Benthic detritus dynamics in a mountain stream

The dynamics of sedimentary detritus was examined over a 17-month period in a third-order mountain stream in Colorado. Autumnal and vernal peaks in total detritus standing crop (197 g and 165 g AFDW m⁻², respectively) were associated with leaf abscission and ice out. The flushing action of snowmelt runoff rapidly reduced benthic detritus to 14 g AFDW m⁻². Detrital size classes differentially contributed to temporal fluctuations in total detritus. The autumnal maximum was largely comprised of coarse particulate organic matter (CPOM), primarily leaf litter. The large increase following ice out was mainly due to fine detritus (FPOM, 0.45–75 μm) which accounted for > 50% of benthic detritus during 9 months of the year. Evidence suggests that changes in standing crop and particle size distribution during the extended period of ice cover (November-March) resulted from in situ processing rather than transport. Despite major differences in climate, geochemistry and hydrology, the Colorado stream exhibited patterns of detrital dynamics remarkably similar to an eastern deciduous forest stream.     

PRODUCTIVITY OF PODOSTEMUM CERATOPHYLLUM IN THE NEW RIVER,VIRGINIA

Productivity of Podostemum ceratophyllum, the dominant aquatic macrophyte in the New River, was measured at four sites representing soft- and hardwater reaches of the river. Available dissolved inorganic carbon (DIC) was 4–5 times greater in the hardwater reach. The difference in available DIC was reflected in standing crop and productivity of P. ceratophyllum. Maximum standing crops of P. ceratophyllum at the two hardwater sites (Sites 1 and 2) were 244.8 ± 30.7 g ash-free dry wt (AFDW) m^−-2 and 193.8 ± 18.7 g AFDW m^−-2 compared to 128.5 ± 14.9 g AFDW m^−-2 and 101.3 ± 6.9 g AFDW m^−-2 for the softwater sites (Sites 3 and 4). Productivity, based on differences in standing crops, was: Site 1, 1.08 ± 0.12 g C m^−-2 d^−-1; Site 2, 0.86 ± 0.08 g Cm^−-2d^−-1; Site 3,0.58 ± 0.06 g C m^−-2 d^−-1; Site 4,0.45 ± 0.03 g C m^−-2 d^−-1. Corresponding values for productivity as ¹⁴C uptake were: 2.77 ± 0.44 g C m^−-2 d^−-1; 2.10 ± 0.45 g C m^−-2 d^−-1; 0.34 ± 0.04 g C m^−-2 d^−-1; 0.28 ± 0.03 g C m^−-2 d^−-1. Productivity/biomass (P/B) based on ¹⁴C uptake and standing crop revealed that P. ceratophyllum productivity was inhibited at the softwater sites perhaps due to carbon limitation. Because of its abundance and its high productivity, P. ceratophyllum is hypothesized to contribute significantly to the New River organic matter budget.     

Transport of coarse particulate organic matter in an Idaho river,USA

I investigated organic matter transport in the Henry's Fork of the Snake River, Idaho, USA, from August 1987 to November 1988. Mean discharge during the study was 15 m³ s^–1. Screens were used to sample very coarse (> 6 mm) transported aquatic macrophyte material (VCTMM). Drift nets were used to sample coarse (1–6 mm) and fine (0.25–1 mm) transported particulate organic matter (CTOM and FTOM). Mean monthly concentration of VCTMM was 0.064 mg AFDWl^–1 and was significantly higher than CTOM (0.024 mg AFDW l^–1) and FTOM (0.036 mg AFDW l^–1). VCTMM concentration was highest in December (0.163 mg AFDW l^–1) and lowest in May (0.018 mg AFDW l^–1). The sample position along a transect across the channel had a significant effect on the amount of transported organic matter collected in many months. The concentration of debris from individual species tracked the standing stock of that species during the growing season. In Fall, a dramatic increase in VCTMM corresponded to a decrease in macrophyte standing stock. FTOM and CTOM concentrations were highest in January (CTOM: 0.048; FTOM: 0.111 mg AFDW l^–1), lowest in November 1988 (<0.006 mg AFDW l^–1), were not correlated with discharge, and were inversely correlated with the standing stocks of macrophytes upstream, probably because macrophyte beds influenced the retentiveness of the channel. Standing stock of aquatic macrophytes was highest in September–October (5.2 kg wet weight m ^–2) and lowest in February (1.7 kg wet weight m^–2). Annual movement of particulate organic matter past the sampling point was about 45 000 kg AFDW, of which 21 000 kg was VCTMM, 8 000 kg was CPOM, and 16 000 kg was FPOM.     

