Variation in routine metabolism of juvenile muskellunge: evidence for seasonal metabolic compensation in fishes

Metabolic rate of age 0 muskellunge Esox masquinongy ranged from 0·10 at 5° C to 0·24 mg O₂ g^-1 h^-1 at 25° C and was significantly higher in spring and autumn than during winter months at comparable water temperatures. Reduced metabolic rate in winter was consistent with the metabolic compensation hypothesis, implying that metabolism of muskellunge varies independently of acclimation temperature and gonadogenesis. Moreover, seasonal variation in metabolic rate has important implications for energy budget studies. Single-season estimates of esocid metabolism may be inadequate to describe annual energy requirements; the magnitude of errors will depend on the time of year metabolic rate was measured. As a result, it is suggested that seasonal variation in metabolic rate be incorporated into energy budget determinations for fishes.     

The effects of temperature and season on the O2 consumption of third-instar larvae of the goldenrod gall fly (Eurosta solidaginis)

Abstract. Third-instar larvae of the goldenrod gall fly ( Eurosta solidaginis Fitch) live inside ball galls on goldenrod plants from summer to the following spring.Because galls are highly exposed to the weather, larvae experience substantial variations in body temperature.This study documents the oxygen consumption of gall fly larvae with regard to the effects of ambient temperature, seasonal conditioning, and prior exposure to subzero temperature.The body mass of larvae doubles between the late summer and the autumn; it subsequently undergoes a modest decline by early winter.The O₂, consumption of field-acclimatized larvae increases with ambient temperature, especially between 0 and 10°C (Q₁₀= 2.6-3.4).The thermal sensitivity of metabolism declines at higher ambient temperatures, most notably during the autumn/early winter.After exposure to 15°C for 1 week, autumn and early winter larvae maintain much lower rates of O₂ consumption than do late summer specimens.Prior exposure to -5°C for 24 h did not influence the O₂ consumption of larvae.Low thermal sensitivity of O₂ consumption, especially at higher ambient temperatures, is an energy-sparing mechanism during seasonal inactivity.Indeed, the persistence of this metabolic pattern in larvae exposed to 15°C suggests that they have entered a state of diapause.     

山西省辛安泉泉域三种泉溪大型藻类植物附生藻类的研究

对三种泉溪大型藻类(脆弱刚毛藻, 弧形串珠藻和普生轮藻)藻体上附生藻类的种类、分布及环境因子与寄主本身对其的影响进行了研究。2004 年7 月至2005 年4 月在山西省辛安泉泉域采集三种大型藻类植物(每个季节采集一次)。研究结果表明, 三种大型藻类藻体上的附生藻类种类数不同, 普生轮藻上的附生藻类有49 种, 脆弱刚毛藻上37 种, 弧形串珠藻上6 种, 其中硅藻门种类数最多。这说明寄主本身结构的复杂程度在一定意义上会影响附生藻类的种类数及多样性。就季节变化而言, 三种大型藻类藻体上的附生藻类也表现出了明显的季节性,大致趋势为: 春季>秋季>夏季>冬季, 同时, 灰关联分析结果也表明, 环境因子在一定程度上对附生藻类也有影响, 最关键的影响因子是流速、水深和电导。这说明影响大型藻类植物藻体上附生藻类的影响因素是多方面的, 不仅有寄主本身, 同时流速、水深和电导这三种环境因子也起到了很关键的作用。       

The seasonal biomass and productivity of the submerged macrophytes in a polluted Wisconsin stream

SUMMARY 1. We measured biomass and light/dark bottle productivity of macrophytes in a Wisconsin stream throughout one growing season. Except for a brief period in early spring when a Cladophora glomerata -filamentous algal community was dominant, Potamogeton pectinatus was the dominant macrophyte species in Badfish Creek.
2. Maximum community biomass was 710 g DW m⁻², with a maximum above ground biomass of 620 g DW m⁻² and a maximum below ground biomass of 120 g DW m⁻². Annual productivity was estimated at 1435 g DW m⁻² year⁻¹, with a calculated P/B of 2.01.
3. In situ net production averaged 2.83g C g AFDW⁻¹ h⁻¹ Net positive carbon gain by the P. pectinatus community occurred when water temperatures were above 15°C, and daylength at least 12h. This is correlated to the onset of tuber germination in spring, and the point of maximal biomass decline in autumn.     

