Spatial and seasonal patterns of periphyton biomass and productivity in the northern Everglades,Florida, U.S.A.

We sampled periphyton in dominant habitats at oligotrophic and eutrophic sites in the northern Everglades during the wet and the dryseasons to determine the effects of nutrient enrichment on periphytonbiomass, taxonomic composition, productivity, and phosphorus storage. Arealbiomass was high (100–1600 g ash-free dry mass [AFDM]m⁻²) in oligotrophic sloughs and in stands of the emergentmacrophyte Eleocharis cellulosa, but was low in adjacent stands of sawgrass,Cladium jamaicense (7–52 g AFDM m⁻²). Epipelon biomasswas high throughout the year at oligotrophic sites whereas epiphyton andmetaphyton biomass varied seasonally and peaked during the wet season.Periphyton biomass was low (3–68 g AFDM m⁻²) and limitedto epiphyton and metaphyton in open-water habitats at eutrophic sites andwas undetectable in cattail stands (Typha domingensis) that covered morethan 90% of the marsh in these areas. Oligotrophic periphytonassemblages exhibited strong seasonal shifts in species composition and weredominated by cyanobacteria (e.g., Chroococcus turgidus, Scytonema hofmannii)during the wet season and diatoms (e.g. Amphora lineolata, Mastogloiasmithii) during the dry season. Eutrophic assemblages were dominated byCyanobacteria (e.g., Oscillatoria princeps) and green algae (e.g., Spirogyraspp.) and exhibited comparatively little seasonality. Biomass-specific grossprimary productivity (GPP) of periphyton assemblages in eutrophic openwaters was higher than for comparable slough assemblages, but areal GPP wassimilar in these eutrophic (0.9–9.1 g C m⁻²d⁻¹) and oligotrophic (1.75–11.49 g C m⁻²d⁻¹) habitats. On a habitat-weighted basis, areal periphytonGPP was 6- to 30-fold lower in eutrophic areas of the marsh due to extensiveTypha stands that were devoid of periphyton. Periphyton at eutrophic siteshad higher P content and uptake rates than the oligotrophic assemblage, butstored only 5% as much P because of the lower areal biomass.Eutrophication in the Everglades has resulted in a decrease in periphytonbiomass and its contribution to marsh primary productivity. These changesmay have important implications for efforts to manage this wetland in asustainable manner. This revised version was published online in July 2006 with corrections to the Cover Date.     

Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.     

High within-stream replication is needed to predict litter fluxes in wet–dry tropical streams

1. Streams draining forested landscapes are fuelled by terrestrial plant litter, which can be transported downstream or retained and broken down locally. However, fluxes of plant litter in streams can vary at multiple spatio-temporal scales, affecting the availability of this key resource in heterotrophic stream food webs.
2. To explore this question we quantified several processes related to litter dynamics (i.e. litter inputs, storage, losses by transport and losses by breakdown) by sampling litter at multiple sites in three streams of the Brazilian Cerrado biome (which has a tropical wet–dry climate) for 2 years. We assessed the relative contribution of different spatial (among and within streams) and temporal scales (annual, seasonal and monthly) to total variability of these processes (hereafter fluxes).
3. Spatial and temporal variability of fluxes were both high, but spatial variation was 1.67-fold greater than temporal variation (61 versus 37%, respectively), especially at the within-stream scale (50% overall); an exception was litterfall, which varied less spatially than temporally (24 versus 76%). Temporal variation of litter storage (and hence availability to consumers) was mostly seasonal and due to differences in net transport.
4. Inputs and transport were higher in the wet than the dry season (wet versus dry season, 1.45 versus 0.92 and 1.43 versus 0.06 g litter m⁻² day⁻¹), while breakdown was similar between both seasons (0.88 versus 0.94 g litter m⁻² day⁻¹). Storage (i.e. accumulation) rate was positive and negative in the dry and wet season, respectively, indicating that litter was stored in the dry season and exported in the wet season. The transitional dry–wet season showed the highest inputs, breakdown and storage (3.21, 1.63 g litter m⁻² day⁻¹ and 145 g litter m⁻²), while the wet–dry season showed lower inputs (as in the dry season), higher transport (as in the wet season) and lower breakdown and storage than the other seasons (0.93, 0.65, 0.31 g litter m⁻² day⁻¹ and 24 g litter m⁻²).
5. Our results underscore the role of variation in biophysical drivers of litter fluxes within streams (e.g. pool–riffle configuration, substrate features, biological communities), and suggest that high within-stream replication is necessary to study litter fluxes at larger scales and over time. The seasonal patterns suggested potential changes in litter dynamics under future climate scenarios in the tropics, including increased storage due to reduced transport in a drier climate.

