Recovery from the impact of light reduction on the seagrass Amphibolis griffithii, insights for dredging management

A large-scale, manipulative experiment was conducted to examine the extent and rate of recovery of meadows of the temperate Australian seagrass, Amphibolis griffithii to different light-reduction scenarios typical of dredging operations, and to identify potential indicators of recovery from light reduction stress. Shade cloth was used to mimic different intensities, durations and start times of light reduction, and then was removed to assess the recovery. The meadow could recover from 3 months of light stress (5-18% ambient) following 10 months re-exposure to ambient light, even when up to 72% of leaf biomass was lost, much faster recovery rates than has previously been observed for large seagrasses. However, when the meadow had been shaded for 6-9 months and more than 82% of leaf biomass was lost, no recovery was detected up to 23 months after the light stress had ceased, consistent with other studies. Five potential indicators of recovery were recommended.     

Temperature extremes reduce seagrass growth and induce mortality

Extreme heating (up to 43 °C measured from five-year temperature records) occurs in shallow coastal seagrass meadows of the Great Barrier Reef at low tide. We measured effective quantum yield (_?PSII${?}_{\text{PSII}}$), growth, senescence and mortality in four tropical seagrasses to experimental short-duration (2.5 h) spikes in water temperature to 35 °C, 40 °C and 43 °C, for 6 days followed by one day at ambient temperature. Increasing temperature to 35 °C had positive effects on _?PSII${?}_{\text{PSII}}$ (the magnitude varied between days and was highly correlated with PPFD), with no effects on growth or mortality. 40 °C represented a critical threshold as there were strong species differences and there was a large impact on growth and mortality. At 43 °C there was complete mortality after 2–3 days. These findings indicate that increasing duration (more days in a row) of thermal events above 40 °C is likely to affect the ecological function of tropical seagrass meadows.     

Using Rn to estimate submarine groundwater discharge (SGD) and the associated nutrient fluxes into Xiangshan Bay,East China Sea

Continuous radon (²²²Rn) monitoring was conducted at two stations (site A and site B) with different perpendicular distance from the shoreline in Xiangshan Bay, East China Sea. Based on a ²²²Rn balance model (various sources and sinks of ²²²Rn in coastal water), the average rate of SGD was estimated to be 0.69 cm/day and 0.23 cm/day for site A and site B, respectively. The results from a nutrient analysis of the groundwater indicate that the associated nutrients fluxes loading through the SGD pathway were 4.27 × 10⁶ mol/day for DIN, 2.24 × 10⁴ mol/day for DIP and 1.82 × 10⁶ mol/day for DSi, respectively, which were comparable to or even higher than the levels observed in the local streams. Therefore, adequate attention should be paid to the importance of SGD as one source of nutrients during the eutrophication control process in this area.     

Remote forcing of water levels by tropical cyclones in southwest Australia

Tropical cyclones (termed hurricanes and typhoons in other regions), are extreme events associated with strong winds, torrential rain and storm surges (in coastal areas) and cause extensive damage as a result of strong winds and flooding (caused by either heavy rainfall or ocean storm surges) in the immediate area of impact. The eastern Indian Ocean, particularly in the northwest region of Australia, is impacted by up to 10 tropical cyclones during the cyclone season, although direct impact of cyclones along the west and southwest coastlines is rare. However, the sub-tidal frequency component of sea level records along the west and south coasts of Western Australia indicates lagged correspondence with the occurrence of tropical cyclones. It is demonstrated that the tropical cyclones generate a continental shelf wave which travels along the west and south coasts of Australia up to 3500 km with speeds of 450–500 km day⁻¹ (5.2–5.8 ms⁻¹) with maximum trough to crest wave height of 0.63 m, comparable with the mean daily tidal range in the region. The shelf wave is identified in the coastal sea level records, initially as a decrease in water level, 1–2 days after the passage of the cyclone and has a period of influence up to 10 days. Amplitude of the shelf wave was strongly affected by the path of the tropical cyclone, with cyclones travelling parallel to the west coast typically producing the most significant signal due to resonance and superposition with local forcing. Analysis of water levels from Port Hedland, Geraldton, Fremantle and Albany together with cyclone paths over a ten year period (1988–1998) indicated that the tropical cyclones paths may be classified into 6 different types based on the amplitude of the wave.     

