Shifts in allochthonous input and autochthonous production in streams along an agricultural land-use gradient

Relative contributions of allochthonous inputs and autochthonous production vary depending on terrestrial land use and biome. Terrestrially derived organic matter and in-stream primary production were measured in 12 headwater streams along an agricultural land-use gradient. Streams were examined to see how carbon (C) supply shifts from forested streams receiving primarily terrestrially derived C to agricultural streams, which may rely primarily on C derived from algal productivity. We measured allochthonous input, chlorophyll a concentration, and periphyton biomass in each stream, and whole-stream metabolism in six streams. Our results suggest a threshold between moderate- and heavy-agriculture land uses in which terrestrially derived C is replaced by in-stream algal productivity as the primary C source for aquatic consumers. A shift from allochthonous to autochthonous production was not evident in all heavy-agriculture streams, and only occurred in heavy-agriculture streams not impacted by livestock grazing. We then compared our findings to rates of allochthonous input and GPP in streams with minimal human influences in multiple biomes to assess how land-use practices influence C sources to stream ecosystems. The proportion of C derived from allochthonous versus autochthonous sources to heavy-agriculture streams was most similar to grassland and desert streams, while C sources to forested, light-, and moderate-agriculture streams were more similar to deciduous and montane coniferous forest streams. We show that C source to streams is dependent on land use, terrestrial biome, and degradation of in-stream conditions. Further, we suggest that within a biome there seems to be a compensation such that total C input is nearly equal whether it is from allochthonous or autochthonous sources.     

Nitrogen dynamics in two antarctic streams

The many glacier meltwater streams of southern Victoria Land flow through catchments where life forms are almost entirely microbial. Allochthonous inputs of nitrogen from two study streams near McMurdo Sound were derived mostly from the melting glaciers (ca. 100–200 mg N m^–3) with some originating from N₂-fixation by heterocystous cyanobacteria (max. 939 mg N m^–2 year^–1). Thirty to fifty per cent of the glacier derived N was dissolved organic N and a major proportion of this was identified as urea N which was utilised by the rich algal and cyanobacterial mats in the streams. A nutrient budget for Fryxell Stream was estimated, quantifying uptake of urea-N and dissolved inorganic N and the release of dissolved organic (non urea) and particulate N by the stream communities. An index of in-stream nitrogen processing, the Net Uptake Length Constant in these streams was compared with that from temperate climates and was found to be similar. Despite the influence of low temperatures on microbial activity (mean daily water temperature = 5 °C) nutrient removal rates from these antarctic streams are high because of the large standing stock of microbial biomass there.     

Differences in carbon fluxes between forested and cultivated micronesian tropical peatlands

Taro is a staple crop that is often grown in wetlands throughout the Indo-Pacific, but the long-term impacts of its cultivation on wetland ecosystem functions are unknown. The objective of this study was to determine how cultivating taro affects carbon cycling by comparing key pathways in a forested peatland and an adjacent cultivated taro patch. Leaves decomposed rapidly at both sites with roughly 73% remaining after 2 weeks, 53% after 8 weeks, 38% after 17 weeks, and 17% after 36 weeks. Root decomposition proceeded much more slowly with roughly 93% remaining after 2 weeks, 80% after 8 weeks, 71% after 17 weeks, and 66% after 36 weeks. Annual litterfall was 1181 g m^–2 year^–1 and 849 g m^–2 year^–1 for the forested and cultivated sites, respectively. For the two sites combined, litterfall consisted of 78% leaves, 10% reproductive material, 3% branches, and 9% miscellaneous material. Fine root biomass was greater in the forested site than the cultivated site, averaging 205 g m^–2 and 34 g m^–2, respectively. Fine root production was much greater in the forested than the cultivated site, averaging 226 g C m^–2 year^–1 and 48 g C m^–2 year^–1, respectively. Soil respiration averaged 99 mg C m^–2 h^–1 and 55 mg C m^–2 h^–1 at the forested and cultivated sites, respectively. We found that the major change to carbon fluxes in the cultivated site was less carbon was entering the peatland, particularly less root production. Alterations to the carbon cycle caused by cultivation would probably not be permanent, because taro patches are periodically abandoned and allowed to regenerate naturally.     