     

Abundance,Population Structure and Production of Scrobicularia plana and Abra tenuis (Bivalvia: Scrobicularidae) in a Mediterranean Brackish Lagoon,Lake Ichkeul,Tunisia

Abundance, growth and production of the deposit‐feeding bivalves were studied in the Ichkeul wetland, northern Tunisia, from July 1993 – April 1994. Scrobicularia plana (Da Costa , 1778) occurred at annual mean densities (biomasses) of 299 ± 65 to 400 ± 100 individuals/m² (22.54 ± 3.00 to 34.27 ± 3.96 g ash‐free dry mass (AFDM)/m²) depending on the study area. The annual mean density of Abra tenuis (Montagu , 1803) amounted to 640 ± 74 individuals/m² during the whole study period, in contrast the biomass rose from 2.87 g AFDM/m² in July to 10.29 g AFDM/m² in April. Both species were largely dominated by age class I. Although not very successful, recruitment presented a two‐period pattern: the main period at the beginning of spring, and a secondary one in late summer/autumn. S. plana rarely exceeded 40 mm and lived for only 2 years, while most individuals of A. tenuis lived for only 15–18 months growing to a length of 12 mm. The annual bivalve deposit‐feeder production for the whole lagoon system (90 km²) was 8.24 g AFDM/m² (5.26 g C/m², 0.65 g N/m²). The annual P/ ratio was about 0.4 and therefore in the same order of magnitude as estimates from other brackish coastal waters. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Saltcedar (Tamarix ramosissima) invasion alters organic matter dynamics in a desert stream

1. We investigated the impacts of saltcedar invasion on organic matter dynamics in a spring‐fed stream (Jackrabbit Spring) in the Mojave Desert of southern Nevada, U.S.A., by experimentally manipulating saltcedar abundance. 2. Saltcedar heavily shaded Jackrabbit Spring and shifted the dominant organic matter inputs from autochthonous production that was available throughout the year to allochthonous saltcedar leaf litter that was strongly pulsed in the autumn. Specifically, reaches dominated by saltcedar had allochthonous litter inputs of 299 g ash free dry mass (AFDM) m^?2 year^?1, macrophyte production of 15 g AFDM m^?2 year^?1 and algal production of 400 g AFDM m^?2 year^?1, while reaches dominated by native riparian vegetation or where saltcedar had been experimentally removed had allochthonous litter inputs of 7–34 g AFDM m^?2 year^?1, macrophyte production of 118–425 g AFDM m^?2 year^?1 and algal production of 640–900 g AFDM m^?2 year^?1. 3. A leaf litter breakdown study indicated that saltcedar also altered decomposition in Jackrabbit Spring, mainly through its influence on litter quality rather than by altering the environment for decomposition. Decomposition rates for saltcedar were lower than for ash (Fraxinus velutina), the dominant native allochthonous litter type, but faster than for bulrush (Scirpus americanus), the dominant macrophyte in this system.     

Lateral input of particulate organic matter from bank slopes surpasses direct litter fall in the uppermost reaches of a headwater stream in Hokkaido, Japan