Environmental response to sewage treatment strategies: Hong Kong's experience in long term water quality monitoring

In many coastal cities around the world, marine outfalls are used for disposal of partially treated wastewater effluent. The combined use of land-based treatment and marine discharge can be a cost-effective and environmentally acceptable sewage strategy. Before 2001, screened sewage was discharged into Victoria Harbour through many small outfalls. After 2001, the Hong Kong Harbour Area Treatment Scheme (HATS) was implemented to improve the water quality in Victoria Harbour and surrounding waters. Stage I of HATS involved the construction of a 24 km long deep tunnel sewerage system to collect sewage from the densely populated urban areas of Hong Kong to a centralized sewage treatment plant at Stonecutters Island. A sewage flow of 1.4 million m³ d⁻¹ receives Chemically Enhanced Primary Treatment (CEPT) followed by discharge via a 1.2 km long outfall 2 km west of the harbor. The ecosystem recovery in Victoria Harbour and the environmental response to sewage abatement after the implementation of HATS was studied using a 21-year data set from long term monthly water quality monitoring. Overall, the pollution control scheme has achieved the intended objectives. The sewage abatement has resulted in improved water quality in terms of a significant reduction in nutrients and an increase in bottom DO levels. Furthermore, due to the efficient tidal mixing and flushing, the impact of the HATS discharge on water quality in the vicinity of the outfall location is relatively limited. However, Chl a concentrations have not been reduced in Victoria Harbour where algal growth is limited by hydrodynamic mixing and water clarity rather than nutrient concentrations. Phosphorus removal in the summer is suggested to reduce the risk of algal blooms in the more weakly-flushed and stratified southern waters, while nutrient removal is less important in other seasons due to the pronounced role played by hydrodynamic mixing. The need for disinfection of the effluent to reduce bacterial (E. coli) concentrations to acceptable levels is also confirmed and has recently been implemented.     

Additive toxicity of herbicide mixtures and comparative sensitivity of tropical benthic microalgae

Natural waters often contain complex mixtures of unknown contaminants potentially posing a threat to marine communities through chemical interactions. Here, acute effects of the photosystem II-inhibiting herbicides diuron, tebuthiuron, atrazine, simazine, and hexazinone, herbicide breakdown products (desethyl-atrazine (DEA) and 3,4-dichloroaniline (3,4-DCA)) and binary mixtures, were investigated using three tropical benthic microalgae; Navicula sp. and Cylindrotheca closterium (Ochrophyta) and Nephroselmis pyriformis (Chlorophyta), and one standard test species, Phaeodactylum tricornutum (Ochrophyta), in a high-throughput Maxi-Imaging-PAM bioassay (Maxi-IPAM). The order of toxicity was; diuron > hexazinone > tebuthiuron > atrazine > simazine > DEA > 3,4-DCA for all species. The tropical green alga N. pyriformis was up to 10-fold more sensitive than the diatoms tested here and reported for coral symbionts, and is recommended as a standard tropical test species for future research. All binary mixtures exhibited additive toxicity, and the use of herbicide equivalents (HEq) is therefore recommended in order to incorporate total-maximum-load measures for environmental regulatory purposes.     

Phytoplankton abundance, biomass and diversity within and between Pantanal wetland habitats

During the high water season, the flooding reduces environmental heterogeneity in aquatic ecosystems of the Pantanal wetland. When the water level recedes, lakes and channels are formed as individual systems. Therefore, we expected the spatial heterogeneity during the low water phase resulting in changes on biological communities leading to high phytoplankton abundance, biomass and diversity within and between habitats. To test this hypothesis, we analyzed eight freshwater systems (five oxbow lakes, two channels, and the river) during the low water period. Phytoplankton biomass, abundance, diversity (alpha, beta, gamma) and diversity metrics as richness (species per sample), Shannon diversity (H′) and evenness were measured in all systems along with nutrient concentrations, zooplankton and bacteria abundances. We found 97 species as gamma diversity. The alpha diversity was unexpectedly low in comparison to most other South American floodplain systems (38 species in river, 24 in channels and 29 in lakes). Also, the systems are relatively similar in composition (beta diversity, 28%). Connectivity differences between systems highlighted differences in phytoplankton abundance and biomass (fresh weight) ranging from 1428 ind mL⁻¹ (river) to 3710 ind mL⁻¹ (lakes) and from 0.71 mg L⁻¹ (river) to 2.9 mg L⁻¹ (lakes), respectively. However, our results did not indicate significant differences in phytoplankton species richness between the systems during the low water. Our main conclusions are that local factors may be responsible for changes on phytoplankton community and the time of isolation during the low water phase was not sufficient to promote changes in phytoplankton diversity between the habitats.     

