Effect of young-of-the-year walleye (Percidae: Stizostedion vitreum) on plankton dynamics and water quality in ponds

To understand the impact of young-of-the-year (YOY) fish on food web dynamics and water quality, we stocked larval walleye (9 mm TL) (Stizostedion vitreum) in six experimental ponds using two fish densities (10 and 50 fish m^–3) with three replicates. At high fish density, the average abundances of cladocerans and copepods and the Secchi depth were lower whereas abundances of rotifers and algae, gross primary productivity (GPP), pH and total phosphorus concentration were higher than at low fish density. Fish impact on bacterial abundance, dissolved oxygen, nitrogen and phosphorus concentrations, however, was not significant. The within treatment measurements of all variables except GPP were significantly different over time. Our results indicate that YOY walleye predation at high density can affect plankton community by reducing large zooplankton biomass and water clarity, and increasing phytoplankton abundance. The impact of YOY piscivorous fish on plankton should be considered when biomanipulation is applied for improvement of water quality.     

From triphenyltins to integrated management of the `pest' snail Cerithidea cingulata in mangrove-derived milkfish ponds in the Philippines

The potamidid snail Cerithidea cingulata is considered a pest in brackishwater milkfish ponds in the Philippines and has been controlled by the triphenyltin (TPT) compounds Aquatin and Brestan. But TPT is also toxic to other invertebrates, fishes, algae, bacteria and people, and high TPT residues occur in sea foods including milkfish. Thus, control of snails in milkfish ponds should be shifted from reliance on TPT to an integrated pest management (IPM) strategy. To formulate a responsible IPM, studies were conducted on C. cingulata in ponds and mangroves and the available data were synthesized with the relevant information from the literature. The deposit-feeding C. cingulata is a native resident of mangrove areas and becomes a problem in mangrove-derived ponds where the lack of competitors and predators results in 'ecological release' and population explosion. Snail densities ranged 1–470 m^–2 in the mangroves and 100–5000 m^–2 in ponds. In ponds, snails ranged 2–40 mm in shell length; those 25 mm long and 8 mm wide weighed 1 g on average, and had 150 mg flesh. Snails matured at 20 mm long and reproduced the whole year with a peak in Mar–Sep at water temperatures of 24–36 °C. Enriched sediments and stagnant water in ponds allowed fast growth and reproduction, low dispersal and high recruitment of snails. Snails were very tolerant to hypoxia and adverse conditions, but were killed within a week by sun-drying or by salinities of 48–70 and within 3 d by ammonium phosphate at 10 g l^–1 or urea at 5 g l^–1. IPM of snails requires changes in mind sets and perspectives of milkfish farmers and industry supporters and changes in farm practices and management. Snails must be viewed as a resource from which income can be made and employment can be generated. Harvest of snails for shellcraft and other enterprises also effectively removes the spawning population. Complete draining and sun-drying of ponds after harvest kills the adult snails and the egg strings on the bottom. Snails in puddles in the ponds may be killed by the usual nitrogen fertilizers and lime applied during pond preparation. Water input may be timed with periods of low veliger counts in the supply water. These IPM recommendations have yet to be verified.     

Influence of daylight and incubation interval on water column respiration in tropical fish ponds

Water column respiration (WCR) was measured in dark BOD bottles for 2, 4 and 8 h intervals during 22 h periods in two 1000 m² ponds stocked withOreochromis niloticus at 1 m^–2, and fertilized weekly with chicken litter at 750 kg total solids ha^–1. Mean WCR ranged from a low of 0.39 mg l^–1 for 8 h nocturnal intervals to a high of 0.62 mg l^–1 for 2-h diurnal intervals. WCR was significantly influenced by daylight and time into the diurnal or nocturnal period when it was determined. Mean WCR was significantly greater during the day than during the night (P<0.01). During the day, 2 h incubation intervals resulted in significantly higher WCR than 4 h or 8 h intervals (P<0.01); length of incubation interval did not significantly influence nocturnal WCR (P>0.05). Higher WCR during the day and during short diurnal incubation intervals was attributed to greater availability of photosynthetic respiration substrate. Diurnal, diel, or nocturnal WCR could be best estimated by a single 2, 4 or 8 h incubation interval, respectively, beginning at 0800 h.     