Biomass and oxygen dynamics of the epiphyte community in a Danish lowland stream

SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus , in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.
2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O₂, g-¹ organic DW h⁻¹) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.
3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.     

Benthic algae support zooplankton growth during winter in a clear-water lake

We used stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes to assess the importance of benthic algae for the zooplankton individual growth in winter in a shallow, clear subarctic lake. The δ¹³C values of calanoid ( Eudiaptomus graciloides ) and cyclopoid ( Cyclops scutifer ) zooplankton in autumn suggest a food resource of pelagic origin during the ice-free period. The zooplankton δ¹³C values were high in spring compared to autumn. E. graciloides did not grow over winter and the change in δ¹³C was attributed to a decrease in lipid content during the winter. In contrast, the increase in δ¹³C values of C. scutifer over the winter was explained by their growth on organic carbon generated by benthic algae. The δ¹⁵N of the C. scutifer food resource during winter was low compared to δ¹⁵N of the benthic community, suggesting that organic matter generated by benthic algae was mainly channelled to zooplankton via ¹⁵N-depleted heterotrophic bacteria. The results demonstrate that benthic algae can sustain zooplankton metabolic demands and growth during long winters, which, in turn, may promote zooplankton growth on pelagic resources during the summer. Such multi-chain omnivory challenges the view of zooplankton as mainly dependent on internal primary production and stresses the importance of benthic resources for the productivity of plankton food webs in shallow lakes.     

Combined effect of light and temperature on triacylglycerol accumulation in Dicranum elongatum

In the subarctic moss Dicranum elongatum Schleich & Schwaegr., the level of total lipids and triacylglycerols (TAG) was high in late winter and spring and low in autumn and winter. Four-week exposure of field material to continuous light (135μmol m⁻²s⁻¹) at 1°C resulted in a considerable increase in the amount of TAG in the autumn material acclimated to low temperatures and rhythmic light in the field. In contrast, the same treatment did not cause any increase in TAG in the spring material, acclimated to low temperatures and continuous light in the field. Results from experiments, in which moss cultivated for 4 months at 9°C on 12-h photoperiods (135μmol m⁻²s⁻¹) was kept for 3 weeks at low temperatures (9°C and −3°C) either in continuous light (135 or 70 μmol m⁻²s⁻¹) or with 12-h photoperiods (135 μmol m⁻²s⁻¹), indicated that the TAG level was higher at higher light intensity. At 9°C it was also higher in continuous light of both intensities than in rhythmic light. These results strongly suggest that decreasing irradiance and decreasing daylength limits the accumulation of TAG in D. elongatum during autumn in the subarctic.     

Seasonal changes in vertical distribution, biomass and faecal production of tubificids in the profundal region of a shallow Japanese lake

Two tubificid species Limnodrilus hoffmeisteri and L. claparedeianus formed more than 93% of the total number of oligochaetes in the profundal. Limnodrilus spp. worms were found down to 33 cm in the sediment but in great numbers in the upper zone in June and October. Worms confined to the top 15 cm of sediment accounted for 53-92% of the total number. There were two annual maxima in population density and biomass, one in late spring (66000 inds m⁻², 17 g wet wt m⁻²) and the other in mid autumn (97000 inds m⁻², 176 g wet wt m⁻²). Two regression lines describing the effect of temperature on faecal production rate were obtained; Log F = 0.0604 T (°C) −0.7660 (below 15°C), Log F = 0.0266 T – 0.2170 (above 15°C). In total 26.8 kg dry wt m⁻² of sediment was defecated annually by Limnodrilus spp. The sediment in the 0–10 cm stratum may pass through the guts of the worms 2.3 times a year. Sedimentation rates in profundal region were very low with respect to the faecal production rates of the tubificids.     

