Nutrient dynamics in milpa agriculture and the role of weeds in initial stages of secondary succession in Belize,C.A.

Nutrient levels, soil moisture and soil organic matter were monitored monthly during a three year period in a site cleared for shifting agriculture (milpa) and in adjacement undisturbed high bush forest soils in Belize. Throughout the study there was no evidence of a decline in available nutrient levels in the cleared site. Sharp changes in P and Mn were considered indicative of wetting and drying periods. Corn yields for the rainy season crop in the first two years were average for the region ca 2500 kg ha⁻¹ dry wt, but declined dramatically in the third year. Total weed biomass for the rainy season crop conversely increased every year and reduced grain yield probably through competition for nutrients (especially N), light and water. Substantial amounts of nutrients were also immobilized in the weed biomass, which served the agroecological purpose of preventing nutrient loss by leaching.     

Spatial and temporal variation in fish community structure of a marine embayment in Zanzibar,Tanzania

Spatial and temporal variation in the fish community structure were studied in a tropical non-estuarine embayment in Chwaka Bay, Zanzibar (Tanzania). Fish samples were collected bi-monthly (at each spring low tide) for 1 year (November 2001–October 2002) from a range of bay habitats ranging from mangroves deep inside the bay to seagrass beds close to the mouth of the bay. Additionally, environmental variables were examined to determine their relationship with the fish community structure. Being a non-estuarine embayment, the environmental variables as well as the fish community structure in each habitat remained relatively constant for most part of the year; however, a marked decline was observed during the rainy period (April–May). Significant variations in fish community variables (density, biomass and species richness) and in water temperature and salinity were observed during the rainy season in all habitats, with larger changes in the mangrove and mud/sand flats habitats than in the seagrass beds. Seasonal variations in water clarity and dissolved oxygen were not significant, though. Many species disappeared from the mangrove and mud/sand flats habitats during the rainy season and those which persisted showed a remarkable decrease in density. Moreover, the results indicate that mangroves were the preferred settling habitats for Gerres filamentosus, Gerres oyena, Lethrinus lentjan and Monodactylus argenteus, especially during the dry period (December–February) before the rainy season. This observation is contrary to what has been reported from some other tropical regions where greater abundance and species richness was observed during the rainy season. A significant relationship was found between density of fish and temperature, salinity and turbidity. Since salinity was the most conspicuously changing environmental variable with seasons, we propose that salinity, alone or in combination with low visibility and temperature, was probably the most important environmental factor structuring the fish assemblage in the mangrove and mud/sand flats habitats, particularly during the rainy season. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: J. A. Cambray     

The fate of traditional extensive (gei wai) shrimp farming at the Mai Po Marshes Nature Reserve, Hong Kong

Extensive shrimp farming around Deep Bay, Hong Kong,began in the mid-1940‘s after the construction of intertidal ponds (gei wai) among the coastal mangroves. The ponds are increasingly being seen as an example of how wetlands can be used sustainablysince they are naturally stocked with shrimp postlarvae (e.g. Metapenaeus ensis) and young fish (e.g. Mugil cephalus) flushed into the ponds from Deep Bay. Once inside, these shrimps and fish feed on naturally occurring detritus on the pond floor. The only gei wai remaining in the Territory, are those at the WWF Hong Kong Mai Po Marshes Nature Reserve, adjacent to Deep Bay. Analysis of the shrimp production between 1990–1995 showed that there were two seasonal peaks, from April–June (Recruitment-I)and from July–October (Recruitment-II). The second peak was significantly lower than the first (p <0.001), especially from those gei wai in the southern part of the reserve which are much closer to a polluted river. The average harvest from each gei wai had also significantly declined from40.9 ±6.0 kg ha⁻¹ yr⁻¹ in 1990 to15.1 ±3.6 kg ha⁻¹ yr⁻¹ in 1995 (p <0.01). This decline can be attributed to the abundance of predatory fish in the gei wai, and increasing water pollution in Deep Bay which adversely affects the amount of shrimp larvae for stocking the gei wai, as well as the quality of water for flushing the ponds during the rearing and harvesting seasons. Despite this, those gei wai which are not-commercially viable can still support many non-commercial, more pollution tolerant fish and shrimp species. As a result, the management of the segei wai has been altered such that their objective is to provide feeding habitat for piscivorous waterbirds, which is also in line with the aims of the nature reserve. This revised version was published online in August 2006 with corrections to the Cover Date.     