In forested streams, surrounding riparian forests provide essential supplies of organic matter to aquatic ecosystems. We focused on two pathways of particulate organic matter inputs: direct input from upper riparian forests and indirect lateral input from bank slopes, for which there are limited quantitative data. We investigated the inputs of coarse particulate organic matter (CPOM) and carbon and nitrogen in the CPOM into the uppermost reaches of a headwater stream with steep bank slopes in Hokkaido, Japan. CPOM collected by litter traps was divided into categories (e.g., leaves, twigs) and weighed. Monthly nitrogen and carbon inputs were also estimated. The annual direct input of CPOM (ash-free dry mass) was 472 g m⁻², a common value for temperate riparian forests. The annual lateral CPOM input was 353 g m⁻¹ and 941 g m⁻² when they were converted to area base. This value surpassed the direct input. Organic matter that we could not separate from inorganic sediments contributed to the total lateral input from the bank slopes (124 g m⁻¹); this organic matter contained relatively high amounts of nitrogen and carbon. At uppermost stream reaches, the bank slope would be a key factor to understanding the carbon and nitrogen pathways from the surrounding terrestrial ecosystem to the aquatic ecosystem.     

Allochthonous litter inputs, organic matter standing stocks, and organic seston dynamics in upland Panamanian streams: potential effects of larval amphibians on organic matter dynamics

Allochthonous inputs of detritus represent an important energy source for streams in forested regions, but dynamics of these materials are not well studied in neotropical headwater streams. As part of the tropical amphibian declines in streams (TADS) project, we quantified benthic organic matter standing stocks and organic seston dynamics in four Panamanian headwater streams, two with (pre-amphibian decline) and two without (post-decline) healthy amphibian assemblages. We also measured direct litterfall and lateral litter inputs in two of these streams. Continuous litterfall and monthly benthic samples were collected for 1 year, and seston was collected 1–3 times/month for 1 year at or near baseflow. Direct litterfall was similar between the two streams examined, ranging from 934–1,137 g DM m⁻² y⁻¹. Lateral inputs were lower, ranging from 140–187 g DM m⁻¹ y⁻¹. Dead leaves (57–60%), wood (24–29%), and green leaves (8–9%) contributed most to inputs, and total inputs were generally higher during the rainy season. Annual habitat-weighted benthic organic matter standing stocks ranged from 101–171 g AFDM m⁻² across the four study reaches, with ∼4 × higher values in pools compared to erosional habitats. Total benthic organic matter (BOM) values did not change appreciably with season, but coarse particulate organic matter (CPOM, >1 mm) generally decreased and very fine particulate organic matter (VFPOM, 1.6–250 μm) generally increased during the dry season. Average annual seston concentrations ranged from 0.2–0.6 mg AFDM l⁻¹ (fine seston, <754 μm >250 μm) and 2.0–4.7 mg AFDM l⁻¹ (very fine, <250 μm >1.6 μm), with very fine particles composing 85–92% of total seston. Quality of fine seston particles in the two reaches where tadpoles were present was significantly higher (lower C/N) than the two where tadpoles had been severely reduced (P = 0.0028), suggesting that ongoing amphibian declines in this region are negatively influencing the quality of particles exported from headwaters. Compared to forested streams in other regions, these systems receive relatively high amounts of allochthonous litter inputs but have low in-stream storage. Handling editor: J. Padisak     

Energy flow and subsidies associated with the complex life cycle of ambystomatid salamanders in ponds and adjacent forest in southern Illinois

Breeding adults and metamorphosing larval amphibians transfer energy between freshwater and terrestrial ecosystems during seasonal migrations and emergences, although rarely has this been quantified. We intensively sampled ambystomatid salamander assemblages (Ambystoma opacum,A. maculatum, and A. tigrinum) in five forested ponds in southern Illinois to quantify energy flow associated with egg deposition, larval production, and emergence of metamorphosed larvae. Oviposition by female salamanders added 7.0–761.4 g ash-free dry mass (AFDM) year⁻¹ to ponds (up to 5.5 g AFDM m⁻² year⁻¹). Larval production ranged from 0.4 to 7.4 g AFDM m⁻² year⁻¹ among populations in three ponds that did not dry during larval development, with as much as 7.9 g AFDM m⁻² year⁻¹ produced by an entire assemblage. Mean larval biomass during cohort production intervals in these three ponds ranged from 0.1 to 2.3 g AFDM m⁻² and annual P/B (production/biomass) ranged from 4 to 21 for individual taxa. Emergent biomass averaged 10% (range=2–35%) of larval production; larval mortality within ponds accounted for the difference. Hydroperiod and intraguild predation limited larval production in some ponds, but emerging metamorphs exported an average of 70.0±33.9 g AFDM year⁻¹ (range=21.0–135.2 g AFDM year⁻¹) from ponds to surrounding forest. For the three ponds where larvae survived to metamorphosis, salamander assemblages provided an average net flux of 349.5±140.8 g AFDM year⁻¹ into pond habitats. Among all ponds, net flux into ponds was highest for the largest pond and decreased for smaller ponds with higher perimeter to surface area ratios (r ² =0.94, P<0.05, n=5). These results are important in understanding the multiple functional roles of salamanders and the impact of amphibian population declines on ecosystems. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Allochthonous input and retention in a small mountain stream,South Africa