Phytoplankton ecology in an Antarctic lake

SUMMARY. The ecology of the phytoplankton of Heywood Lake, Signy Island, South Orkney Islands, Antarctica was investigated during 1969–72. The lake, which is ice-covered for 8–10 months per year, is moderately eutrophic due to enrichment by seal excreta.
The annual cycle of the phytoplankton is described. During the winter (approximately May-September), very few algal cells could be detected in the water column and ¹⁴C fixation was below measurable limits. In spring (October-November), a rapidly-growing population of algae caused a large increase in the chlorophyll- a concentration (maximum value 170 mg m⁻²) but carbon fixation remained low, with values <500 mg C m⁻² day⁻¹. The algae contributing to this peak were mainly small chlorophytes and chrysophytes. The summer open-water period (December-March) was characterized by a different phytoplankton population dominated by cryptophytes. Chlorophyll levels were lower ( c . 40 mg m⁻²) but ¹⁴C fixation rates >3 g C m⁻² day⁻¹ were measured on bright days. Values for Assimilation Number were very high (maximum value 10.5 mg C h⁻¹ mg⁻¹ (chlorophyll- a ) in January (1971) though temperatures never exceeded 8°C. In autumn, the phytoplankton regressed to winter levels. Both spring and summer algal populations probably overwinter as resting stages.     

Seasonal dynamics of invertebrate drift in a Hong Kong stream

Drift samples were taken with paired nets on 19 occasions over a 12-month period in Tai Po Kau Forest Stream (TPKFS), Hong Kong. Mean drift density (±1 S.E.) was 277·9 ± 25·0 individuals 100 m^-3; peaks in density were apparent during autumn and spring. One hundred and two taxa were recovered from the drift, and the total number of taxa drifting was positively related to water temperatures. Over 99% of the aquatic animals collected in drift samples were insects, 10 taxa of which constituted 67·3% of the entire catch. Baetid mayflies dominated the composition of the drift, comprising 40·4% of individuals caught.
Seasonal changes in the drift of individual taxa were evident, reflecting significant relationships between drift densities and water temperature: Simulium T₁ (Diptera). Anisocentropus maculatus (Trichoptera) and Amphinemura chui (Plecoptera) drifted most in winter, whereas Chimarra T₁, Polymorphanisus astictus (Trichoptera), Helodes #1 and cf. Rhantus sp. (Coleoptera) were most numerous in summer. Drifting mayflies showed spring ( Indobaetis sp., Cinygmina T₁, Serratella T₂), autumn ( Baetiella sp., Pseudocloeon T₂), or spring and autumn ( Baetis nr pseudofrequentus ) peaks which were not clearly related to water temperature. In only two cases ( A. maculatus and P. astictus ) was TPKFS drift seasonality associated with life-cycle events. Overall, there was no evidence of community-level trends in the periodicity of stream drift in this seasonal tropical habitat.     

Seasonal shifts in seagrass bed primary producers in a cold-temperate estuary: Dynamics of eelgrass Zostera marina and associated epiphytic algae

The seasonal dynamics of seagrass and epiphytic algal primary production were measured in an eelgrass (Zostera marina) bed in the Akkeshi-ko estuary, Hokkaido, Japan (43°02′N, 144°52′E). During spring and early summer, eelgrass biomass increased, with a high production (maximum: 2.89 g C m⁻² day⁻¹), but the production and biomass of epiphytic algae remained low. In contrast, epiphytic algae bloomed in August, with a high production (5.21 g C m⁻² day⁻¹), but eelgrass production ceased and its biomass subsequently decreased. Therefore, the major primary producers in this eelgrass bed switched seasonally from eelgrass in spring and early summer to epiphytic algae in late summer and autumn. Epiphytic algae maintained similar productivity because of the change of photosynthetic kinetics and the dominant epiphytic diatom changed from highly adhesive species to less adhesive or filamentous small species during the bloom. This suggests that the change of epiphyte density and biomass was due to change of its loss rate, possibly due to herbivorous grazing rate. Moreover, competition between epiphytic algae and eelgrass for nutrients and light may also affect the dramatic seasonal changes in the major primary producers.     