Factors affecting seasonal variations in demersal fish assemblages at an ecocline in a tropical–subtropical estuary

Seasonal changes of fish species composition in terms of biomass, density and number of species in three areas of the main channel of the Paranaguá Estuary (axis east–west) are described in relation to seasonal fluctuations in salinity, water temperature and dissolved oxygen in the main channel. Two hundred and thirty‐four samples were collected monthly, between July 2000 and June 2001, in the main channel. Seventy‐nine species of 29 families were captured with a total estimated mean density and biomass of 1513 individuals ha^?1 and 34 kg ha^?1, respectively. The number of species and total mean density differed significantly among areas and seasons, but the total mean biomass differed only significantly throughout the ecocline (areas) of the Paranaguá Estuary. For the most abundant species, the mean densities of Stellifer rastrifer, Aspistor luniscutis, Menticirrhus americanus, Sphoeroides testudineus, Cynoscion leiarchus and Symphurus tesselatus (with the exception of Cathorops spixii and Genidens genidens) differed significantly among seasons. The mean biomass of these species, with the exception of G. genidens, S. rastrifer, A. luniscutis and S. testudineus, also differed significantly for the factor seasons. Area was a significant factor for the eight most abundant species (density and biomass), except S. testudineus (density), G. genidens, C. leiarchus and S. tesselatus (biomass). The season v. area interaction term was significant for C. leiarchus (density). Most of these differences occurred during the rainy season when fishes concentrated principally in the middle of the estuary, where the salinity remained stable. It is suggested that the salinity stability in the middle of the estuary is the main reason why the most estuarine resident fish species move downstream and remain there, regardless of the increased freshwater runoff. Moreover, canonical correspondent analysis output detected that during the late rainy season, the variable dissolved oxygen (P < 0·01) was the most important environmental variable, responsible for structuring patterns of fishes assemblages in the west–east axis of Paranaguá Estuary. During the end of the dry season, both salinity (P < 0·01) and dissolved oxygen (P < 0·05) were responsible for this ecological feature in the estuary. Finally, it was possible to detect that juveniles and adults of some important species respond differently to seasonal fluctuations of the ecocline‐determining environmental factors. This behaviour is suggested as a strategy to avoid competition and predation during the rainy season in the middle estuary. The Paranaguá Estuary did not fit with the pre‐existing models described in the tropical and subtropical estuarine fish literature since its main channel fish assemblages remained within its bounds even during the rainy season.     

Flux of organic carbon in a riverine mangrove wetland in the Florida Coastal Everglades

Short-term (daily) and seasonal variations in concentration and flux of dissolved organic carbon (DOC) were examined over 15 tidal cycles in a riverine mangrove wetland along Shark River, Florida in 2003. Due to the influence of seasonal rainfall and wind patterns on Shark River’s hydrology, samplings were made to include wet, dry and transitional (Norte) seasons. We used a flume extending from a tidal creek to a basin forest to measure vertical (vegetated soil/water column) and horizontal (mangrove forest/tidal creek) flux of DOC. We found significant (p < 0.05) variations in surface water temperature, salinity, conductivity, pH and mean concentration of DOC with season. Water temperature and salinity followed seasonal patterns of air temperature and rainfall, while mean DOC concentration was highest during the dry season (May), followed by the wet (October) and ‘Norte’ (December) seasons. This pattern of DOC concentration may be due to a combination of litter production and inundation pattern of the wetland. In contrast to daily (between tides) variation in DOC flux between the mangrove forest and tidal creek, daily variations of mean water quality were not significant. However, within-tide variation of DOC flux, dissolved oxygen content and salinity was observed. This indicated that the length of inundation and water source (freshwater vs. saltwater) variation across tidal cycles influenced water quality and DOC flux in the water column. Net DOC export was measured in October and December, suggesting the mangrove forest was a source of DOC to the adjacent tidal creek during these periods. Net annual export of DOC from the fringe mangrove to both the tidal creek and basin mangrove forest was 56 g C m⁻² year⁻¹. The seasonal pattern in our flux results indicates that DOC flux from this mangrove forest may be governed by both freshwater discharge and tidal range.     