Physiological energetics of the fourth instar of Chinese horseshoe crabs (Tachypleus tridentatus) in response to hypoxic stress and re-oxygenation

Hypoxia associated with eutrophication is a potential threat to the Chinese horseshoe crab Tachypleus tridentatus which inhabits intertidal sand flats in Asia. This study investigated the effect of dissolved oxygen level (DO) (6, 4 and 2 mg O₂ l⁻¹) on the physiological energetics in the juvenile T. tridentatus. They were exposed to various oxygen levels for three days and then transferred to normoxia for three days to examine the recovery from low oxygen stress. Feeding rate, respiration rate and scope for growth were reduced at lower DO levels while absorption efficiency and excretion rate were independent of DO levels. Although full recovery of the physiological responses and scope for growth from hypoxis stress was observed when normoxia resumed, their long term survival in suboptimal habitats with frequent occurrence of hypoxia deserves a close monitoring as hypoxia may be even more common in future in a warming world.     

Effects of nitrogen and phosphorus on phytoplankton composition and biomass in 15 subtropical, urban shallow lakes in Wuhan, China

This study aims at investigating the composition and biomass of the phytoplankton community in 15 urban shallow eutrophic lakes as well as the effects of main environmental factors, including nutrient concentrations and the ratio of nitrogen to phosphorus, temperature, COD, BOD, water depth, etc. on the phytoplankton community structure. Lake water samples were taken and analyzed on a bimonthly basis during the period from March 2004 to March 2006. The redundancy analysis (RDA) and regression analysis (RA) were performed to identify the effects of nutrients on the phytoplankton community and biomass in these typical urban lakes. The results indicate that most of these urban lakes were hypertrophic due to high concentrations of total phosphorus (TP) and total nitrogen (TN), with mean levels of 490 and 5380 mg m⁻³, respectively. The phytoplankton community was dominated by Microcystis aeruginosa and Euglena caudate in summer and Cryptomonas ovata and Cyclotella meneghiniana in winter. The mean biomass of the phytoplankton reached 456.87 mg L⁻¹ in summer months and the annual level was 189.24 mg L⁻¹. Temperature and TP content were found to be the principal limiting factors for phytoplankton growth on an annual basis. On the other hand, the results of RDA and RA demonstrate that the dominant phytoplankton species were not nutrient-limited during summer months. Low TN:TP ratios (<10) were detected accompanied with fewer occurrences of N-fixing cyanobacteria and other filamentous algae in most lakes in summer, which implies that low N:P ratio does not always shifts the dominance of phytoplankton community to the N-fixing cyanobacteria. Moreover, TP always had higher correlation with chlorophyll a (Chl-a) than TN, even when the TN:TP ratios of most samples were lower than 10. Therefore, it is concluded that the TN:TP ratio is not always a suitable index to determine whether nitrogen or phosphorus limits the phytoplankton biomass in urban shallow eutrophic lakes.     

Assessing the impact of land use change on hydrology by ensemble modelling (LUCHEM) IV: Model sensitivity to data aggregation and spatial (re-)distribution

This paper analyses the effect of spatial resolution and distribution of model input data on the results of regional-scale land use scenarios using three different hydrological catchment models. A 25 m resolution data set of a mesoscale catchment and three land use scenarios are used. Data are systematically aggregated to resolutions up to 2 km. Land use scenarios are spatially redistributed, both randomly and topography based. Using these data, water fluxes are calculated on a daily time step for a 16 year time period without further calibration. Simulation results are used to identify grid size, distribution and model dependent scenario effects. In the case of data aggregation, all applied models react sensitively to grid size. WASIM and TOPLATS simulate constant water balances for grid sizes from 50 m to 300–500 m, SWAT is more sensitive to input data aggregation, simulating constant water balances between 50 m and 200 m grid size. The calculation of scenario effects is less robust to data aggregation. The maximum acceptable grid size reduces to 200–300 m for TOPLATS and WASIM. In case of spatial distribution, SWAT and TOPLATS are slightly sensitive to a redistribution of land use (below 1.5% for water balance terms), whereas WASIM shows almost no reaction. Because the aggregation effects were stronger than the redistribution effects, it is concluded that spatial discretisation is more important than spatial distribution. As the aggregation effect was mainly associated with a change in land use fraction, it is concluded that accuracy of data sets is much more important than a high spatial resolution.     