Effect of algal deposition on acetate and methane concentrations in the profundal sediment of a deep lake (Lake Constance)

Abstract In the profundal sediment ot Lake Constance (143 m depth) the temperature is constant at 4 °C. Despite the constant temperature, CH₄ concentrations changed with season between about 120 μM in winter and about 750 μM in summer, measured down to 30 cm depth. The acetate concentration profiles also varied between seasons. In summer, acetate concentration reached a maximum at about 100 μM in 2 or 4 cm depth. In winter, maximal concentrations of about 5 μM were observed over the entire depth. Input of organic material in late spring may be the reason for the seasonal change of both compounds. To simulate such a sedimentation event, intact sediment cores were covered with suspensions of Porphyridium aerugenium or Synechococcus sp. The addition of the phytoplankton material resulted in a drastic increase of acetate concentrations with a maximum at 2 cm depth, similar to in situ acetate concentrations measured in summer. The same applies for CH₄ for which increased concentrations were observed down to 6 cm depth. H₂ concentrations, on the other hand, showed no distinct increase. Treatment of intact sediment cores with ¹⁴C-labeled Synechococcus cells resulted in the formation of ¹⁴C-acetate, ¹⁴CH₄ and ¹⁴CO₂. Maximum concentrations of ¹⁴CH₄ were found at 4 cm depth, i.e. just above the depth to which ¹⁴C-acetate penetrated. The results show that phytoplankton blooms may cause a seasonal variation of acetate and CH₄ in profundal sediments of deep lakes despite the constant low temperature. They also indicate that acetate is the dominant substrate for methanogenic bacteria in the profundal sediments of Lake Constance.     

Effects of cooling water discharge on the structure and dynamics of epilithic algal communities in the northern Baltic

The Forsmark Biotest Basin is a shallow coastal ecosystem that receives brackish cooling-water discharge from a nuclear power plant. The effects of the discharge on epilithic algal communities were investigated by analysing samples taken every third week throughout one year at 11 sites differentially affected by temperature and/or flow rate enhancement. Community variation was summarized in a canonical correspondence analysis (CCA) of species abundances as a function of site and date. The temperature increase favoured blue-green algae at the expense of red and brown algae. Blue-green algae were however abundant in summer in stagnant water, whether heated or not, and some red and brown algae became abundant in winter in heated sites with flowing water. Green algae and diatoms increased in biomass in the heated sites, but not in relative cover-abundance. The absence of ice and snow cover at sites with heated and/or flowing water caused autumn species to persist into winter, because of the higher light intensity (compared with natural conditions) and the absence of the mechanical abrasion by ice. The thermal discharge lowered species diversity (Shannon-Weaver index) both in summer and winter at sites with flowing water, but not at sites with quiescent or stagnant water. CCA showed alternate periods of stability and rapid change within the seasonal cycle. Individual species were placed according to their optimum; red and brown algae in winter/spring, green algae in spring/summer, blue-green algae in summer, and diatoms at various times. Exceptions to this pattern were species endo- or epiphytic on species of a different group. Analysis of the effects of temperature, flow rate and ice cover on the seasonal pattern of particular species showed that different species respond in individualistic ways to different combinations of these environmental variables.     

Seasonal variation in foliar carbon exchange in Pinus radiata and Populus deltoides: respiration acclimates fully to changes in temperature but photosynthesis does not

We investigated seasonal variation in dark respiration and photosynthesis by measuring gas exchange characteristics on Pinus radiata and Populus deltoides under field conditions each month for 1 year. The field site in the South Island of New Zealand is characterized by large day-to-day and seasonal changes in air temperature. The rate of foliar respiration at a base temperature of 10 °C ( R ₁₀) in both pine and poplar was found to be greater during autumn and winter and displayed a strong downward adjustment in warmer months. The sensitivity of instantaneous leaf respiration to a 10 °C increase in temperature ( Q ₁₀) was also greater during the winter period. The net effect of this strong acclimation was that the long-term temperature response of respiration was essentially flat over a wide range of ambient temperatures. Seasonal changes in photosynthesis were sensitive to temperature but largely independent of leaf nitrogen concentration or stomatal conductance. Over the range of day time growth temperatures (5–32 °C), we did not observe strong evidence of photosynthetic acclimation to temperature, and the long-term responses of photosynthetic parameters to ambient temperature were similar to previously published instantaneous responses. The ratio of foliar respiration to photosynthetic capacity ( R _d/ A _sat) was significantly greater in winter than in spring/summer. This indicates that there is little likelihood that respiration would be stimulated significantly in either of these species with moderate increases in temperature – in fact net carbon uptake was favoured at moderately higher temperatures. Model calculations demonstrate that failing to account for strong thermal acclimation of leaf respiration influences determinations of leaf carbon exchange significantly, especially for the evergreen conifer.     