Beaver invasion alters terrestrial subsidies to subantarctic stream food webs

North American beavers (Castor canadensis) were introduced to Tierra del Fuego Island in 1946 for their fur, and have since spread across the archipelago and onto the South American mainland. We assessed the impact of invasive beavers on streams of these forested watersheds by quantifying the trophic basis of production (TBP) and consumptive organic matter flows of benthic macroinvertebrate assemblages. TBP was determined in two streams: clear- and black-water. Stable isotopes were used across four streams to further elucidate food web structure and dominant pathways. TBP and stable isotopes showed that terrestrially derived organic matter (amorphous detritus, leaves, and wood) supported a majority of secondary production in the benthic food webs at all sites (forested reaches, beaver ponds, and sections downstream of ponds with foraged riparian zones). The magnitude of these flows was enhanced in beaver-modified sites compared with forested habitats (4.0–5.3× increase g AFDM m⁻² year⁻¹ in pond habitats, 1.1–2.1× increase in downstream habitats). Diatoms were the only autochthonous resource identified in macroinvertebrate guts, but their contribution to secondary production was small. Consumptive flows mirrored trends in TBP (i.e., dominance of terrestrial sources and greater magnitude in beaver ponds). Collector–gatherer consumption of amorphous detrital material dominated food web flows in all habitats, but was higher in beaver ponds relative to other habitats. Food web structure was simplified in beaver ponds; only two of the five possible functional groups contributed >1% of total organic matter flow in ponds (collector–gatherers and predators). Consumptive flows to predators increased in ponds, and stable isotopes of nitrogen and carbon (δ¹⁵N and δ¹³C) corroborated a relatively greater importance of predators (greater trophic distance), as well as less diversity of basal resources (less variation in δ¹³C) in ponds. Our findings indicate that invasive beaver’s engineering activities resulted in greater flows of terrestrial organic matter subsidies to in-stream food webs, which had a relatively greater change in the clear-water than in the black-water stream. Owing to the fact that these streams were naturally dependent on allochthonous resources for a majority of production and material flows, changes wrought by beavers to streams in forested environments are probably less than in watersheds with inherently greater dependence on autochthonous production such as the adjacent steppe biome.     

Top-down and bottom-up processes in grassland and forested streams

The influence of predatory fish on the structure of stream food webs may be altered by the presence of forest canopy cover, and consequent differences in allochthonous inputs and primary production. Eight sites containing introduced brown trout (Salmo trutta) and eight sites that did not were sampled in the Cass region, South Island, New Zealand. For each predator category, half the sites were located in southern beech (Nothofagus) forest patches (range of canopy cover, 65–90%) and the other half were in tussock grassland. Food resources used by two dominant herbivores-detritivores were assessed using stable isotopes. ¹³C/¹²C ratios were obtained for coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), algal dominated biofilm from rocks, and larvae of Deleatidium (Ephemeroptera) and Olinga (Trichoptera). Total abundance and biomass of macroinvertebrates did not differ between streams with and without trout, but were significantly higher at grassland sites than forested sites. However, taxon richness and species composition differed substantially between trout and no-trout sites, irrespective of whether streams were located in forest or not. Trout streams typically contained more taxa, had low biomass of predatory invertebrates and large shredders, but a high proportion of consumers with cases or shells. The standing stock of CPOM was higher at forested sites, but there was less FPOM and more algae at sites with trout, regardless of the presence or absence of forest cover. The stable carbon isotope range for biofilm on rocks was broad and encompassed the narrow CPOM and FPOM ranges. At trout sites, carbon isotope ratios of Deleatidium, the most abundant invertebrate primary consumer, were closely related to biofilm values, but no relationship was found at no-trout sites where algal biomass was much lower. These results support a role for both bottom-up and top-down processes in controlling the structure of the stream communities studied, but indicate that predatory fish and forest cover had largely independent effects.     