Assessment of Hurricane Ivan impact on chlorophyll-a in Pensacola Bay by MODIS 250 m remote sensing

The impact of Hurricane Ivan on water quality in Pensacola Bay was investigated by MODIS 250 m remote sensing of chlorophyll-a concentrations at different time slots before and after the hurricane event. Before the hurricane, the mean chlorophyll-a in the Bay was 5.3 μg/L. Heavy rainfall occurred during the hurricane landfall. The 48 h rainfall reached 40 cm and the peak storm surge reached 3 m on 9/16. After the rainstorm and during the storm surge on 9/17/2004, the mean chlorophyll-a concentration substantially increased to 14.7 μg/L. 26.3% water area was in the poor-water-quality condition (chl-a > 20 μg/L). This indicates that heavy nutrient loads from urban stormwater runoff and storm-surge inundation simulated chlorophyll bloom. After the end of the storm surge on 9/18/2004, the mean chlorophyll dropped to 2.0 μg/L, suggesting the effective flushing of polluted water from the bay to the Gulf of Mexico by the storm-surge. The good water quality condition lasted for almost several weeks after the storm surge. The peak river flow, arriving on the 4th day after the peak storm surge, did not alter the good water quality situation in the bay. This indicates that urban stormwater runoff rather than the river inflow is the major pollutant source for water quality in Pensacola Bay during the hurricane.     

Estuarine macrofauna responses to continuous in situ nutrient addition on a tropical mudflat

A field experiment to assess the effects of continuous nutrient addition on the macrobenthic community was carried out on an estuarine mudflat on the northeast coast of Brazil. The experiment began on 5 October 2005 and ended on 8 February 2006. Macrofauna was compared at approximately four-week intervals in triplicate plots with three levels (Control – C, Low Dose – LD and High Dose – HD) of weekly fertilizer additions for 17 weeks. Inorganic fertilizer (N–P–K) was applied on nine randomly defined quadrangular plots (4 m² each). All measurements were calculated from species abundances. Multivariate analyses as well as the univariate indices (richness, abundance and Shannon–Wiener index) showed statistically significant differences between the enriched and control areas during the period of the experiment. The expected gradual response based on the succession model of Pearson and Rosenberg was not observed. The nutrient doses used were high enough to cause severe decreases in abundance, richness and evenness, and an increase in dominance.     

Intra-annual variability in nutrients in the Godavari estuary,India

Daily variations in nutrients were monitored for 15 months (September 2007–November 2008) in the Godavari estuary, Andhra Pradesh, India, at two fixed locations. River discharge has significant influence on nutrients loading to the estuary, which peaks during June–August (peak discharge period; monsoon) whereas exchanges at the sediment–water interface, groundwater and rainwater contribute significantly during other period. Despite significant amount of nutrients brought by discharge to the study region, phytoplankton biomass, in terms of chlorophyll-a (Chl a), did not increase significantly due to high suspended load and shallow photic depth. Nutrients showed downward gradient towards downstream of the estuary from upstream due to dilution by nutrient poor seawater and biological uptake. The N:P ratios were higher than Redfield ratio in both upstream and downstream of the estuary during no discharge period suggesting PO₄ to be a limiting nutrient for phytoplankton production, at levels <0.10 μmol L⁻¹. On the other hand, Si:N ratios were always more than unity during entire study period at both the stations indicating that Si(OH)₄ is not a limiting nutrient. Our results suggest that suspended matter limits phytoplankton biomass during peak discharge period whereas PO₄ during no discharge period.     