Colonization and succession of submerged macrophytes in shallow Lake Væng during the first five years following fish manipulation

Colonization of submerged macrophytes and changes in species composition were studied in shallow Lake Væng during the first five years (1987–91) following fish manipulation in 1986–1988 and a resultant significant improvement in lake water transparency. No submerged macrophytes were present in the lake from 1981–1986, during which time the summer mean Secchi depth ranged from 0.6 and 0.8 m. From 1987 to 1990, Secchi depth increased from 0.9 m to 1.8 m and macrophyte coverage consequently increased (1 % of the lake area in 1987, 2% in 1988, 50% in 1989, 80% in 1990 and 90% in 1991). At the same time, the macrophytes became taller, and the weedbeds more dense. The macrophytes colonized from the exposed and deeper part of the lake towards the sheltered and more shallow part of the lake, a colonization pattern that was confirmed by transplantation experiments. The delay in colonization of the shallow parts may be caused by waterfowl grazing. The vegetation was initially dominated by Potamogeton crispus L., but there was a gradual change during 1988–1989 and Elodea canadensis Michx became exclusively dominant in 1990–1991.     

High aquatic niche overlap in the newts Triturus cristatus and T. marmoratus (Amphibia,Urodela)

We studied spatial niche metrics of large-bodied newts (Triturus cristatus and T. marmoratus) in three breeding ponds in western France. Adults and larvae were sampled with underwater funnel traps. Larvae were identified to the species with diagnostic microsatellite DNA markers. The distribution of adult T. cristatus and T. marmoratus across pond regions differed in one out of six cases, no differences were observed between larvae (two ponds studied). Niche overlap and niche breadth indices across resource states defined as pond regions or individual traps were high (Schoener's C: pond regions 0.60–0.98, traps 0.35–0.71; Levins' B: pond regions 0.71–0.98, traps 0.35–0.76). Adults of large-bodied newts significantly differed in resource use from small-bodied newts (T. helveticus). The results are discussed in view of the occurrence of interspecific breeding attempts, and the unpredictable ecological characteristics of newt breeding ponds.     

Characteristics of plankton communities in Chinese integrated fish ponds: effects of excessive grazing by planktivorous carps on plankton communities

Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 10⁷ and 2.20 × 10⁷ cells ml^–1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m^–3 Chla on average) and rotifers (5372 and 20733 ind. 1^–1 on average) exceeded the maximum values reported for natural waters.The average [³H]thymidine uptake rates were 694 and 904 pmoles 1^–1 h^–1 (13.4 and 20.6 µgC 1^–1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [³H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m^–3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m^–2 d^–1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1^–1 and 445 µg 1^–1 and those of rotifers were 763 µg 1^–1 and 1186 µg 1^–1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.     

Primary productivity and related parameters in three different types of inland waters in Sri Lanka

Gross and net primary production together with chlorophyll-a biomass were investigated with respect to depth and diurnal changes in three categories of inland waters (reservoirs, temporary ponds, brackish water lagoons) in Sri Lanka. Ten field sites, in both the dry and wet zones of the island, were investigated. Bimodal productivity profiles were recorded in two of the three reservoirs studied. The diel pattern of net photosynthetic rate varied between sites although peak photosynthetic efficiency occurred at solar noon. Surface photoinhibition was characteristic of the reservoirs and brackish water lagoons but not of the temporary ponds. Mean gross primary production was 3.02 g C m^–2 d^–1 but was higher in the temporary ponds than in the reservoirs. The gross primary production in the brackish water Koggala Lagoon at 0.08 g C m^–2 d^–1 is a record low for tropical lagoons and was 2.5 times less than the two other lagoons investigated. Variability in net primary production between sites was similar to the variation in gross production with a relatively low mean value for tropical inland waters of 0.495 C m^–2 d^–1. Mean maximum photosynthetic rate was 0.30 mg C m^–3 h^–1 but was lower in the reservoirs than in the temporary ponds and lagoons.     