Environmental Variables and Planktonic Communities in Two Ponds of El Hondo Wetland (SE Spain)

The annual cycle of physical and chemical variables and plankton dynamics was studied in two shallow ponds (East and West Ponds) of the El Hondo wetland, an ecosystem of international importance. Water conductivity increased up to 31–49 mS cm^–1 as water level decreased due to high evaporation and minimal water inputs. Initially considered mesohaline, the waters were reclassified as polyhaline during the hot season. EP was subject to successive desiccation‐flooding cycles, and flooding of the dried sediment caused the release of high concentrations of nitrogen and phosphorus compounds, which quickly depleted. The algal species composition was typical of eutrophic ecosystems, and the chlorophyll content indicated that EP was eutrophic and WP mesotrophic. Phytoplanktonic species richness and diversity were low in both ponds. Algal assemblages, in terms of biovolume, were mainly dominated by Dinophyceae in EP and by Cryptophyta in WP. The zooplankton community was dominated by Rotifers (Brachionus and Hexarthra), although Copepods and Ciliates were also important. Different water inputs to the ponds, partial drying in EP during the warm season with the subsequent higher increment of salinity, and the presence of dense populations of submerged macrophytes in WP, explain the differences in plankton communities found between the two ponds.     

A recently landlocked brackish-water lagoon of Lake Tanganyika: physical and chemical characteristics,and spatio-temporal distribution of phytoplankton

Monthly measurements of physical and chemical characteristics were made at two localities in the eastern part of a recently landlocked lagoon of Lake Tanganyika. Variables analysed were: temperature, pH, conductivity, sodium, potassium, magnesium, calcium, carbonate, bicarbonate, chloride and sulphate. Large seasonal fluctuations of salinity were recorded (1.68–8.21 g l^–1). The seasonal water input controlled algal seasonality mainly through its effect on salinity and indirectly through its influence on nutrient concentration by dissolution and dilution of the excrements of the numerous cattle and other organic matter. Phytoplankton was mainly composed of Cyanophyta and Euglenophyta. Euglenophyta dominated during the dry periods with high salinity and probably very high nutrient levels, while Cyanophyta preferred moderate salt and nutrient concentrations during the rainy periods. The phytoplanktonic community was composed of a large number of perennial and a reduced quantity of annual organisms.A spatial study of the recently landlocked lagoon revealed an ascending salinity gradient, principally due to a sodium bicarbonate/carbonate enrichment, between locations near the lake and more inland situated stations. These facts point to a lake water supply and a salt concentration by evaporation in the swamps. Proportionally lower magnesium, calcium and potassium values were recorded at high salinities, due to chemical precipitation and biotic factors.A blue-green algal bloom was observed in the eastern water-body (salinity: 4.64 g l^–1); simultaneously an important development of diatoms dominated the western water-body (salinity: 2.18 g l^–1). No significant differences in morphometry, exposure, water temperature or nutrient levels (nitrate, nitrite, ammonia, orthophosphate) were observed. The relatively low salinity and high nutrient concentration in the western water-body probably favoured diatom development during the rainy season. The relatively higher salinity in the eastern water-body during the rainy season was probably responsible for the dominance of blue-green algae through its negative influence on silica concentration and notwithstanding the high inorganic nitrogen concentration.     

Ecology and distribution of diatoms in Biscayne Bay,Florida (USA): Implications for bioassessment and paleoenvironmental studies

The spatial and temporal distribution of planktonic, sediment-associated and epiphytic diatoms among 58 sites in Biscayne Bay, Florida was examined in order to identify diatom taxa indicative of different salinity and water quality conditions, geographic locations and habitat types. Assessments were made in contrasting wet and dry seasons in order to develop robust assessment models for salinity and water quality for this region. We found that diatom assemblages differed between nearshore and offshore locations, especially during the wet season when salinity and nutrient gradients were steepest. In the dry season, habitat structure was primary determinant of diatom assemblage composition. Among a suite of physicochemical variables, water depth and sediment total phosphorus (STP) were most strongly associated with diatom assemblage composition in the dry season, while salinity and water total phosphorus (TP) were more important in the wet season. We used indicator species analysis (ISA) to identify taxa that were most abundant and frequent at nearshore and offshore locations, in planktonic, epiphytic and benthic habitats and in contrasting salinity and water quality regimes. Because surface water concentrations of salts, total phosphorus, nitrogen (TN) and organic carbon (TOC) are partly controlled by water management in this region, diatom-based models were produced to infer these variables in modern and retrospective assessments of management-driven changes. Weighted averaging (WA) and weighted averaging partial least squares (WA-PLS) regressions produced reliable estimates of salinity, TP, TN and TOC from diatoms (r² = 0.92, 0.77, 0.77 and 0.71, respectively). Because of their sensitivity to salinity, nutrient and TOC concentrations diatom assemblages should be useful in developing protective nutrient criteria for estuaries and coastal waters of Florida.     