Allochthonous input and benthic coarse particulate organic matter (CPOM) standing stocks were investigated in a first-order stream in South Africa between May 1984 and April 1985. Monthly falls into the stream of all litter types (total) ranged from 11 (September) to 79 g m^–2 (March). Total annual litter fall was 426 g dry weight, which corresponds to 1.2 g m^–2 d^–1. Flowers, fruits and seeds contributed 37 g m^–2, woody debris, 122 g m^–2, and leaves 267 g m^–2 to this total. Leaf fall from native trees, which accounted for approximately 57% of total litter input (244 g m^–2 a^–1), was significantly higher in summer than in winter. The summer peak in leaf fall recorded is far smaller and more protracted than the autumnal peak recorded for many Northern Hemisphere streams.Monthly total standing stocks of CPOM ranged from 14 g dry weight m^–2 in January to 69 g m^–2 in August, and a mean total CPOM standing stock of 41 g m^–2 mth^–1 was estimated. This comprised 18 g m^–2 mth^–1 soft litter, and 23 g m^–2 mth^–1 hard litter. CPOM standing stocks showed no seasonal trends, and with the exception of two species, standing stocks of endemic leaf species reflected their contributions to the total litter fall. Contrary to earlier reports for streams in the Fynbos Biome, Window Stream has CPOM standing stocks well within the ranges reported for low-order streams worldwide.     

Temporal and Spatial Patterns in Inputs and Stock of Organic Matter in Savannah Streams of Central Brazil

Coarse particulate organic matter (CPOM) inputs from riparian vegetation into streams and CPOM benthic stock vary naturally in space and time, but most studies in the tropical savannah (Cerrado) have been done over a small temporal scale (<1 year), which does not allow for the determination of inter-annual patterns. We found that CPOM collected over two years differed temporally and spatially, whereas there was no significant variation between years for the benthic stock, which indicates high stability in the energy balance of streams. The largest monthly inputs occurred between August and October, at the end of the dry season and the onset of the rainy season, which was partially explained by precipitation. Other factors such as photoperiod, which was not studied, could also have important roles in this pattern. Spatial differences in CPOM between streams were attributed to topography and channel morphology. The plant density was lowest in the stream with a more irregular topography and a deeper channel, which results in drier riparian soil. The benthic stock was highest in the stream with a flat channel, where the lower water speed facilitates the accumulation of CPOM in the stream bed. Inter-annual differences in CPOM were attributed more to the differences in the beginning of the dry and wet periods between years than to the average values of precipitation. Longer-term studies are needed to clarify this temporal pattern.     

Seeds of the annual grasses Schizachyrium spp. as a food resource for tropical granivorous birds

Seed dynamics of the annual tropical grasses Schizachyrium fragile (R. Br.) A. Camus, S. pachyarthron C. Gardner and S. pseudeulalia (Hosok.) S.T. Blake were studied with the aim of documenting fluxes in granivore food resources. In S. fragile, seed production began in the early dry season, and seed output was primarily influenced by seedling survival. Following seed-fall, there were 651 S. fragile seeds/m² (393 kernels/m²⁾ and 1014 S. pachyarthron seeds/m² (593 kernels/m²) across the study area, with a combined kernel biomass of 14.1 × 10³ g/ha. Seed densities remained stable through the dry season, then declined rapidly after wet season rain. Initial wet season rains of up to 25 mm, 40 mm and 50 mm resulted in seed-bank germination of < 5%, 57%, and 93%, respectively. Some seeds were buried by rain and resurfaced or germinated later, but about 30% was lost. Widespread and abrupt depletion of the seed-bank is likely to occur at the start of about 8% of wet seasons on central Cape York Peninsula, leaving little seed, either for subsequent germination or as food for granivores. Burning early in the dry season, when most seeds were still on the plants, reduced seed densities by 85%. The proportion of seeds with sound kernels was reduced in areas burnt by dry season fires, both directly after the fires and, subsequently, as a result of preferential granivore activity. Areas burnt in the dry season were thus depleted of seed earlier than were unburnt areas. Food availability for granivores should therefore be optimized by fire regimes that include a range of burning histories, including fires in both early dry and early wet seasons as well as keeping other areas unburnt.     