湖北汉阳沉湖泛区浮游物的现存量与化学组成

本文报道了武汉市汉阳县沉湖泛区1983年5至9月浮游物(eston)的现存量与化学组成的动态,并根据该泛区的生态特征,分别对枯水期、灌江期和丰水期浮游物的干重(DW)、无灰干重(AFDW)、灰分重(Ash)、碳(C)和氮(N)量的变动特性进行了比较研究,而且还在此基础上,进一步探讨了评价浮游物营养质量的指标。沉湖泛区1983年5至9月浮游物干重为5.24mg/L,其中无灰干重为2.55mg/L,灰分重为2.69mg/L,它们均以灌江初期为高;碳、氮量分别为0.830和0.193mg/L,前者高峰出现在丰水后期,后者出现在灌江初期;浮游物干重、无灰干重、灰分重、碳和氮量均以灌江结束期为最低。浮游物的无灰干重、灰分重、碳和氮量分别占干重的51.60、47.50、18.30和3.90%;碳和氮量分别占无灰干重的36.40和7.70%。沉湖泛区浮游物的无灰干重/灰分重平均为1.24,碳/氮平均为4.74。这两个参数可分别作为评价沉湖泛区浮游物营养质量的指标。沉湖泛区浮游物干重、无灰干重、碳和氮量间,浮游物无灰干重与其灰分重、碳、氮量间,浮游物碳、氮量间均存在着极显著的相关关系,表明它们之间相应的动态是一致的。       

Relationships Between Abundance of Insects and Standing Crop of Coarse Detritus in a Pennsylvania Stream

Relationships between insect abundance and standing crop of coarse detritus, which was estimated both volumetrically and gravimetrically, were studied in a second-order stream (Little Sugar Creek, Pennsylvania) between June 1977 and June 1978. For individual taxa, coefficients of determination (R²) for relationships between insect abundance and standing crop of detritus were significant (P≦0.05) in 39% (by volume) and 32% (by weight) of the cases. Deposit feeding collectors accounted for only 41% of the significant R² values. For total fauna, R² values were significant (P≦0.05) on 7 of the 11 dates (by volume) and on 8 of the 11 dates (by weight).     

Stable musselMytilus edulis beds in the Wadden Sea—They’re just for the birds

Predation by eiders, oystercatchers and herring gulls on natural mussel bedsMytilus edulis was studied in the Königshafen, a sheltered bay in the Wadden Sea. About 15 ha (2.5%) of the Königshafen were covered with mussel patches of a biomass of about 1300 g AFDW m^?2. The biomass on the mussel beds was dominated by old mussels and found to be constant over several years. Birds annually removed 30% of the standing stock. Eiders were by far the most important predators and consumed 346 g AFDW m^?2, followed by oystercatchers with 28 g AFDW m^?2 and herring gulls with 3.6 g AFDW m^?2. Birds consumed a substantial part of the annual production of the mussel beds which was estimated from literature data to be approx. 500 to 600 g AFDW m^?2. As other predators were absent, the production of the mussels was sufficient to sustain the high predation rate by birds. Stable mussel beds form a short and efficient link between primary production and bird predation which is unusual for the Wadden Sea, where the main part of primary food supply is thought to be unavailable for higher trophic levels.     