Microbial Flora of Pond-Reared Brown Shrimp (Penaeus aztecus)

Agar plate counts and microbial types are reported for brown shrimp reared in 2-acre natural marshland and in 0.5-acre artificial ponds during June to October 1970. Bacterial counts of pond-reared shrimp ranged from 5 × 10⁴ to 5.5 × 10⁶ per g. At final harvest in October, bacterial counts ranged from 2 × 10⁵ to 5.5 × 10⁶ per g. In marsh ponds, bacterial counts of shrimp and pond water were lowest in August when both water temperature and salinity were high. Coryneform bacteria and to a lesser extent Vibrio were the predominant isolates from fresh pond shrimp. Shrimp stored at 3 to 5 C for 7 days were acceptable as judged by appearance and odor. Between 7 and 14 days of refrigerated storage, bacterial counts increased sharply and about 50% of the samples became unacceptable. Refrigerated storage of pond shrimp caused increases in coryneform bacteria and micrococci and decreases in Vibrio, Flavobacterium, Moraxella, and Bacillus species. Pseudomonas species were not significant in fresh or stored pond shrimp. The microbial flora of pond water usually was dominated by coryneform bacteria, Flavobacterium, Moraxella, and Bacillus species.     

Species assemblages of cyclopoid Copepoda from Mantecal,Venezuela

A 14 year monitoring of species composition and abundance of cyclopoid copepods throughout the rainy and dry seasons in a flooded savanna at Mantecal, Apure State, Venezuela, is presented. Two characteristic habitats within these water bodies were selected: the open center of small ponds free of emergent vegetation and the surrounding flooded grassland. In total, 13 species were registered, all of which appeared in the flooded grassland and 11 of which occurred in the open water. Ten species were common for the two habitats during the dry season. The highest abundance was found in flooded grassland during the dry season. Eight species showed abundances greater than 1.0 individual per liter in flooded grassland during both seasons; 3 and 7 species were this abundant in the open water during rainy and dry seasons, respectively. Mesocyclops meridianus and Microcyclops varicans were the most abundant species in both habitats and seasons, while Thermocyclops decipiens was the dominant species during rainy season in open water. Principal component analysis indicates that the important species were positively correlated between them (sharing habitat and season).     

Bacterial Productivity Distribution During a Rainy Year in an Estuarine System

Bacterial density and productivity were investigated along four salinity gradients within the estuary Ria de Aveiro. Bacterial variables and environmental parameters were measured at three to four stations spanning the entire salinity gradient of the four channels. The rather high variation in bacterial productivity (0.16–7.6 μg C L⁻¹ h⁻¹) along the profiles of salinity indicates that bacterial activity shows a reactive behavior to environmental changing. Bacterial density (0.5–11.2 × 10⁹ cells L⁻¹) with a comparative smaller variation showed a more conservative behavior, mainly reflecting the phytoplankton distribution. Contrary to expectation, minimal values of bacterial productivity were not observed in November–December but in June. In fact, in November–December, the deep zone near the mouth showed the highest values of bacterial activity. At the upper stations, the highest values were observed in October. The relatively high values of bacterial production during the cold rainy season suggest that allochthonous substrates leached out from the surroundings by rain controlled the distribution of bacterial activity in the estuarine system. The substantial decrease in salinity during the rainy season negatively affected bacterial productivity, namely in the marine zone, where water column was highly stratified. Salinity seems to play an indirect role in the regulation of estuarine bacteria because there are different bacterial communities adapted to a wide salinity range.     