Spatial variation in basic chemistry of streams draining a volcanic landscape on Costa Rica's Caribbean slope

Spatial variability in selected chemical, physical and biological parameters was examined in waters draining relatively pristine tropical forests spanning elevations from 35 to 2600 meters above sea level in a volcanic landscape on Costa Rica's Caribbean slope. Waters were sampled within three different vegetative life zones and two transition zones. Water temperatures ranged from 24–25 °C in streams draining lower elevations (35–250 m) in tropical wet forest, to 10 °C in a crater lake at 2600 m in montane forest. Ambient phosphorus levels (60–300 µg SRP L^–1; 66–405 µg TP L^–1) were high at sites within six pristine drainages at elevations between 35–350 m, while other undisturbed streams within and above this range in elevation were low (typically <30.0 µg SRP L^–1). High ambient phosphorus levels within a given stream were not diagnostic of riparian swamp forest. Phosphorus levels (but not nitrate) were highly correlated with conductivity, Cl, Na, Ca, Mg and SO₄. Results indicate two major stream types: 1) phosphorus-poor streams characterized by low levels of dissolved solids reflecting local weathering processes; and 2) phosphorus-rich streams characterized by relatively high Cl, SO₄, Na, Mg, Ca and other dissolved solids, reflecting dissolution of basaltic rock at distant sources and/or input of volcanic brines. Phosphorus-poor streams were located within the entire elevation range, while phosphorus-rich streams were predominately located at the terminus of Pleistocene lava flows at low elevations. Results indicate that deep groundwater inputs, rich in phosphorus and other dissolved solids, surface from basaltic aquifers at breaks in landform along faults and/or where the foothills of the central mountain range merge with the coastal plain.     

Microbial activity under alpine snowpacks, Niwot Ridge, Colorado

Experiments were conducted during 1993 at Niwot Ridge in the Colorado Front Range to determine if the insulating effect of winter snow cover allows soil microbial activity to significantly affect nitrogen inputs and outputs in alpine systems. Soil surface temperatures under seasonal snowpacks warmed from –14 °C in January to 0 °C by May 4th. Snowmelt began in mid-May and the sites were snow free by mid June. Heterotrophic microbial activity in snow-covered soils, measured as C0₂ production, was first identified on March 4, 1993. Net C0₂ flux increased from 55 mg CO₂-C m^–2 day^–1 in early March to greater than 824 mg CO₂-C m-2 day^–1 by the middle of May. Carbon dioxide production decreased in late May as soils became saturated during snowmelt. Soil inorganic N concentrations increased before snowmelt, peaking between 101 and 276 mg kg^–1 soil in May, and then decreasing as soils became saturated with melt water. Net N mineralization for the period of March 3 to May 4 ranged from 2.23 to 6.63 g N m^–2, and were approximately two orders of magnitude greater than snowmelt inputs of 50.4 mg N m^–2 for NH₄ ⁺ and 97.2 mg N m^–2 for NO₃ ^–. Both NO₃ ^– and NH₄ ⁺ concentrations remained at or below detection limits in surface water during snowmelt, indicating the only export of inorganic N from the system was through gaseous losses. Nitrous oxide production under snow was first observed in early April. Production increased as soils warned, peaking at 75 g N₂O-N m^–2 day^–1 in soils saturated with melt water one week before the sites were snow free. These data suggest that microbial activity in snow-covered soils may play a key role in alpine N cycling before plants become active.     

Autochthonous primary production in southern Amazon headwater streams: Novel indicators of altered environmental integrity

The riparian forest reduces the amount of light entering streams, which limits autochthonous primary production. The aim of this study was to evaluate temporal variation of autochthonous primary production in pristine and altered streams, with the goal of identifying indicators of change in environmental integrity in the southern Brazilian Amazon. We evaluated free algal biomass in the water column, the presence of periphyton, and the richness and cover of aquatic herbaceous plants in 20 streams (10 pristine and 10 altered, i.e., with riparian deforestation) during the dry period, at the beginning of the rainy period, and at the end of the rainy period. In altered streams, we recorded the presence of macroscopic periphyton and the amount of algal biomass varied between the dry and flood seasons. Variations in hydrological periods did not contribute to changes in algal biomass in pristine streams; we did not observe the presence of macroscopic periphyton these streams. In altered streams, 23 aquatic herbaceous species were identified, versus only four in the pristine streams. Results showed that riparian deforestation contributes to increased autochthonous primary production, which is also influenced by different hydrological periods, with algae and aquatic herbaceous plants responding differently to dry and rainy periods. The responses of these primary producers confirm their role as important bioindicators of change in the environmental integrity of southern Amazonian streams.     