Effects of alumina refinery wastewater and signature metal constituents at the upper thermal tolerance of: 1. The tropical diatom Nitzschia closterium

Ecotoxicological studies, using the tropical marine diatom, Nitzschia closterium (72-h growth rate), were undertaken to assess potential issues relating to the discharge from an alumina refinery in northern Australia. The studies assessed: (i) the species’ upper thermal tolerance; (ii) the effects of three signature metals, aluminium (Al), vanadium (V) and gallium (Ga) (at 32 °C); and (iii) the effects of wastewater (at 27 and 32 °C). The critical thermal maximum and median inhibition temperature for N. closterium were 32.7 °C and 33.1 °C, respectively. Single metal toxicity tests found that N. closterium was more sensitive to Al compared to Ga and V, with IC₅₀s (95% confidence limits) of 190 (140-280), 19,640 (11,600-25,200) and 42,000 (32,770-56,000) μg L⁻¹, respectively. The undiluted wastewater samples were of low toxicity to N. closterium (IC₅₀s > 100% wastewater). Environmental chemistry data suggested that the key metals and discharge are a very low risk to this species.     

Water and nutrient fluxes off Northwest Africa

A historical data set is used to describe the coastal transition zone off Northwest Africa during spring 1973 and fall 1975, from 17° to 26°N, with special emphasis on the interaction between subtropical (North Atlantic Central Waters) and tropical (South Atlantic Central Waters) gyres. The near-surface geostrophic circulation, relative to 300 m, is quite complex. Major features are a large cyclonic pattern north of Cape Blanc (21°N) and offshore flow at the Cape Verde front. The large cyclone occurs in the region of most intense winds, and resembles a large meander of the baroclinic southward upwelling jet. The Cape Verde frontal system displays substantial interleaving that may partly originate as mesoscale features at the coastal upwelling front. Property–property diagrams show that the front is an effective barrier to all properties except temperature. The analysis of the Turner angle suggests that the frontal system is characterized by large heat horizontal diffusion as a result of intense double diffusion, which results in the smoothing of the temperature horizontal gradients. Nine cross-shore sections are used to calculate along-shore geostrophic water-mass and nutrient transports and to infer exchanges between the coastal transition zone and the deep ocean (import: deep ocean to transition zone; export: transition zone to deep ocean). These exchanges compare well with mean wind-induced transports and actual geostrophic cross-shore transport estimates. The region is divided into three areas: southern (18–21°N), central (21–23.5°N), and northern (23.5–26°N). In the northern area geostrophic import is roughly compensated with wind-induced export during both seasons. In the central area geostrophic import is greater than wind-induced export during spring, resulting in net import of both water (0.8 Sv) and nitrate (14 kmol s⁻¹), but during fall both factors again roughly cancel. In the southern area geostrophy and wind join to export water and nutrients during both seasons, they increase from 0.6 Sv and 3 kmol s⁻¹ during fall to 2.9 Sv and 53 kmol s⁻¹ during spring.     

Assessing underwater noise levels during pile-driving at an offshore windfarm and its potential effects on marine mammals

Marine renewable developments have raised concerns over impacts of underwater noise on marine species, particularly from pile-driving for wind turbines. Environmental assessments typically use generic sound propagation models, but empirical tests of these models are lacking. In 2006, two 5 MW wind turbines were installed off NE Scotland. The turbines were in deep (>40 m) water, 25 km from the Moray Firth Special Area of Conservation (SAC), potentially affecting a protected population of bottlenose dolphins. We measured pile-driving noise at distances of 0.1 (maximum broadband peak to peak sound level 205 dB re 1 μPa) to 80 km (no longer distinguishable above background noise). These sound levels were related to noise exposure criteria for marine mammals to assess possible effects. For bottlenose dolphins, auditory injury would only have occurred within 100 m of the pile-driving and behavioural disturbance, defined as modifications in behaviour, could have occurred up to 50 km away.     

Plankton community as an indicator of water quality in tropical shrimp culture ponds

The plankton was examined as an indicator of water quality in 14 shrimp Litopenaeus vannamei farms in Brazil in 2003. The ponds were categorized by high stocking density (>30 PL m⁻²) of phytoplankton, consisting of 51 species with concentrations ranging from 365,218 ± 416,615 cells mL⁻¹ to 1,961,675 ± 3,160,172 cells mL⁻¹. Diatoms contributed to almost 70% of the species number and high densities resulted from Cyanophyta blooms, mainly Pseudanabaena cf limnetica. Forty zooplankton taxa were registered and were essentially composed of typical marine euryhaline species and suspension-feeders. Copepoda dominated (45%) the make-up, followed by Protozoa (18%), Rotifera (12%), and Mollusca (12%) larvae. Zooplankton varied from 972 ± 209 ind m⁻³ to 4235 ± 2877 ind m⁻³. Enhanced nutrient input affected plankton density and composition. Diatom and Copepoda dominance was replaced by cyanobacteria, protozoan, and rotifers as nutrient concentrations increased with the cultured period, indicating that plankton structure is affected by eutrophic conditions.     