Solute dynamics in a North Brazilian mangrove: the influence of sediment permeability and freshwater input

Although water in mangrove sediments influences nutrient cycling in both, mangrove forest and estuary, little information exists on seasonal and vertical distribution of dissolved organic and inorganic compounds in the sediment column. We studied the influence of sediment texture and chemistry, permeability (K), tides, and rainfall on dissolved organic carbon (DOC) and nitrogen (DON), dissolved inorganic phosphate (DIP) and salinity in creek and sediment waters of a mangrove in Pará, Brazil. Water samples were taken from boreholes and piezometers in the mangrove forest and from an adjacent tidal creek at neap and spring tides, during the dry and rainy season. Forest sediment was analysed for carbon (C), nitrogen (N), salinity and permeability. Clay, C and N decreased with depth. Sediment permeability (K) was lowest (<0.1 m day⁻¹) in the upper, clay-rich and crab-burrow-free mud layer. In the deeper, fine sand strata, K ranged from 0.7 to 1.8 m day⁻¹. Tidal range in the creek was 3.5 and 5.5 m for neap and spring tides, respectively. Salinity, DOC, DON and DIP in creek water were inversely related to tidal height. Piezometer data revealed significant water level changes in deeper, sandy sediment layer, which followed, time-lagged, the tidal fluctuations. In contrast, tide did not affect the water level in the upper sediment due to low permeability. Compared with creek water, sediment water was enriched in DOC, DON and DIP because of organic matter input and mineralization. In deeper layers, solute concentration was most likely affected by sorption processes (DOC and DIP) and reduction reactions (DIP). During the rainy season, DOC and DON in creek and sediment water were higher than in the dry season. DIP appeared invariant to seasonal changes. In the rainy season, salt flushing from surface sediments resulted in higher salinities at intermediate sediment depths, while in the deeper layers salinity was lower due to exchange with water from the tidal creek.     

Seasonal variations in growth, condition and gonads of Dormitator latifrons (Richardson) in the Chone River Basin, Ecuador

Growth in length, condition, and gonads of a food fish, Dormitator latifrons , were studied in the Chone River basin, Ecuador, in 1981. The river was bordered by floodplains in the upstream freshwater zone, and by mangrove swamps and shrimp farms in the downstream estuarine zone. The climate was marked by wet (January to April) and dry (May to December) seasons. During the dry season, an earth dam in the river prevented movement of water and fish between upstream and downstream zones. At the end of the dry season, most of the upstream floodplains were dry, and the main fish refuges were in downstream areas in deep pools in the river upstream. During the floods, fish migrated from downstream areas towards upstream floodplains. Growth rates and condition increased when water levels were high or salinity was low and decreased when water levels were low or salinity was high. Seasonal changes in gonads and abundance of juveniles indicated that reproduction occurred during the floods, but there was some reproduction in the dry season. Reproduction occurred in upstream and downstream zones and appeared to be stimulated by a complex of factors, including water levels, currents and salinity. The yield in flood plain sites was estimated as c. 115 kg ha⁻¹ in 1981.     

Microcystin in plants that treat water for human consumption in a tropical environment: the Metropolitan Area of Costa Rica

We measured microcystin levels in water of the Metropolitan Area of Costa Rica by competitive inhibition ELISA and we quantified total coliforms, fecal coliforms, Escherichia coli (by a Most Probable Number method) and aerobic count. We wanted to identify any cyanotoxin correlation with these parameters, as a public health risk. We sampled in the rainy season of 2003 (April-October) and in the dry season of 2004 (February-March) (30 samples/season). We sampled pre-treated, semi-treated and treated water. Microcystin levels < 0.5 ppb were found in the rainy season (and > 0.5 ppb in the dry season). Dry season levels exceeded World Health Organization limits (1.0 ppb). Cyanotoxins occurred in the Tres Rios plant. We did not find a correlation between these microbiologic parameters of water quality and microcystin levels in water.     