Biophysics of underwater hearing in the clawed frog,Xenopus laevis

Anesthetized clawed frogs (Xenopus laevis) were stimulated with underwater sound and the tympanic disk vibrations were studied using laser vibrometry. The tympanic disk velocities ranged from 0.01 to 0.5 mm/s (at a sound pressure of 2 Pa) in the frequency range of 0.4–4 kHz and were 20–40 dB higher than those of the surrounding tissue. The frequency response of the disk had two peaks, in the range of 0.6–1.1 kHz and 1.6–2.2 kHz, respectively. The first peak corresponded to the peak vibrations of the body wall overlying the lung. The second peak matched model predictions of the pulsations of the air bubble in the middle ear cavity. Filling the middle ear cavity with water lowered the disk vibrations by 10–30 dB in the frequency range of 0.5–3 kHz.Inflating the lungs shifted the low-frequency peak downwards, but did not change the high-frequency peak. Thus, the disk vibrations in the frequency range of the mating call (main energy at 1.7–1.9 kHz) were mainly caused by pulsations of the air in the middle ear cavity; sound transmission via the lungs was more important at low frequencies (below 1 kHz). Furthermore, the low-frequency peak could be reversibly reduced in amplitude by loading the larynx with metal or tissue glue. This shows that the sound-induced vibrations of the lungs are probably coupled to the middle ear cavities via the larynx. Also, anatomical observations show that the two middle ear cavities and the larynx are connected in an air-filled recess in submerged animals.This arrangement is unique to pipid frogs and may be a structural adaptation to connect all the air spaces of the frog and improve low-frequency underwater hearing. Another function of the recess may be to allow cross-talk between the two middle ear cavities. Thus, the ear might be directional. Our pilot experiments show up to 10 dB difference between ipsi- and contralateral stimulus directions in a narrow frequency range around 2 kHz.     

The effect of non-algal turbidity on the relationship of Secchi depth to chlorophyll a

Data from four reservoirs representative of different trophic states and with different apparent optical properties were analyzed to determine the relationship of Secchi depth to algal biomass as measured by chlorophyll a. In the eutrophic reservoir Secchi depth was determined partially by the chlorophyll a content (r² = 0.31) but only when chlorophyll a data from bloom conditions are included. In the two mesotrophic reservoirs, Secchi depth was entirely determined by non-algal turbidity. In the oligotrophic reservoir, Secchi depth was determined neither by chlorophyll a nor non-algal turbidity and was probably determined by dissolved color. When data from the four reservoirs were pooled (N = 205), 53% of the variation in Secchi depth was explained by: SD = 2.55–0.52 ln (Turbidity) + 0.005 (Chlorophyll a). It is apparent that attempts to estimate algal biomass for trophic state classification or other management practices from Secchi depth data are inappropriate even where moderate amounts of non-algal turbidity are present.     

Effect of osmotic gradient on ADH-induced intramembranous particle aggregates in toad bladder

Summary Paired toad urinary bladders were prepared without or with an osmotic gradient (175 mosm) across them, stimulated for 2.5 (n=6), 5 (n=6), 30 (n=6) or 60 (n=6) min with ADH (20 mU/ml), and studied by freeze-fracture electron microscopy. Water permeability at these times was assessed in additional bladders (n=6 for each case) after tissue fixation according to the technique of Eggena. After both 60 and 30 min of ADH stimulation, the presence of a gradient compared with the absence of one was associated with fewer aggregates (242±35vs. 382±14 ×235 m^–2 at 60 min,P<0.01; 279±36vs. 470±51 ×235 m^–2 at 30 min,P<0.01) and lower water permeability (8.4±1.1vs. 18.8±1.8g×min^–1×cm^–1 ×mosm ^–1 at60min,P<0.005; 9.2±1.0vs. 22.0±2.1 g ×min^–1×cm^–2×mosm ^–1 at 30 min,P<0.001). In addition, with a gradient both maximum water permeability and maximum aggregate frequency were reached nearly together; a similar correspondence occurred without a gradient. We conclude that in the presence of an osmotic gradient both the ADH-associated aggregates and the water permeability response to ADH are prevented from reaching the higher levels observed in bladders not exposed to a gradient.     