Effect of riparian land use on contributions of terrestrial invertebrates to streams

Since terrestrial invertebrates are often consumed by stream fishes, land-use practices that influence the input of terrestrial invertebrates to streams are predicted to have consequences for fish production. We studied the effect of riparian land-use regime on terrestrial invertebrate inputs by estimating the biomass, abundance and taxonomic richness of terrestrial invertebrate drift from 15 streams draining catchments with three different riparian land-use regimes and vegetation types: intensive grazing — exotic pasture grasses (4 streams), extensive grazing — native tussock grasses (6 streams), reserve — native forest (5 streams). Terrestrial invertebrate drift was sampled from replicated stream reaches enclosed by two 1 mm mesh drift nets that spanned the entire channel. The mean biomass of terrestrial invertebrates that entered tussock grassland (12 mg ash-free dry mass m^–2 d^–1) and forest streams (6 mg AFDM m^–2 d^–1) was not significantly different (p > 0.05). However, biomass estimated for tussock grassland and forest streams was significantly higher than biomass that entered pasture streams (1 mg AFDM m^–2 d^–1). Mean abundance and richness of drifting terrestrial invertebrates was not significantly different among land-use types. Winged insects contributed more biomass than wingless invertebrates to both pasture and tussock grassland streams. Winged and wingless invertebrates contributed equally to biomass entering forest streams. Land use was a useful variable explaining landscape-level patterns of terrestrial invertebrate input for New Zealand streams. Evidence from this study suggests that riparian land-use regime will have important influences on the availability of terrestrial invertebrates to stream fishes.     

Carbon balance of a tropical savanna of northern Australia

Through estimations of above- and below-ground standing biomass, annual biomass increment, fine root production and turnover, litterfall, canopy respiration and total soil CO₂ efflux, a carbon balance on seasonal and yearly time-scales is developed for a Eucalypt open-forest savanna in northern Australia. This carbon balance is compared to estimates of carbon fluxes derived from eddy covariance measurements conducted at the same site. The total carbon (C) stock of the savanna was 204±53 ton C ha^–1, with approximately 84% below-ground and 16% above-ground. Soil organic carbon content (0–1 m) was 151±33 ton C ha^–1, accounting for about 74% of the total carbon content in the ecosystem. Vegetation biomass was 53±20 ton C ha^–1, 39% of which was found in the root component and 61% in above-ground components (trees, shrubs, grasses). Annual gross primary production was 20.8 ton C ha^–1, of which 27% occurred in above-ground components and 73% below-ground components. Net primary production was 11 ton C ha^–1 year^–1, of which 8.0 ton C ha^–1 (73%) was contributed by below-ground net primary production and 3.0 ton C ha^–1 (27%) by above-ground net primary production. Annual soil carbon efflux was 14.3 ton C ha^–1 year^–1. Approximately three-quarters of the carbon flux (above-ground, below-ground and total ecosystem) occur during the 5–6 months of the wet season. This savanna site is a carbon sink during the wet season, but becomes a weak source during the dry season. Annual net ecosystem production was 3.8 ton C ha^–1 year^–1.     

