. The TECS included the total aboveground carbon stocks and the entire soil profile (to as deep as 3 m). TECS significantly increased along the elevation and salinity gradients: 217 ± 60 Mg C/ha for seagrass (low elevation/high salinity), 417 ± 70 Mg C/ha for low marsh, 551 ± 47 Mg C/ha for high marsh, and 1,064 ± 38 Mg C/ha for tidal forest (high elevation/low salinity). Soil carbon stocks accounted for >98% of TECS in the seagrass and marsh communities and 78% in the tidal forest. Soils in the 0–100 cm portion of the profile accounted for only 48%–53% of the TECS in seagrasses and marshes and 34% of the TECS in tidal forests. Thus, the commonly applied limit defining TECS to a 100 cm depth would greatly underestimate both carbon stocks and potential greenhouse gas emissions from land‐use conversion. The large carbon stocks coupled with other ecosystem services suggest value in the conservation and restoration of temperate zone tidal wetlands through climate change mitigation strategies. However, the findings suggest that long‐term sea‐level rise effects such as tidal inundation and increased porewater salinity will likely decrease ecosystem carbon stocks in the absence of upslope wetland migration buffer zones.     

Current rectification in a tropical coral reef system

Acoustic Doppler current profiles (ADCP) and conductivity-temperature-depth (CTD) profiles were recorded in the Veracruz Reef System (VRS) to elucidate the effect of topography on current circulation and zooplankton distribution. Measurements showed the baroclinic behavior of the tropical coral reef system with vertical temperature and salinity gradients of 0.4°C m⁻¹ and 0.5 psu m⁻¹, respectively. Under sustained southerly wind conditions, a cyclonic eddy generated by subtidal and tidal current rectification was observed between the two groups of coral colonies studied. Current rectification was attributed to the shallowness of the coral colonies and to a small cape close to Anton Lizardo Village. The cyclonic eddy, with low surface temperature, produced an increment in the vertically integrated acoustic scattering volume (biovolume), suggesting that this is a high-productivity area.     

Effects of clay discharges on streams

The impact of clay discharges on benthic invertebrates was investigated by comparison of communities upstream and downstream of alluvial gold mining on 6 streams on the West Coast of the South Island, New Zealand. Mean turbidity was increased by 7–154 NTU above background (mean 1.3–8.2 NTU) by the mine discharges during the 2 months before sampling. Patterns of increase in suspended solids (strongly correlated with turbidity, r=0.95) were similar. Invertebrates densities were significantly lower at all downstream sites, ranging from 9 to 45% (median 26%) of densities at matched upstream sites. Downstream densities as a proportion of those upstream were negatively correlated with the logarithm of the turbidity loading (r=–0.82, P<0.05). The densities of the common taxa were also generally lower downstream of mining. Taxonomic richness was significantly lower at downstream sites in the four streams receiving higher turbidity loads (mean turbidity increase = 23–154 NTU). Lower epilithon biomass and productivity, and degraded food quality at the downstream sites probably explain the lowered invertebrate densities. At some sites, reduced bed permeability and interstitial dissolved oxygen, and avoidance reactions of invertebrates (i.e., increased drift), may have also contributed to lower invertebrate densities.Formerly DSIR Marine and Freshwater.     

Marine and estuarine gradients — An overview

The Proceedings of the ECSA 21 symposium on Marine and Estuarine Gradients are reviewed. It is emphasised that this is probably the first time that a full set of papers on the tidal freshwater sections of estuaries has appeared. There is however some ambiguity in the terminology applied to such waters, and a more consistent terminology is proposed. In particular the estuary is defined as reaching upstream as far as the tidal limit, irrespective of salinity. A wide variety of gradients within estuaries and coastal waters are considered, but it is apparent that the crucial spatial gradients are based on salinity, oxygen and turbidity, and that many other gradients are co-variables with these parameters. Temperature is also important for temporal gradients.     