Chemical,physical and biological changes associated with chara succession in farm ponds

Four farm ponds in various stages of Chara vulgaris succession -from the initial invasion of the alga to its nearly complete replacement-were followed each week from ice cover to ice cover. Chemical, physical, and biological parameters were analyzed in order to determine differences between ponds and to discover the biological and physicochemical factors associated with succession in Chara vulgaris ponds. The data were analyzed statistically by intraclass (single factor) analysis of variance, the Student's t-test, and stepwise multiple regression analysis.The initial invasion of C. vulgaris appeared to be preceded by a lowering of the nutrient levels of the pond by Zygnematales growth and by the substrate-stabilizing growth of Rhizoclonium sp. Chara-dominated ponds had significantly lower free CO₂, bicarbonate alkalinity, and total hardness readings. Carbonate alkalinity, low phytoplankton productivity, high dissolved O₂, low phosphate, and high Secchi disk readings were also characteristic of ponds in which C. vulgaris was dominant.In ponds where C. vulgaris appeared in the process of being replaced, the physicochemical factors were more closely associated with phytoplankton productivity and diversity and less closely with submerged vegetation species.Succession from C. vulgaris domination to a Najas-Potamogeton community appeared to involve shading of the former by the latter and was accelerated by the smothering effect of decaying debris caused by Gloeotrichia sp.In the final stages of succession, turbid conditions resulting from the lack of substrate-stabilizing vegetation and increased phytoplankton growth appear to have discouraged C. vulgaris growth early in the season.     

Effects of different forms of Acacia saligna leaves inclusion on feed intake, digestibility and body weight gain in lambs fed grass hay basal diet

The study was undertaken to determine the effects of different forms of Acacia saligna leaves inclusion on feed intake, digestibility and body weight gain in lambs fed grass hay basal diet. Twenty local ‘Farta’ yearling male lambs weighing 17.4 ± 0.10 kg (mean ± SD) were randomly allocated to four dietary treatments consisting of daily feeding of lambs in T1 with 700 g grass hay (dry matter basis), and daily free choice feeding of lambs in T2, T3, and T4 with 700 g grass hay + 700 g fresh, wilted, and dried A. saligna leaves, respectively. Inclusion of different forms of A. saligna leaves improved (P<0.01) the daily intake of total DM by 8–9% and that of OM and CP by 7 and 38%, respectively compared to the control. The daily intake of NDFom was lower (P<0.01) by 1–2.3%, whereas that of the ADFom intake declined (P<0.01) by 1.3–3.4% for lambs on wilted and dried forms of A. saligna leaves. Inclusion of A. saligna leaves replaced the intake of the grass hay by 26–27% of the total diet and reduced (P<0.05) apparent digestibility coefficient of the total DM by 9, 7, and 4%, OM by 7, 6 and 4%, and CP by 12, 12, and 9%, but improved (P<0.05) apparent digestibility coefficient of NDFom by 3–4% compared to the control diet. Dried A. saligna leaves improved (P<0.05) apparent digestibility of DM, OM and CP compared to fresh A. saligna leaves. Lambs fed on hay alone lost (P<0.05) body weight, while the lambs supplemented with A. saligna leaves gained (P<0.05) weight. The results of this study revealed that A. saligna leaves could be best included in the grass hay based feeding in dried form at the rate of 265 g/kg DM for improved nutrient utilization and growth performance of yearling lambs.     

Clearance rates and prey selectivity of the predaceous cladoceran Polyphemus pediculus

Littoral plankton communities have rarely been included in food web dynamics and predator/prey behavior studies of planktonic communities of fresh water ecosystems. Polyphemus pediculus, a typically littoral predaceous cladoceran, is common in lakes and ponds throughout the northern temperate zone.Diel clearance rates of Polyphemus were studied with a differential count method. A multi-chambered in situ feeding trap manipulated Polyphemus densities at three levels above the control ambient densities. Polyphemus generally selected small prey (Vorticella and Keratella) over large prey (nauplii) and more vulnerable prey (Conochilus and Polyarthra) over smaller prey with long protective structures (Kellicottia). Polyphemus clearance rates were inversely correlated with body volume of prey. Polyphemus clearance rates ranged from 4.2 to 4.5 ml h^–1 for Keratella and Vorticella respectively to <1 ml h^–1 for copepod nauplii. Selectivity coefficients were highest for small species, Vorticella and Keratella.     