Effects of nutrient availability on water hyacinth standing crop and detritus deposition

Nutrient-enriched water hyacinths were stocked in outdoor tanks and cultured under both high nutrient (HN) and low nutrient (LN) regimes for 10 months. Seasonal changes in standing crop biomass and morphology of LN water hyacinths were similar to those of HN water hyacinths, despite a ten-fold between-treatment difference in N availability and a two-fold difference in average plant N concentrations (1.0 and 2.0% for LN and HN plants, respectively). Tissue N accumulated by the LN plants prior to stocking helped support standing crop development during the 10 month study. In both HN and LN treatments, the rate of detritus deposition, or the sloughing of dead plant tissues from the mat, was lower than the actual detritus production rate because of the retention of dead ‘aerial’ tissues (laminae and petioles) in the floating mat. The retention of laminae and petioles may serve as a nutrient conservation mechanism, since nutrients released from decomposing tissues in the mat-water environment may be assimilated by adjacent plants. The average rate of detritus deposition (both dry matter and N) by LN water hyacinths (1.2 g dry wt. m⁻² day⁻¹ and 0.017 g N m⁻² day⁻¹) was lower than that of HN plants (3.0 g dry wt. m⁻² day⁻¹ and 0.075 g N m⁻² day⁻¹) during the study. Low detrital N losses by the water hyacinth probably enhance the survival of this species in aquatic systems which receive nutrient inputs intermittently.     

Seasonal dynamics of moss-dwelling chironomid communities

Chironomid communities of mosses in a small upland stream in central Germany were highly dynamic across the year with respect to their abundance, biomass and dominant taxa. During 1988 semi-submersed mosses near a main spring and those occurring some 700 m downstream were compared with permanently submersed mosses in immediate vicinity of the downstream site. All the chironomids sampled were conspicuously small, with nearly 98% being less than 5 mm in length. A total of 65 chironomid species from 26 genera were found, with a higher diversity occurring near the source and a change in dominant taxa along the upper stream section. The mean abundance in permanently submersed mosses (250 larvae/10 cm², n = 125) was about five times higher than in semi-submersed mosses. The maximum value of 830 larvae/10 cm² (n = 1) is the highest chironomid density ever reported, which is explained by the sampling method used. The mean standing crop was also highest in permanently submersed mosses (1.5 mg AFDW/10 cm² (n = 125)), even though the highest individual value was recorded in semi-submersed mosses near the spring (10.4 mg AFDW/10 cm²). The evidence suggested that the dominance of chironomid taxa depended mainly on the location of the moss along the stream, whereas abundance and biomass were determined mainly by constancy in the ambient discharge as well as the factors influenced by this (e.g. temperature, detritus deposition). A trend was seen towards a seasonal succession among the chironomid taxa colonizing lotic mosses.     

Life history,population dynamics and production of Pontoporeia hoyi (Crustacea,Amphipoda) in relation to the trophic gradient of Lake George,New York

The life history characteristics, population dynamics and production of Pontoporeia hoyi in Lake George, New York, were studied from May 1981 through October 1982. P. hoyi, in terms of both density and standing crop, is the most prevalent member of the deep water macrobenthos of Lake George. It reproduces in the winter, with young being released in the late winter-early spring. At the southernmost study site, young released in the spring grew to 6–7 mm in length and bred during their first winter. At the remaining sites, P. hoyi required two years to complete its life cycle. This difference in life history characteristics can be related to food availability and temperature differences. The open waters of the south end of Lake George are not only more productive but are also more closely associated with the littoral zone, providing a wealth of bacteria-rich detritus for benthic deposit feeders. The greater food availability in the south basin of Lake George is reflected in significantly larger brood sizes and smaller size at maturity for P. hoyi populations from the south end of the lake.The southernmost study site has significantly greater P. hoyi density and standing crop than all other sites. The cohort of the year dominated density and standing crop at the southern site while the cohort of the previous year dominated standing crop at the other sites. Peak abundance ranged from 600 · m^–2 at the north site to 2 900 · m^–2 at the south site. Cohort production ranged from 2g · m^–2 at the north site to 15g · m^–2 at the south site.     