Seasonal changes in some photosynthetic properties of Ceratonia siliqua (carob tree) leaves under natural conditions

Rates of photosynthesis and leaf conductance of the leaves of carob trees ( Ceratonia siliqua L.) growing in natural conditions were measured during the course of the seasons to define the effects of the main climatic factors limiting growth in the region: temperature during the winter and water in the summer. The highest photosynthetic rates were measured in spring and autumn and could reach 25 μmol m₋₂ s₋₁ with optimal temperature and available water. Due to lower temperatures (4 to 6°C in the night) these values were frequently around 15 μmol m₋₂ s₋₁ during winter, but the strongest depression was due to prolonged drought in summer. However a reduction in photosynthesis rate down to 5 μmol m₋₂ s₋₁ occurred only after depletion of all the available water in the soil layer up to a depth of 50 cm. In the end of the summer, leaf conductance and water potential were in the order of 20 mmol m₋₂ s₋₁ and −3 MPa respectively. Compared to other trees that make up the Mediterranean sclerophyll forest, the photosynthetic activity of carob is high, and the tree tolerates a considerable depletion of soil water.     

Changes in stream benthic organic matter following watershed disturbance

Benthic organic matter was collected quarterly from streams draining a 9-yr-old clearcut, an 18-yr-old "old-field", a 25-yr-old successional forest, and two reference watersheds at Coweeta Hydrologic Laboratory in the Appalachian Mountains of North Carolina. USA. Samples were separated into large benthic organic matter (LBOM >1 mm) and fine benthic organic matter (FBOM <1 mm). An additional survey of large (>5 em diam.) and small (1–5 cm diam.) wood was conducted. Standing stocks of LBOM ranged from 124 to 235 g AFDM m⁻² (ash tree dry mass) and were significantly higher in streams draining reference watersheds and the successional forest than in either the recent clearcut or old-field. Reference sites exhibited LBOM peaks in late autumn and spring. No seasonal patterns were observed in disturbed streams. Standing stocks of FBOM averaged 113 to 387 g AFDM m⁻², and the stream draining the successional forest had significantly higher FBOM levels than the other sites. In reference streams, FBOM abundance peaked in spring. In disturbed streams, FBOM standing stocks were highest in summer or late autumn. Standing stocks of large wood ranged from 0 to 3956 g AFDM m⁻² and were significantly higher in the reference streams than in streams draining the old-field or successional forest. Small wood averaged 11 to 342 g AFDM m⁻² and was significantly lower in the stream draining the old-field than at the other sites. Comparisons of organic matter inputs with standing stocks indicated that disturbed streams at Coweeta receive less material and process it faster than reference streams. Disturbed streams also appear to be less retentive than reference streams and exhibit a gradual decline in FBOM during the winter when large, long-duration storms combined with low particle generation rates deplete accumulated FBOM.     

Seasonal variation in freezing tolerance of the New Zealand alpine cockroach Celatoblatta quinquemaculata

1. The cold hardiness of the alpine cockroach Celatoblatta quinquemaculata was investigated. This species is found at 1360 m a.s.l. beneath schist slabs on the Rock and Pillar Range (Central Otago, New Zealand). Cockroaches were collected monthly from January to December 1996, and their LT₅₀ and supercooling points determined.
2. Celatoblatta quinquemaculata was freezing tolerant throughout the year, with a lower lethal temperature in winter of – 8.9 °C. Celatoblatta quinquemaculata was also found frozen under rocks in the field when the under-rock temperature was below – 3 °C, and could survive being frozen at – 5 °C for 4 days in the laboratory.
3. There was a marked decrease in LT₅₀ temperature from – 5.5 °C in April to – 7.5 °C in May. This coincides with decreasing temperatures from summer through autumn to winter, during which temperatures beneath snow-covered rocks may reach – 7.3 °C.
4. Supercooling points fluctuated during the year, with an increase from – 4.2 °C in autumn to – 3.4 °C in winter. Supercooling point was highest in spring, and changes in supercooling point do not appear to be related to changes in LT₅₀.
5. Recordings of environmental temperatures from the Rock and Pillar Range suggest that cockroaches may undergo up to twenty-three freeze–thaw cycles in the coldest month of the year, and that they may remain frozen for periods of up to 21 h. Maximum cooling rates recorded in the field (0.01 °C min^–1) were 100-fold slower than laboratory cooling rates, so survival estimates from laboratory experiments may be underestimates.     