Soil erosion and deposition effects on surface characteristics and pearl millet growth in the West African Sahel†

It is well known that surface mulched crop residues (CR) lead to large yield increases of pearl millet (Pennisetum glaucum L.) on acid sandy soils of the West African Sahel. This effect is generally attributed to mulch-induced changes in chemical properties of the surface soil and the protection of millet seedlings from erosive sand storms. However, previous research has failed to separate the anti-erosive effects of CR on plant growth from chemical effects due to the release of nutrients during CR decomposition. To this end a mulching trial with surface applied millet stalks at a rate of 2000 kg ha^-1, an equivalent 10% surface coverage obtained by inert polyethylene (PE) tubes and a bare control treatment was conducted from 1992 to 1994 on an acid sandy soil in southwest Niger. Across treatments, sand flux at 0.1 m height was more than twice as high in the rainy seasons than in the dry months and mulching reduced sand flux by between 25 and 50% during rainy season storms compared with 67% during the dry season. Over the 21 months measurement period, cumulative erosion by wind and water was almost 270 t ha^-1 of soil in unmulched control plots. In mulched plots, in contrast, between 160 and 200 t ha^-1 of soil was deposited. Surface soil temperature at 0.01 m depth reached above 40 °C in bare plots but was up to 4 °C lower with CR. Mulch reduced soil penetration resistance at 0–0.02 m and 0–0.05 depth by more than half and decreased runoff leading to higher water contents at flowering and grain filling in the upper 0.3 m soil layers in 1993 and throughout the entire profile in 1994, a year with particularly high rainfall. Both mulch types were similarly effective in increasing final stand density of millet in the first two years between 5 and 23% compared with bare control plots. Relative to the bare control CR mulch effects on total dry matter of millet at harvest increased from 35% in 1992 and 108% in 1993 to 283% in 1994, whereas PE mulch led to respective relative increases in dry matter of only 6, 44 and 13%. In 1992 and 1993, CR mulch increased total nutrient uptake of millet at harvest by between 34 and 86% for nitrogen (N), between 31 and 162% for P and between 56 and 126% for potassium (K). These differences were mostly the result of differences in total dry matter and only to a smaller part due to changed nutrient concentrations in plants. This revised version was published online in June 2006 with corrections to the Cover Date.     

Film mulch with irrigation and rainfed cultivations improves maize production and water use efficiency in Ethiopia

Improving productivity of maize (Zea mays L.) and water use efficiency is of great significance for agriculture in Ethiopia. In this study, the effects of ridge‐furrow with film mulch cultivation were tested on maize yields in Melkassa, Ethiopia. Three field experiments (drip irrigation, furrow irrigation and rainfed) were conducted each with randomised complete block design with three replicates. The drip irrigation experiment was conducted in the dry season and constituted three film mulch methods (non‐mulch, transparent film mulch and black film mulch) with three irrigation levels (357, 435 and 515 mm). The furrow irrigation experiment was also conducted in the dry season and constituted two film mulches (non‐mulch and transparent film mulch) with three irrigation levels (484, 674 and 865 mm). The rainfed experiment was conducted in the rainy season and constituted three mulches (non‐mulch, transparent film mulch and black film mulch) with two farming methods (ridge‐furrow farming and flat farming). In the drip irrigation experiment, the highest maize yields (5.9 ± 0.6 t ha^?1) and irrigation water use efficiency (9.6 ± 1 kg ha^?1 mm^?1) were recorded in the treatment using black film mulch with high irrigation, with increases of 68% and 68.4% compared to using non‐mulch treatment at that irrigation level. In the furrow irrigation experiment, maize yields and irrigation water use efficiency reached 7 (± 0.8) t ha^?1 and 9.1 (± 1.9) kg ha^?1 mm^?1 in the treatment using transparent film mulch with medium irrigation (674 mm), with increases of 46% and 46.8% compared to that with non‐mulch treatment. In the rainfed experiment, the film mulch rather than farming method had positive effects on the maize yields and rainwater use efficiency. The average maize yield reached 8.5 (± 0.7) t ha^?1 in the film mulch treatments, with an increase of 39% than using the non‐mulch treatment. Compared with that of non‐mulch treatment, the net income in the film mulch treatments increased by 94% in the furrow experiment and 31% in the rainfed experiment. Our results indicate that the ridge‐furrow with film mulch system can be recommended for water‐saving irrigation with low cost in dry seasons, and film mulch with flat farming can be recommended in rainy seasons for maize production in Ethiopia. This study provides strong evidence that maize productivity can be effectively improved in Ethiopia and other similar areas of the world using this simple and cost‐effective technology.     