Particulate organic matter in a mountain stream in the south-western Cape,South Africa

The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a second-order, perennial mountain stream in the south-west Cape, South Africa, between April 1983 and January 1986. Although the endemic, riparian vegetation is sclerophyllous, low and evergreen, inputs of allochthonous detritus to the stream (434 to 500 g m^–2y^–1) were similar to those recorded for riparian communities worldwide, as were calorific values of these inputs (9548 to 10 032 KJ m^–2y^–1). Leaf fall of the riparian vegetation is seasonal, occurring in spring (November) as discharge decreases, resulting in retention of benthic organic matter (BOM) on the stream bed during summer and early autumn (maximum 224 g m^–2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g m^–2). Therefore, BOM was predictably plentiful for about half of each year and predictably scarce for the other half. Coarse BOM (CBOM) and fine BOM (FBOM) constituted 46–64% of BOM standing stock, ultra-fine BOM (UBOM) 16–33% and leaf packs 13–24%. The mean annual calorific value of total BOM standing stock was 1709 KJ m^–2. Both standing stocks and total calorific values of BOM were lower than those reported for streams in other biogeographical regions. Values of C:N ratios decreased with decrease in BOM particle size (CBOM 27–100; FBOM 25–27; UBOM 13–19) with no seasonal trends. The stream is erosive with a poor ability to retain organic detritus. Its character appears to be dictated by abiotic factors, the most important of which is winter spates.     

Path analysis of standing crop and environmental variables in the field layer of two Belgian riverine forests

Seasonal variation in standing crop, litter, pH and relative light intensity was measured in the field layer of two riverine forests near Bruges, Belgium. At each site the living fraction in 10 randomly selected 0.25 m² quadrats was separated into various components and graphed in relation to the environmental variables. The effect of the environmental variables on standing crop was assessed using path analysis, based on the standardized partial regression coefficients obtained in separate stepwise multiple regressions. In order to linearize the relation between the standing crop and the environmental variables, and to meet other statistical assumptions, a log-transformation was applied to the standing crop data. On an annual basis, 44% of the total variation in log-standing crop was explained by the variables litter, relative light intensity and pH, while seasonally the coefficient of determination ranged from 0.23 (November) to 0.72 (April). A general path diagram, based on all data, indicated predominant direct effect of litter (path coefficient: ?0.55) and pH (0.26). Relative light intensity had only an indirect effect through its impact on litter and pH. Considerable seasonal changes occurred in the importance of litter, relative light intensity and pH.     

Population dynamics and production of the tropical freshwater shrimp Caridina nilotica (Decapoda: Atyidae) in the littoral of Lake Sibaya

SUMMARY. Seasonal changes in population structure, standing stock levels and production of Caridina nilotica were studied at three sites in the littoral margins of subtropical Lake Sibaya between January 1975 and March 1976. Average population density at these sites declined from a maximum of c. 1400 to a minimum of c. 350 individuals per m² (3.4–0.4 g m^?2 dry wt) during the study, possibly as a result of emigration into peripheral vegetation inundated by rising lake levels. Shrimps bred perennially and, although egg stocks and instantaneous birth rates (b) were highest during summer, no corresponding increases in populaton density were observed, suggesting that the seasonally higher birth rates were offset by higher mortality rates. Population size structure and size-specific sex ratios did not change seasonally to any marked extent. Relative abundance declined with size and females grew larger than males. Clutch size increased linearly as a function of female carapace length. Estimates of overall mean annual somatic production (g m^?2 year^?1 dry wt) for the three sites between January 1975 and January 1976 ranged between c. 132 (egg-ratio method), 37.5 (summation of growth increments) and 24 (Hynes-Hamilton method) at an annual mean standing stock level of 2.7 g m^?2 dry wt (calorific value, 20.34 kJ g^?1 dry wt). Production at sites 1, 2 and 3 decreased in line with declining annual mean standing stocks (5.32, 3.67 and 0.23 g m^?2, respectively). The growth increment method gave an overall mean annual P/B value of 13.9. Egg production amounted to a further 5.6, 3.6 and 0.1 g m^?2 year^?1 dry wt (calorific value, 28.01 kJ g^?1) at sites 1, 2 and 3, or 2.7 g m^?2 year^?1 on average.     

Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s^?1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish. 2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass. 3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream^?1 day^?1 and 10.4 g detritus stream^?1 day^?1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream^?1 day^?1 and <1–22 individuals m^?3 water; <1–286 g detritus stream^?1 day^?1 and <0.1–1.7 g detritus m^?3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m^?2 year^?1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment.     

Typha productivity in a Texas pond: Implications for energy and nutrient dynamics in freshwater wetlands

Above-and below-ground biomass of Typha angustifolia L. was sampled monthly for 18 months from a small Texas pond. Maximum above-ground biomass was 2559±284 g AFDW (ash-free dry weight) m⁻² in 1983 and 2895±217 g AFDW m⁻² in 1984. Peak below-ground biomass for these 2 years was 2506±278 g AFDW m⁻² and 2314±226 g AFDW m^t-2, respectively. Stepwise multiple linear regression analyses revealed that mean above-ground biomass accrual was related to duration of growing season, cumulative precipitation, cumulative degree days and/or cumulative pan evaporation. The same was not true for below-ground biomass increases. Analysis of covariance revealed that the rates of above-ground biomass production were not significantly different (F test, p < 0.05) between the 1983 and 1984 growing seasons. Below-ground biomass turnover times for 1983 and 1984 were 2.47 and 1.21 years, respectively.     

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.     

Resource dynamics and detritivore production in an acid stream

1. Life history patterns and production of eight shredder‐detritivore species were studied in relation to the detritus dynamics of a small acidic stream in England. Mean annual detritus inputs (direct and lateral sources combined) were approximately 400 g m^?2 year^?1 and showed significant seasonal and annual variation. 2. Detritus standing stock did not increase significantly during times of high input, reflecting low retention efficiency. However, the mean detritus standing stock was relatively large (108 g m^?2) reflecting a slow decomposition rate typical of acid streams. 3. Four species were univoltine with highly synchronous patterns of emergence and recruitment (Leuctra inermis, Leuctra hippopus, Capnia vidua and Amphinemura sulcicollis). Two species were univoltine with extended patterns of emergence and recruitment (Nemoura cinerea, Potamophylax cingulatus). Leuctra nigra was apparently semivoltine, while Protonemura meyeri showed two successive cohorts in the second year of the study, suggesting either bivoltinism or cohort splitting. 3. Secondary production of the dominant shredders was 1.67 g m^?2 year^?1 in 1997 and 1.99 g m^?2 year^?1 in 1998, which is low compared with other small European streams. This was probably because of an impoverished invertebrate community and poor food quality associated with acid conditions. Food availability probably did not account for the low production as the detritus standing stock far exceeded the estimated shredder ingestion of 42–50 g m^?2 year^?1. 4. Despite low overall shredder production, species‐specific production was high, possibly because of competitive release in this species‐poor acid stream. Periods of high production and growth showed no relationship with detritus availability but were closely related to life history.     

Herbs and grasses as an allochthonous resource in open-canopy headwater streams

1. The organic matter dynamics of streams dominated by herbs and grass on their banks are poorly understood, despite the fact that such streams are common worldwide. Further, herbs and grasses can provide large quantities of detritus to stream food webs, and particularly small streams can be heavily shaded by overhanging vegetation, perhaps limiting in‐stream primary production. 2. We quantified the standing crop of edge vegetation and associated macroinvertebrate communities along three headwater streams with herbaceous and grass riparian vegetation on agricultural land in the Piedmont of Maryland, U.S.A., measured the decomposition of four common species of herbs and grasses using experimental leaf packs, and removed edge vegetation experimentally to determine the effect of shading on benthic algal production. 3. Large standing crops of plant material (average range: 68–276 g ash‐free dry mass per m⁻²), composed largely of monocotyledons, were found at all three study streams. These values are similar to those for coarse particulate organic matter in deciduous forested streams in the eastern U.S.A. In addition, diverse assemblages of shredding macroinvertebrates were observed at all three study sites. 4. Decomposition of the herbs was faster than that of the grasses, and both decomposed faster than most deciduous tree leaf litter. The decomposition rates of the herbs and grasses were significantly related to leaf quality as measured by leaf nitrogen content. Macroinvertebrate shredders colonized all experimental leaf packs, and the colonization of the herbs was faster than that of the grasses. 5. The accrual of chlorophyll‐a after the removal of shading vegetation was faster than that measured prior to removal as well as that in an unmanipulated control reach. 6. Given that the standing crop of organic matter in streams with herbs and grass along their banks was similar to that in forested streams, that the organic matter was rich in nitrogen and used by detritivores, and riparian shading limited algal growth, we suggest that herbaceous and grass plant material may be an important allochthonous food resource in such systems.     