The use of monitoring data for identifying factors influencing phytoplankton bloom dynamics in the eutrophic Taw Estuary, SW England

Using the Taw Estuary as an example, data routinely collected by the Environment Agency for England and Wales over the period 1990-2004 were interrogated to identify the drivers of excessive algal growth. The estuary was highly productive with chlorophyll concentrations regularly exceeding 100 μg L⁻¹, mostly during periods of low freshwater input from the River Taw when estuarine water residence times were longest. However, algal growth in mid estuary was often inhibited by ammonia inputs from the adjacent sewage treatment works. The reported approach demonstrates the value of applying conventional statistical analyses in a structured way to existing monitoring data and is recommended as a useful tool for the rapid assessment of eutrophication. However, future estuarine monitoring should include the collection of dissolved organic nutrient data and targeted high temporal resolution data because the drivers of eutrophication are complex and often very specific to a particular estuary.     

Caffeine and agricultural pesticide concentrations in surface water and groundwater on the north shore of Kauai (Hawaii, USA)

Caffeine has been associated with wastewater pollution in temperate and subtropical locations, but environmental caffeine concentrations in tropical locations have not been reported. The objectives of this study were to measure caffeine and agricultural pesticide (carbaryl, metalaxyl, and metribuzin) concentrations in environmental waters on the tropical north shore of Kauai (Hawaii, USA) and assess whether patterns in caffeine concentration were consistent with a wastewater caffeine source. Groundwater, river, stream and coastal ocean samples were collected in August 2006 and February 2007. Caffeine was detected in all August 2006 samples and in 33% of February 2007 samples at concentrations up to 88 ng L⁻¹. Metribuzin was detected in five samples collected in February 2007. Carbaryl and metalaxyl were not detected in any sample. Caffeine was not detected in offshore ocean samples or river samples upstream of human development. A positive correlation between caffeine and enterococci suggested a possible wastewater caffeine source.     

Life history, growth and production of Theodoxus fluviatilis in Lake Esrom, Denmark

A population of Theodoxus fluviatilis L in the littoral zone of Lake Esrom was investigated from November 1977 to February 1979. The population was sampled every month in the winter period and twice during the rest of the year. Biomass was estimated as ash-free dry weight (AFDW) of the organic matter both of the soft parts of the animal and the shell itself. The relation between AFDW (c) and shell length (l) was log c=2.9509×log (l)−1.7120. The population comprised more than 1 year-class, which could be separated by shell length, by a narrow band on the shells and the growth of algae on the shell. The life cycle lasted years. The oldest animals had a shell length of 7.0-7.5 mm. A few individuals who were estimated to be years had a shell length up to 8.6 mm. Population density varied between 575 and 2115 individuals m⁻² on the stony substratum. The average was 1160 individuals m⁻². Mortality was low during the summer period. In winter many animals died due to the effect of ice and stormy weather on the stony substratum. Growth of the animals was estimated from the shell length. Maximum growth was observed from May to August with no growth during the winter. Egg capsules were found on the stones all year round. New capsules were found from late May to the middle of November. Most freshly laid capsules were observed in May-June and August-September. Capsules from the late summer hatched in spring and capsules laid in the spring hatched in August-September. The average annual net production for the whole population was estimated by three methods. The Allen curve method gave 1.895 AFDW m⁻², the growth-increment method gave 1.784 mg AFDW m⁻² and the Hynes method 2.284 mg AFDW m⁻². Corresponding estimated P/B ratios were 1.29, 1.30 and 1.57. Annual net-production of the four investigated year-classes was 16 mg AFDW m⁻² year⁻¹ for 1975, 224 mg AFDW m⁻² year⁻¹ for 1976, 1.258 mg AFDW m⁻² year⁻¹ for 1977 and 287 mg AFDW m⁻² year⁻¹ for 1978. P/B ratios for the three oldest year-classes were, respectively, 0.32, 0.50 and 1.67. A comparison with other investigations on gastropod life cycles, reproduction and P/B ratios is made and differences discussed. Variations are correlated to temperature, and food quality and quantity.