Effects of organic matter availability on the life history and production of a top vertebrate predator (Plethodontidae: Gyrinophilus palleucus) in two cave streams

1. Surface ecosystems provide the primary source of organic matter to many cave communities. Variation in the strength of connectivity to the surface suggests that some caves may be more resource‐limited than others. To test this, we examined diet, prey availability and production of an obligate cave salamander Gyrinophilus palleucus (Plethodontidae), a top predator, in two south‐eastern U.S.A. caves with different levels of organic matter (Tony Sinks cave, 165 g AFDM m^?2; Bluff River cave, 62 g AFDM m^?2). 2. We quantified density, biomass, growth rate, production and diet of G. palleucus monthly for 21 months. Diet composition, differences in prey communities and seasonal patterns in prey consumption were also analysed. 3. Salamander density, biomass and secondary production were significantly greater in the high organic matter cave (0.10 m^?2, 0.18 g AFDM m^?2, 0.12 g AFDM m^?2 year^?1) than in the low organic matter cave (0.03 m^?2, 0.03 g AFDM m^?2, 0.01 g AFDM m^?2 year^?1). Although growth rates were not statistically different between the two cave salamander populations, low recaptures probably influenced this result. 4. Isopoda prey were the major contributor to salamander production in the high organic matter cave (69%). In the low organic matter cave, production was provided by isopods (41%) and oligochaetes (20%). The lower number of prey taxa contributing to salamander production in the high organic matter cave suggests the ability to forage more selectively. 5. The differences in foraging strategy, density, biomass and secondary production were probably related to differences in the strength of surface connectivity, which controls organic matter supply. Links between basal resource level and top predator performance show the importance of bottom‐up limitation in the food webs of caves and other detritus‐based ecosystems.     

Effects of irrigation on litterfall,fine root biomass and production in a semideciduous lowland forest in Panama

The effects of irrigation on fine root biomass, root production and litterfall were measured at the community level, in a semideciduous lowland forest in Panama. Biomass of roots less than 2 mm in dia. in the first 10 cm of the soil (measured with soil cores), was higher in irrigated (1.80 Mg ha^-1) than in non-irrigated plots (1.24 Mg ha^-1). During the dry season, productivity of roots (measured with ingrowth cylinders filled with root-free soil), was higher in irrigated (1.6 g m^-2 day^-1) than in control plots (0.3 g m^-2 day^-1). In control plots, root productivity was highly seasonal. Maximum root growth into the root-free soil, occurred during the transitions from dry to wet, and from wet to dry season, possibly as a response to water and/or nutrient pulses. Litterfall was not significantly different between irrigated (3.8 g m^-2 day^-1) and control plots (3.7 g m^-2 day^-1). The results of this study show that root-productivity is limited by the water supply during the dry season, and that water by itself, is not a limiting factor for community-level litter production. This revised version was published online in June 2006 with corrections to the Cover Date.     