Production and standing crop patterns of giant cutgrass (Zizaniopsis miliacea) in a freshwater tidal marsh

Summary Growth cycles and production of aboveground and belowground giant cutgrass (Zizaniopsis miliacea (Michx.) Doell and Asch.) were studied in a freshwater tidal marsh along the lower Savannah River near Savannah, Georgia, USA. Minimum aboveground live standing crop (142 g/m²) occurred in March with a steady increase thereafter to an October maximum (1,039 g/m²) followed by a rapid decline. Giant cutgrass aboveground net primary production was approximately 1,530 g/m²/yr. Rhizomes (358 g/m²/yr) and roots (160 g/m²/yr) yielded an annual belowground production of 518 g/m²/yr. Total above and belowground annual net production was estimated at 2,048 g/m²/yr.Present Address: Institute of Ecology, University of Georgia, Athens, Georgia, 30602, USA     

Biomass yield and nutrient removal by water hyacinth (Eichhornia crassipes) as influenced by harvesting frequency

The effects of harvesting frequency on productivity, nutrient storage and uptake, and detritus accumulation by water hyacinth (Eichhornia crassipes /Mart/ Solms) cultured outdoors in nutrient-enriched waters were evaluated for a period of 13 months. Significant differences in hyacinth standing crop and productivity were measured with harvesting regimes of 1, 3 (harvest at maximum density) and 21 harvests over a 13-month period. The average plant standing crop decreased from 65 to 20 kg (fresh wt) m⁻² for systems with 1 and 21 harvests, respectively. Total harvested plant biomass was 67 kg (fresh wt) m⁻², 110 kg (fresh wt) m⁻² and 162 kg (fresh wt) m⁻² for 1, 3 and 21 harvests, respectively. The mean net productivity increased from 7·7 to 16·5 and 24·5 g (dry wt) m⁻² day⁻¹ for 1, 3 and 21 harvests, respectively. Nutrient storage in water hyacinth biomass (live, dead and detrital) at the end of the study decreased from 93 to 46 and 30 g N m⁻², and from 20 to 12 and 5 g P m⁻², for 1, 3 and 21 harvests, respectively. For the system with one harvest, 46% of the stored N and 25% of the stored P were recovered in dedrital tissue at the bottom of the tank. For the systtem with 21 harvests, only 11% of the stored N and 15% of the stored P were recovered in detrital tissue at the bottom of the tank. Ammonium-N and soluble reactive P concentrations in the water column were significantly higher for the treatment with one harvest compared to the treatments with 3 and 21 harvests.     

Seasonal changes in chemistry,algal populations,chlorophyll a and photosynthetic activity in the R. Delimi,Jos Plateau,Nigeria

Seasonal studies were undertaken (August 1983–July 1984) toevaluate the algal distribution and ecology of the RiverDelimi, Jos Plateau, Nigeria at three stations: upstream, withlittle allochthonous input; midstream, receiving heavy domesticaffluents; downstream. A wide amplitude of variability wasdiscernible in physicochemical factors, and photosyntheticactivity and abundance of suspended algae in relation to thedegree of pollution along the first 7-km stretch of the river.Conductivity (µS cm^-1) showed a 9-fold increase fromupstream (x = 35) to downstream. The standing crop(Chl a) fluctuated markedly, with 6-times higher valuesfor a polluted midstream site (x = 18.2 mg m^-3). Production rate measurements (mg C m^-3 d^-1) showed significant variability with relatively lowvalues (x = 610) recorded for the upstream andhighest (x = 1050) at the midstream site. Apositive correlation was obtained between production rates andChl a for the polluted midstream zone.