Vergleichende Braunwasser-Teichstudien im niederösterreichischen Waldviertel

Summary Three groups of fishponds with brown water were studied during 1967–1970: A — fishponds in natural state on crystalline rocks, B — medium managedfishponds on tertiary formations, C — extensively managed peat-bog-like ponds on Cordioritgneis. The first group consists of extremely poor oligomineral ponds, pH never reaching 7, with an average transparency of 1 m, with maximum alkalinity 0,16 mval/l and with the cladoceran Holopedium gibberum in the plankton. The second group is treated with line and fertilizers, so that pH oscillates near 7, alkalinity between 0,4–1,44 mval/l and feeble water blooms occur in October and November, consisting of Aphanizomenon flos-aquae, Anabaena scheremetievi and Chrysopyxis paludosa. The third group comprises highly brownish colored pond with an average transparency of 69 cm. Many interesting algae were reported from these ponds, among them some new for science. Lists of phytoplankton and zooplankton show great numbers of species. Diatoms are characteristic for the first group, Chlamydomonas species for the second one. The rotifer Trichotria truncata is considered a glacial relict. Many of the Diatoms and Chrysophyceans prefer low light intensity and are thus able to live in the brown-water ponds, where the light is absorbed within a few meters.     

Phytoplankton in Two Ponds in Kathmandu Valley (Nepal)

A qualitative and quantitative analysis of the plankton of two ponds in Kathmandu Valley (Nepal) was made. Vertical and seasonal variations of the plankton of one pond were studied as well as physical and chemical properties of the water (temperature, visibility, electrical conductivity, pH-value, alkalinity). During the dry winter the number of plankton species and individuals was low. In summer, before the onset of the monsoon mass developments of different μ-algae and Ceratium hirundinella occurred.     

The toxigenic marine dinoflagellate Alexandrium tamarense as the probable cause of mortality of caged salmon in Nova Scotia

The toxins associated with paralytic shellfish poisoning (PSP) are potent neurotoxins produced by natural populations of the marine dinoflagellate Alexandrium tamarense. In early June 2000, a massive bloom (>7×10⁵ cells l⁻¹) of this dinoflagellate coincided with an unusually high mortality of farmed salmon in sea cages in southeastern Nova Scotia. Conditions in the water column in the harbour were characterised by the establishment of a sharp pycnocline after salinity stratification due to abundant freshwater runoff. In situ fluorescence revealed a high sub-surface (2–4 m depth) chlorophyll peak related to the plankton bloom. The intense bloom was virtually monospecific and toxicity was clearly related to the concentration of Alexandrium cells in plankton size fractions. Cultured clonal isolates of A. tamarense from the aquaculture sites were very toxic on a per cell basis and yielded a diversity of PSP toxin profiles, some of which were similar to those from plankton concentrates from the natural bloom population. The toxin profile of plankton concentrates from the 21–56 μm size fraction was complex, dominated by the N-sulfocarbamoyl derivative C2, with levels of other PSP toxins GTX4, NEO, GTX5 (=B1), GTX3, GTX1, STX, C1, and GTX2, in decreasing order of relative abundance. Although no PSP toxin was found systemically in the fish tissues (liver, digestive tract) from this salmon kill event, the detection of Alexandrium cells and low levels of PSP toxins in salmon gills provide evidence that the enhanced mortalities were caused by direct exposure to toxic Alexandrium cells and/or to soluble toxins released during the bloom.     

Influence of gizzard shad on phytoplankton size and primary productivity in mesocosms and earthen ponds in the southeastern U.S.

Gizzard shad (Dorosoma cepedianum), a filter feeding omnivore, can consume phytoplankton, zooplankton and detritus and is a common prey fish in U.S. water bodies. Because of their feeding habits and abundance, shad have the potential to affect primary productivity (and hence water quality) directly through phytoplankton grazing and indirectly through zooplankton grazing and nutrient recycling. To test the ability of shad to influence primary productivity, we conducted a 16-day enclosure study (in 2.36-m³ mesocosms) and a 3-year whole-pond manipulation in 2–5 ha earthen ponds. In the mesocosm experiment, shad reduced zooplankton density and indirectly enhanced chlorophyll a concentration, primary productivity, and photosynthetic efficiency (assimilation number). While shad did not affect total phytoplankton density in the mesocosms, the density of large phytoplankton was directly reduced with shad. Results from the pond study were not consistent as predicted. There were few changes in the zooplankton and phytoplankton communities in ponds with versus ponds without gizzard shad. One apparent difference from systems in which previous work had been conducted was the presence of high densities of a potential competitor (i.e., larval bluegill) in our ponds. We suggest that the presence of these extremely high larval bluegill densities (20–350 larval bluegill m^–3; 3–700 times higher density than that of larval gizzard shad) led to the lack of differences between ponds with versus ponds without gizzard shad. That is, the influence of gizzard shad on zooplankton or phytoplankton was less than the influence of abundant bluegill larvae. Differences in systems across regions must be incorporated into our understanding of factors affecting trophic interactions in aquatic systems if we are to be able to manage these systems for both water quality and fisheries.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.     