北部湾近岸海域浮游动物群落结构特征及季节变化

2017年3月(枯水期)、7月(丰水期)和10月(平水期)分别对北部湾近岸海域44个站位的浮游动物进行了调查。结果共检出浮游动物251种和浮游幼体24类,其中枯水期138种(类),丰水期134种(类),平水期191种(类),分属河口低盐、近岸暖温、近岸暖水和外海暖水4个生态类群。优势种9种,其中枯水期以原生动物占绝对优势,丰水期以枝角类、桡足类和浮游幼体类占优势,平水期以十足类和浮游幼体类占优势。浮游动物丰度年均值为789.95个/m~3,呈现出枯水期(1540.19个/m~3)明显高于平水期(457.58个/m~3)和丰水期(372.08个/m~3)的季节变化特征;浮游动物生物量年均值为252.40 mg/m~3,生物量的季节变化与丰度变化不一致,平水期生物量(385.01 mg/m~3)明显高于枯水期(221.41 mg/m~3)和丰水期(150.78 mg/m~3)。多样性指数平水期最高(3.16),丰水期(2.35)次之,枯水期(2.22)最低。枯水期和丰水期北部湾近岸海域浮游动物生物量和丰度水平分布特征基本呈现自河口近岸海域向外海递增的趋势,平水期浮游动物生物量与丰度的空间分布较为均匀。浮游动物的种类组成结构以及优势种的演替对浮游动物的生物量和丰度季节变化有着重要的决定作用。径流导致的悬浮物、营养盐等的变化可能是决定北部湾近岸海域浮游动物生物量和丰度空间分布的主要因素。研究还表明与其他海湾相比,北部湾近岸海域浮游动物群落结构趋于小型化,需加大关注。     

Seasonal and inter-stream variations in the population dynamics, growth and secondary production of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) in monsoonal Hong Kong

1. Balitorid loaches are widespread and highly diverse in Asian streams, yet their life history and ecology have received little attention. We investigated seasonal (wet versus dry season) and spatial variation in populations of algivorous Pseudogastromyzon myersi in Hong Kong, and estimated the magnitude of secondary production by this fish in pools in four streams (two shaded and two unshaded) over a 15‐month period. 2. Mean population densities of P. myersi ranged from 6.0 to 23.2 individuals m⁻², constituting more than half (and typically >70%) of benthic fishes censused. Abundance was c. 25% greater in the wet season, when recruitment occurred. Significant density differences among streams were not related to shading conditions and were evident despite small‐scale variations in P. myersi abundance among pools. Mean biomass varied among streams from 0.85 to 3.87 g ash‐free dry weight (AFDW) m⁻². Spatial and seasonal patterns in biomass and density were similar, apart from some minor disparities attributable to differences in mean body size among populations. 3. All four P. myersi populations bred once a year in June and July, and life spans varied from 24 to 26 months. Populations consisted of three cohorts immediately after recruitment but, for most of the study period, only two cohorts were evident. Cohort‐specific growth rates did not differ significantly among streams but, in all streams, younger cohorts had higher cohort‐specific growth rates. 4. Secondary production of P. myersi estimated by the size‐frequency (SF) method was 2.7–11.5 g AFDW m⁻² year⁻¹ and almost twice that calculated by the increment‐summation (IS) method (1.2–6.6 g AFDW m⁻² year⁻¹). Annual P/B ratios were 1.17 – 2.16 year⁻¹ (IS) and 2.73 – 3.22 year⁻¹ (SF). Highest production was recorded in an unshaded stream and the lowest in a shaded stream, but site rankings by production did not otherwise match shading conditions. Wet‐season production was six times greater than dry‐season production, and daily production fell to almost zero during January and February. Cool temperatures (<17 °C) may have limited fish activity and influenced detectability during some dry‐season censuses. Estimates of abundance and annual production by P. myersi are therefore conservative. 5. Comparisons with the literature indicate that the abundance and production of P. myersi in Hong Kong was high relative to other benthic fishes in tropical Asia, or their temperate counterparts in small streams. Manipulative experiments are needed to determine the influence of P. myersi, and algivorous balitorids in general, on periphyton dynamics and energy flow in Asian streams.     