海南岛西北沿岸海域浮游桡足类的分布及群落特征

为了解昌江沿岸海域生态系统的现状, 探讨海域环境因素对浮游动物的生存环境造成的影响。本文根据2008年11月至2009年7月在海南西部昌江沿岸水域21个测站、4个季度月调查所获的浮游桡足类样品数据, 对该海域浮游桡足类群落结构、分布、季节变化及影响因素进行了分析。本调查共鉴定出桡足类44种, 隶属4目17科24属, 其中秋季25种, 冬季23种, 春季22种, 夏季23种。本次调查共发现优势种6种, 分别是微刺哲水蚤(Canthocalanus pauper)、亚强次真哲水蚤(Subeucalanus subcrassus)、锥形宽水蚤(Temora turbinata)、刺尾纺锤水蚤(Acartia spinicauda)、椭形长足水蚤(Calanopia elliptica)和精致真刺水蚤(Euchaeta concinna), 优势种以近岸暖水种居多。浮游桡足类丰度季节变化明显: 冬季最高, 达409 ind./m³; 秋季次之, 为144 ind./m³, 春季为55 ind./m³, 夏季最低仅为17 ind./m³。其丰度的平面分布显示: 秋、冬季节分别在海区中部和南部形成明显密集区, 春、夏季节则大致呈现由外海向近岸逐渐递减的趋势。浮游桡足类的多样性指数(H')表现为夏季>春季>秋季>冬季, 春、夏季的均匀度指数(J')明显高于秋、冬季。本调查反映出该海区的桡足类群落具有热带—亚热带区系特征, 种类组成季节更替明显, 桡足类种群受海域水温和硅藻的影响明显, 受盐度影响不明显。     

Planktic cyanophytes and their ecology in the shallow Lake Mikri Prespa, Greece

Cyanophytes dominated the phytoplankton of shallow Lake Mikri Prespa during the period May 1990 to September 1992 (76.5 and 52.0% of the total phytoplankton biomass in 1990–1991 and 1991–1992, respectively). Biomass peaks were observed in autumn (from 5.2 to 34.5 g m^-3) when low dissolved inorganic nitrogen, high phosphate phosphorus and low Z_cu to Z_mix ratio prevailed. The dominant species were Microcystis aeruginosa and M. wesenbergii. These represented 64 and 86% of the cyanophyte biomass in 1990–1991 and 1991–1992, respectively and revealed similar patterns of seasonality forming biomass peaks in late summer - autumn. Small chroococcalean cyanophytes (< 2 μm) showed also similar temporal distributions. Of the filamentous cyanophytes, the most important species was Anabaena lemmermannii var. minor which formed peaks in late summer and autumn (2.6 and 1.1 g m^-3 in 1990 and 1992, respectively). Rainfall and the N:P ratio were probably the main factors influencing the seasonality of all of the filamentous cyanophytes in the lake. Cyanophytes, tended to increase at temperatures higher than 16 °C and at inorganic nitrogen concentrations lower than 100 μg 1^--¹. The frequent mixing of the water column did not seem to prohibit the substantial increase of the group.     

Variable growth rates of the tropical estuarine fish barramundi Lates calcarifer (Bloch) under different freshwater flow conditions

Relationships between freshwater flows and growth rates of the opportunistic predatory finfish barramundi Lates calcarifer in a dry tropical estuary were examined using data from a long-term tag-recapture programme. Lagged effects were not investigated. After accounting for length at release, time at liberty and seasonal variation ( e.g. winter, spring, summer and autumn), growth rates were significantly and positively related to fresh water flowing to the estuary. Effects were present at relatively low levels of freshwater flow ( i.e. 2·15 m³ s⁻¹, the 5th percentile of the mean flow rate experienced by fish in the study during time at liberty). The analysis, although correlative, provides quantitative evidence to support the hypothesis that freshwater flows are important in driving the productivity of estuaries and can improve growth of species high in the trophic chain.