Temporal patterns in the intertidal faunal community at the mouth of a tropical estuary

The use of intertidal sandy beaches by fish and macrocrustaceans was studied at different temporal scales at the mouth of a tropical estuary. Samples were taken along the lunar and diel cycles in the late dry and rainy seasons. Fish assemblage (number of species, density and biomass), crustaceans and wrack biomass, showed significant interactions among all studied factors, and the combination of moon phase and diel cycle, resulting in different patterns of environmental variables (depth, water temperature and dissolved oxygen), affected habitat use by the different species. Variances in faunal community were detected between seasons, stimulated by salinity fluctuations from freshwater input during the rainy season. These differences suggest an important cycling of habitats and an increase in connectivity between adjacent habitats (estuary and coastal waters). Moreover, the results showed that this intertidal sandy beach also provides an alternative nursery and protected shallow‐water area for the initial development phase of many marine and estuarine species. In addition, this intertidal habitat plays an important role in the maintenance of the ecological functioning of the estuarine–coastal ecosystem continuum.     

Environmental capacity of receiving water as basis for regulating intensity of milkfish (Chanos chanos Forsskal) culture

This paper presents the results of the assessment of the environmental condition of a receiving water and demonstrates how the environmental capacity of the system can be estimated based on effluent discharge from milkfish ponds and water criteria from scientific literature and other studies. An estuary (average volume, 295 333 m³; average depth, 1.9 m) in Punta Pulao, Dumangas, Iloilo, Philippines served as discharge and irrigation system for commercial milkfish ponds and experimental/verification ponds owned by the Southeast Asian Fisheries Development Center. Total ammonia nitrogen, nitrite, nitrate, phosphate, and chlorophyll a (chl a) were determined monthly (during spring tide) for 4 months. Samples were taken at low and high tides that coincided with the draining and flooding of ponds, respectively. Monthly concentrations of these water quality variables generally increased from the first to fourth month of sampling (April–July 2001) during low tide. Very slight increases were observed during high tide. The magnitude of changes in the diurnal patterns of phosphate, chl a, and dissolved oxygen appeared to be higher at spring tide than at neap tide. This indicates that the inflowing river at low tide (during spring tide) brought effluents containing high amounts of nutrients from ponds located upstream. The water, salt, dissolved inorganic nitrogen (DIN), and phosphate (DIP) budgets of the entire estuary were determined following the one‐box model by LOICZ‐IGBP (2000, LOICZ biochemical budgeting procedure: a tutorial pamphlet. L. T. David, M. L. San Diego‐McGlone, C. J. Crossland and S. V. Smith (Eds). Publ. for LOICZ International Project Office, the Netherlands, 29 pp.). The budgets indicate that the system is net heterotrophic and nitrogen (N) fixing during the dry months, but that there could be no environmental impact during the rainy months because of short water exchange time. Results further suggest that the system is a source of N and phosphorus (P) during the dry and rainy months; the condition is largely influenced by the high amount of nutrient inputs in to the river. Linear regression analysis was performed to determine relationship between nutrient concentrations in the system and total DIN or DIP input to the system at low tide. Environmental capacity in terms of the maximum amount of DIN or DIP input to the system was predicted using regression analysis and following set criteria for nutrients, i.e. nitrite, nitrate, and phosphate. At present, the estuarine water quality has already reached its environmental capacity during the dry months. About 945 ha of commercial milkfish ponds are operating upstream, mostly as extensive systems. If these ponds are converted to semi‐intensive or intensive systems, it is recommended that the pond area be reduced to 122 ha if the DIP criterion is to be followed so as not to exceed the environmental capacity. Exceeding this environmental capacity may affect production through reduction of fish growth, occurrence of diseases, and fish mortalities.     