Population dynamics and production of the freshwater snail Chilina gibbosa Sowerby 1841 (Chilinidae,Pulmonata) in a North-Patagonian reservoir

Chilina gibbosa is an endemic snail widely distributed in Patagonia, Argentina. Due to its importance in the benthic fauna and in the diet of some fish in the oligo-mesotrophic reservoir Ezequiel Ramos Mexía (39° 30′ S, 69° 00′ W), special attention has been given to its life cycle, growth patterns and annual production. Samples were taken monthly at five littoral stations between June 1983 and July 1984. Mean abundance and biomass of C. gibbosa were much higher in vegetated stations dominated by Potamogeton berteroanus (Station 1 : 583 ind. m⁻², 5.95 g AFDM m⁻²) or by Nitella clavata (Station 5 : 275 ind. m⁻², 4.18 g AFDM m⁻²) than bare stations with low transparency or stations with other macrophytes. The snails presented a clustered spatial pattern and their abundance was significantly correlated with macrophyte wet biomass only when this was above 250 g m⁻². Analysis of size distributions showed an annual life cycle with a reproductive period in the summer. However, differences in recruitment and growth occurred probably due to differences in water temperature and food availability. Growth was maximum in summer and almost absent during winter. Hence, shell growth data fit a sigmoid curve well, and growth was somewhat higher at Station 1. Annual production at Stations 1 and 5, estimated by the ‘growth increment summation’ method (28.8 g AFDM m⁻² and 14.18 g AFDM m⁻² respectively), was among the highest recorded for pulmonate gastropods, possibly due to a low interspecific competition. The P : B ratio values were within the range for univoltine gastropods (4.84 and 3.39). The high productivity and turnover rate of these snails grant a high availability of food for the abundant molluscivore, the silverside Patagonina hatcheri.     

The influence of stream flow reduction on the energetics of endemic Hawaiian torrenticolous aquatic insects, <Emphasis Type="Italic">Telmatogeton</Emphasis> Schiner and <Emphasis Type="Italic">Procanace</Emphasis> Hendel

Tropical island stream ecosystems continue to be threatened by increasing anthropogenic demands for freshwater, with many streams dammed or diverted. Stream flow amendments can have substantial effects on aquatic insect populations of tropical archipelagoes. In Hawaiian streams, an endemic Diptera community of the following genera dominates cascades and other torrential habitats: Telmatogeton Schiner (Chironomidae), Procanace Hendel (Canacidae), Scatella Robineau-Desvoidy (Ephydridae). Larval densities, standing stock biomass (SSB, as ash-free dry mass [AFDM]), and monthly secondary production of Telmatogeton and Procanace were measured during two summers of significantly different stream discharge in Iao Valley, Maui, Hawaii. Very few Scatella larvae were collected (<20 larvae for both years combined), so they were omitted from subsequent analyses. Stream discharge decreased approximately 40% from 1994 to 1995, providing a ‘natural test’ of the effects of reduced stream flow on these torrenticolous populations between two years. Combined Telmatogeton and Procanace SSB (total torrential community SSB) was 3176 and 1683 mg AFDM m⁻² for 1994 and 1995, respectively, with Telmatogeton accounting for >95% in both years due to significantly larger body size and high density. The SSB of Telmatogeton significantly decreased from 3138 to 1622 mg AFDM m⁻² from 1994 to 1995 but increased for Procanace (37.6–60.9 mg AFDM m⁻², respectively). Total torrential community secondary production was 31% lower in 1994 (12,833 mg AFDM m⁻² mo⁻¹) compared to 1995 (8855 mg AFDM m⁻² mo⁻¹), reflecting the Telmatogeton proportion of total community production (99%); however, Procanace production increased by 40%. Monthly P/B ratios indicated that biomass turnover was generally high and increased for Telmatogeton from 1994 (3.8) to 1995 (5.1), whereas it remained lower and did not change between years for Procanace (1.7). A natural drought of the Iao Stream valley was associated with structural and functional changes in two endemic aquatic insects; these results are a conservative indication of permanent stream flow reductions from anthropogenic withdrawal (e.g., dams and diversions).     

WATER TRANSPORT PROPERTIES OF VINE AND TREE STEMS IN A TROPICAL DECIDUOUS FOREST

Excised stem segments of vines had higher specific hydraulic conductivities (flow rate per pressure gradient per stem transverse area) than did trees during the dry season in a deciduous forest in Jalisco, México. Vine species averaged from 2.7 to 203 10^-3 m² MPa^-1 s^-1 and tree species from 0.8 to 5.1 10^-3 m² MPa^-1 s^-1. Only three of the 20 species retain their leaves during part or all of the dry season, and these included the vine and the tree with the lowest conductivities within their growth forms. An index of the mean diameter of the six widest vessels per sample was positively and significantly correlated with specific conductivity for the vines but not the trees. Stem density (g cm^-3, dry weight/wet volume) was not correlated with either specific conductivity or vessel diameter index for vines or trees.     