Phytoplankton community response to reservoir aging, 1968–92

The effects of reservoir aging on the phytoplankton community of amidwestern U.S. reservoir constructed in 1965 (Pawnee Reservoir) werestudied by comparing algal biovolume and species composition from April 1992through November 1992 to surveys conducted in 1968–73 and 1990. Meansummer total phosphorus, nitrate-nitrogen, Secchi disk depth, totalsuspended solids, chlorophyll a, and phytoplankton species composition datacharacterized Pawnee Reservoir during 1968–69 as a high nutrient,relatively clear water environment. Phytoplankton biomass was relativelylow, consisting mainly of cyanophytes and non-flagellated chlorophytes.During 1970–73, water clarity was poor, total suspended solids werehigh, and total phosphorus was lower, but was still greater than 100 µgl^–1. The 1970–73 phytoplankton biomass was high and wasdominated by cyanophytes. Mean summer total phosphorus remained >100µg l^–1, water clarity remained poor, but phytoplanktonbiomass decreased significantly during 1990–92. The dramatic drop inchlorophyll a and low mean volatile suspended solids indicated thatinorganic suspended sediments, rather than phytoplankton, accounted for themajority of the turbidity in 1990-92. In addition to lower phytoplanktonbiomass, community composition shifted away from buoyancy-regulatingcyanophytes toward flagellated chlorophytes. These data suggest that asreservoirs located in agricultural watersheds age, (1) inorganic suspendedsediments have a significant effect on the light environment as well asphytoplankton biomass and species composition, (2) the control ofphytoplankton biomass and species composition shifts away from nutrients tolight and suspended sediments, and (3) there is a 1–2 year lag in theresponse of phytoplankton biomass to maximum nutrient loading during thetrophic upsurge period. Thus, sedimentation has been shown to be a primarydeterminant of plankton and benthic macroinvertebrate community compositionas Pawnee Reservoir aged.     

Soil Organic Carbon and Water Retention after Conversion of Grasslands to Pine Plantations in the Ecuadorian Andes

Tree plantations in the high elevations of the tropics constitute a growing land use, but their effect on ecosystem processes and services is not well known. We examined changes in soil organic carbon (C) and water retention in a chronosequence of Pinus radiata stands planted in páramo grasslands in Cotopaxi province, Ecuador. Water retention at 10, 33, and 1,500 kPa declined with stand age, with soils in the oldest pine stands retaining 39%, 55%, and 63% less water than grassland soils at the three pressures tested. Soil organic C in the 0–10-cm depth also declined with stand age, from 5.0 kg m^–2 in grasslands to 3.5 kg m^–2 in 20–25-year-old pine stands (P < 0.001); at greater depth in the A horizon, C contents decreased from 2.8 to 1.2 kg m^–2 (P = 0.047). There were no significant differences among age classes in the AC and C horizons (P = 0.15 and P = 0.34, respectively), where little or no weathering of the primary material has occurred. Inputs of C may be affected by the significantly higher carbon–nitrogen (C:N) ratio of the litter under older pine stands (P = 0.005), whereas outputs are influenced by substrate quality as well as soil environmental factors. Soil ratios at the 0–10 cm depth were significantly higher in grasslands and young pine stands (P < 0.001), whereas carbon–phosphorous (C:P) ratios at 0–10-cm depth followed a similar but not significant trend. However, there was no significant difference in short-term decomposition rates (P = 0.60) when the soils were incubated under uniform temperature and moisture conditions. In páramo ecosystems, where high soil moisture plays an important role in retarding decomposition and driving high C storage, the loss of water retention after afforestation may be the dominant factor in C loss. These results suggest that soil C buildup and water retention respond rapidly to changes in biota and need to be assessed with regard to implications for C sequestration and watershed management.