Influence of mulching on the pattern of growth and water use by spring wheat and moisture storage on a fine textured soil

Eight tonnes ha^–1 of stubble were used to mulch spring wheat (Triticum aestivum) on a fine textured soil with the aim of controlling both transpiration and soil evaporation during the wet pre-anthesis phase to increase moisture supply during grain filling in the eastern wheatbelt of Western Australia. Mulching reduced leaf area per plant by reducing the culm number; consequently the green area index was reduced. Reduced culm number was associated with low soil temperature which at 50 mm depth averaged 7°C lower under the mulched crop relative to the control crop in mid-season. The smaller canopies of the mulched crop used 15 mm less water than those of the control before anthesis; this difference in water-use was due equally to reduced transpiration and soil evaporation. However, the mulched crop was unable to increase ET during grain filling, a response associated with the persistence of low soil temperature for most of the growth period. Hence, total ET for the season was significantly lower (18 mm) under the mulched crop than the control crop. At harvest, mulching did not have significant effects on total above-ground dry matter and grain yields, but it increased water use efficiency for grain yield by 18%, grain weight by almost 17% and available moisture in both uncropped and cropped plots by an average of 43 mm.To determine whether there was any residual effects of soil treatment on moisture storage during the summer fallow period, soil moisture was monitored both in cropped plots and uncropped plots, that were either mulched or unmulched during the growing season, from harvest in October 1988 until next planting in June 1989. Available moisture at next planting was correlated with moisture storage at harvest despite the differences in run-off, soil evaporation and fallowing efficiency (increase in moisture storage as a percentage of rainfall) between treatments during fallowing. Therefore, the mulched treatments had more moisture available (30 mm), mostly as a result of less water use during cropping in the previous growing season, than the unmulched treatment.The study shows that mulching may be used to restrain both transpiration and soil evaporation early in the season to increase availability of soil moisture during grain filling. Secondly, mulching during the previous growing season had little effect on soil moisture during the summer fallow period, however, the moisture saved by mulching during cropping was conserved for the following season. These results indicate the importance of evaluating mulching of winter crops in terms of crop yield in the subsequent growing season as well as in the current season in which the soil was treated.Abbreviations D through drainage - DAS days after sowing of the crop on 31 May 1988 - DM dry matter produced in the above-ground portion of the crop (kg ha^–1) - E₀ evaporation from Class A pan (mm) - E_s evaporation from uncropped soil (mm) - E_sc evaporation from soil beneath the wheat canopy (mm) - ET evapotranspiration (mm) - FE fallowing efficiency (gain in soil moisture storage/rainfall) - GAI green area index (area of green vegetation per unit land area) - GWUE water-use efficiency for grain production (grain yield/total ET, kg ha^–1mm^–1) - K extinction coefficient (see equation 1) - RO run-off of moisture from soil surface during/following rainfall (mm) - SM available soil moisture (mm) at harvest (SMh) or at planting (SMp) - WUE water-use efficiency for total above-ground dry matter yield (see GWUE)     

Physiological ecology of a tropical dragon,Lophognathus temporalis

Lophognathus temporalis is an arboreal lizard from the wet–dry tropics of Australia. During the wet season the field metabolic rate (FMR) of the lizards was 209 kJ kg^?1 d^?1, but during the dry season FMR was only 62 kJ kg^?1 d^?1. Similarly, water flux decreased from 73.6 mL kg^?1 d^?1 in the wet season to 18.5 mL kg^?1 d^?1 in the dry season. Body temperatures (T_b) were significantly lower in the dry season, and operative temperatures, calculated by incorporating microclimatic data with characteristics of the lizards, indicated that the seasonal shift was due to changes in thermoregulatory behaviour rather than limitations of the thermal environment. By combining field measurements of T_b and FMR with laboratory measurements of standard metabolic rate over a range of T_b, we were able to subdivide the FMR into its components and to determine which factors contributed to the seasonal reduction in energy expenditure. During the dry season, lizards used 147 kJ kg^?1 d^?1 less energy than during the wet season, and 24% of this decrease was estimated to be due to the passive effects of lower nighttime T_b, 14% was due to the active selection of lower daytime T_b, 27% was due to the physiological shift to lower standard metabolic rates, and 35% was due to reduced activity in the dry season. Although the population size remained relatively constant (107 lizards ha^?1 during the wet season and 125 lizards ha^?1 during the dry season), the population structure changed, reflecting the seasonal patterns of recruitment and mortality. The number of lizards active at any one time was much lower in the dry season, reflecting the lower levels of activity in this season. The energy expenditure of the population of L. temporalis was 612 kJ ha^?1 d^?1 during the wet season and 113 kJ ha^?1 d^?1 during the dry season.