Variability in annual recruitment success as a determinant of long-term and large-scale variation in annual production of intertidal Wadden Sea mussels (<Emphasis Type="Italic">Mytilus edulis</Emphasis>)

To understand the background of the strong variation and recent decline of stocks and production of mussels (Mytilus edulis) on tidal flats of the Wadden Sea, we analysed long-term (twice-annual for 26 years) and multi-station (15 sites) estimates of numbers, mean individual weights, biomass, and annual production on Balgzand, a 50-km² tidal-flat area in the westernmost part of the Wadden Sea (The Netherlands). Somatic production was estimated from summed growth increments of soft tissues per half-year period and expressed in ash-free dry mass (AFDM). In adults, positive values in spring/summer regularly alternated with negative values in autumn/winter, when up to ∼25% (mean: 14%) of individual weight gains in the preceding season were lost. No weight losses were observed during the first winter of the life of mussels. The 26-year mean of net somatic tissue production P amounted to 5.5 g AFDM m⁻² a⁻¹ at a mean biomass B of 3.2 g AFDM m⁻²; the ratio P/B varied strongly with age composition of the mussel population and ranged between 0.5 and 3.0 a⁻¹ (mean: 1.7). Within the restricted areas of mussel beds, mean biomass and annual production values were two orders of magnitude higher. In the Wadden Sea, mussel beds cover a typical 1% of extensive tidal flat areas. Numerical densities of recruits showed straight-line relationships with subsequent life-time year-class production. Once recruits had reached an age of ∼10 months, their numbers predicted subsequent production within narrow limits. Production per recruit averaged 0.21 g AFDM for 10-mo recruits and was not related to recruit density. Local variation in annual production varied strongly, with maximal values between mid-tide and low-tide level, where recruitment was also maximal. Production per recruit was higher at low than at high intertidal levels. Frequently failing recruitment is indicated as the main cause of declining mussel stocks in the Wadden Sea. As in other bivalve species, a declining frequency of the occurrence of cold winters appears to govern declining recruitment success and consequently declining production.     

Comparison of water level and eutrophication indicators during the wet and dry period in a eutrophic urban lake

Eutrophication modifies lakes' ecological balances and threatens its viability. To date, eutrophication management strategies have been related to nutrient reduction in the lakes' water column. However, nutrient reduction strategies are complicated by the variations of the lake's water level, nutrient concentration, and eutrophication symptom, which are primarily known to be influenced by the local rainfall patterns. Therefore, this study aimed to compare the variability of water level, total phosphorus, and total chlorophyll-a concentrations in Slim River Lake during wet and dry seasons. In this study, water sampling and depth measurements were carried out from six sampling points for 1 year. Water samples were used to quantify total phosphorus and total chlorophyll-a. Our results showed that mean water levels in the studied lake ranged from 1.36 m to 5.46 m in the wet season and from 1.31 m to 5.41 m in the dry season, which implicated no significant difference (p > .05) between seasons in most sampling points. Total phosphorus present at concentrations exceeding 10 mg/L and showed small variations between wet and dry seasons. Mean total phosphorus concentrations varied from 10.55 mg/L to 26.66 mg/L in the wet season and 10.77 mg/L to 21.76 mg/L in the dry season and showed no significant difference between seasons. In addition, mean chlorophyll-a concentrations ranged from 14.35 mg/m³ to 180.13 mg/m³ and from 14.15 mg/m³ to 39.27 mg/m³ in wet and dry seasons, respectively. Chlorophyll-a concentrations showed significant differences (p < .05) between seasons in the deepest sampling points in the lake. The observed seasonal variations in total chlorophyll-a suggest the importance of algae monitoring during the wet season even when no apparent surge of phosphorus concentration is detected.