Relationship of internal macrobioeroder densities in living massive Porites to turbidity and chlorophyll on the Australian Great Barrier Reef

This study investigates the relationship between the density of internal macrobioeroders in living massive Porites and nutrient status. The study was conducted along turbidity and chlorophyll gradients towards river mouths on 12 reefs in four regions of the inshore Great Barrier Reef. Mean internal macrobioeroder densities doubled from 2 to 8 m depth, and at the 8 m sites, densities increased 4- to 7-fold towards the river mouths in all regions. Densities also increased 1.6-fold for each additional 1 NTU turbidity and 650-fold per 1 μg L⁻¹ additional chlorophyll a. The study shows that the density of macrobioeroder boreholes in living massive Porites is a simple bioindicator measure for changing turbidity and chlorophyll concentrations on the Great Barrier Reef for sites from which direct water quality measurements are unavailable.     

Instability of the turbidity maximum in the macrotidal Geum River estuary, western Korea

In the low-salinity area of many macrotidal estuaries, through the combination of tidal pumping and estuarine circulation, an estuarine turbidity maximum (ETM) develops providing favorable conditions for various organisms. To investigate ecological roles of the ETM in East Asian estuaries, we conducted seasonal observations in the Geum (or Keum) River estuary, one of the representative macrotidal estuaries flowing into the Yellow Sea, from 2007 to 2008. The estuary was frequently filled with high-salinity (>10 PSU) and low-turbidity (<100 NTU) water under small or no freshwater discharge from a dam (ca. 8 km upstream from the river mouth). Brackish water was, however, completely pushed out of the estuary within a few hours after an intensive discharge in summer. Chlorophyll a (up to 50 μg l⁻¹) and pheophytin (up to 80 μg l⁻¹) were concentrated in a low-salinity (<1 PSU) and high-turbidity (up to 1000 NTU) area, indicating that the intensive discharge transported both living phytoplankton and resuspended detritus into the area. In contrast, a phytoplankton bloom (chlorophyll a, up to 100 μg l⁻¹) was observed at low salinities under little discharge in winter. The present study demonstrated an absence of the ETM suitable for estuarine-dependent organisms from the present Geum River estuary, indicating potential importance of adequate control of freshwater discharge for the formation and maintenance of the ETM.     

The effects of light intensity and algae-induced turbidity on feeding behaviour of larval striped trumpeter

Striped trumpeter larvae reared in algal cell‐induced turbid water (greenwater) fed equally well in clearwater in a light intensity range of 1–10 μmol s^‐1 m^‐2, when evaluated in terms of both the proportion of larvae feeding and larval feeding intensity. An ontogenetic improvement in photopic visual sensitivity of larvae was indicated by improved feeding at 0·1 μmol s^‐1 m^‐2, from 26±5% of larvae feeding and 0·027±0·005 rotifers consumed per feeding larva min^‐1 on day 8, to 96±2% and 0·221±0·007 rotifers consumed larva^‐1 min^‐1 on day 23 post‐hatching. Algal cell‐induced turbidity was shown to reduce incident irradiance with depth, indicated by increasing coefficients of attenuation (1·4–33·1) with increasing cell densities (0–2×10⁶ cells ml^‐1), though light intensities in the feeding experiment test chambers, at the algal cell densities tested, were within the optimal range for feeding (1–10 μmol s^‐1 m^‐2). Algae‐induced turbidity had different effects on larval feeding response dependent upon the previous visual environment of the larvae. Young larvae (day 9 post‐hatching) reared in clearwater showed decreased feeding capabilities with increasing turbidity, from 98±1% feeding and 0·153±0·022 rotifers consumed larva^‐1 min^‐1 in clearwater to 61±10% feeding and 0·042±0·004 rotifers consumed larva^‐1 min^‐1 at 56 NTU, while older clearwater reared larvae fed well at all turbidities tested. Likewise, greenwater reared larvae had increased feeding capabilities in the highest algal cell densities tested (32 and 66 NTU) compared with those in low algal cell density (6 NTU), and clearwater (0·7 NTU) to which they were naïve.