Small-scale distribution and diel vertical migration of zooplankton in a shallow lake (Lake Naardermeer,the Netherlands)

Groundwater is a major influence on the hydrological, chemical and thermal regime of chalk streams in the southern U.K. However, little is currently known about the nature of the sediment delivery system within these chalk stream systems, even though sediment-related problems have been increasingly cited as a cause of habitat degradation and of declining salmonid stocks. To address this knowledge gap, suspended sediment fluxes were monitored at 4 sites within the Hampshire Avon catchment between February 1999 and August 2000. Maximum suspended sediment concentrations ranged from nearly 45 mg l^–1 to 260 mg l^–1. Over the study period, annual suspended sediment loads ranged from 644 to 6215 t yr^–1 and annual specific sediment yields ranged from 1.4 to 12.5 t km^–2 yr^–1. The results show that, relative to other U.K. rivers, the study chalk streams are characterised by low suspended sediment concentrations and loads and less episodic behaviour.     

Recent variations in sediment organic carbon content in Lake Soyang (Korea)

Four gravity cores were taken from Lake Soyang, which was created by the construction of an artificial dam in 1973. The variation of organic carbon content of the bottom sediments since the construction was interpreted using a variety of sedimentological and geochemical measurements. Based on the textural properties of sediments and the total organic carbon (TOC) content, it is clear that sediment organic carbon has been deposited very consistently throughout the postconstruction period, except for local sudden inputs caused by summer flooding events near fish farms or farmland areas. The autochthonous input resulting from the enhanced primary production led by phytoplanktons each year is likely to become a major problem at downstream sites within Lake Soyang. The vertical variation of δ¹³C values of sediment organic matter also reflects the fact that the level of autochthonous supply of organic carbon becomes high downstream.An erratum to this article can be found at .     

Catchment land cover influences macroinvertebrate food‐web structure and energy flow pathways in mountain streams

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Nitrogen and phosphorus budgets for the sub-tropical Richmond River catchment, Australia

Nitrogen and phosphorus budgets were developed forfour sub-catchments in the Richmond River catchmentfor two study years. The catchment is used for avariety of farming pursuits including dairying, beef,cropping, fruit, nuts, forestry, and sugar cane. Eachsub-catchment varies in hydrology, the proportion ofeach land use, and the population density whichenabled a unique opportunity to study fluxes andstorage associated with a variety of environmentalfactors. Total loadings entering each sub-catchmentvaried from 12 to 57 kg ha^–1yr^–1 fornitrogen and 0.25 to 6.6 kg ha^–1yr^–1 forphosphorus with little inter-annual variation.Averaged across the whole catchment, nitrogen fixation(47%) dominated the inputs; fertiliser (26%) andrainfall (21%) made up the next largest inputs.Fertiliser inputs dominated the phosphorus budget(65.5%); rainfall and manures making up 13% and 12%respectively. Produce dominated the outputs of bothnitrogen and phosphorus from the four sub-catchmentsbeing greater than the riverine export. The deliveryof nitrogen to catchment streams ranged from <1 to24% of the total inputs and the delivery of phosphorus to catchment streams ranged from <1 to 39%. Storage of phosphorus in catchment soils varied between –0.32 and 4.46 kg ha^–1yr^–1. Whendenitrification and volatilisation were estimated using data from other studies, storage of nitrogen ranged from 1 to 24 kg ha^–1yr^–1. Despite theepisodic nature of runoff in the sub-tropical RichmondRiver catchment, the magnitude of nutrient fluxes andstorage appear similar to other catchments of theworld which have mixed land use and relatively lowcatchment nutrient loadings.     

Bacterial Communities in Acidic and Circumneutral Streams

The relationship between pH and the abundance and activity of bacteria in streams was examined as part of a study of the effect of acidification on stream communities. Of the bacterial communities examined, the epilithic community appeared to be the most significantly affected by acidification. Microbial biomass, as quantified by measuring the ATP level, on rock surfaces was significantly correlated with pH. Also, bacterial production by the epilithic bacteria, indicated by incorporation of tritiated thymidine into DNA, was always higher at high-pH sites than at low-pH sites of the same stream order and elevation. Bacterioplankton concentrations varied between 0.53 × 10⁵ and 9.42 × 10⁵ cells · ml⁻¹ in the first- to fourth-order streams examined. The bacterioplankton concentration in one sample from a spring was 0.17 × 10⁵ cells · ml⁻¹. Bacterioplankton concentrations were not correlated with pH but were significantly correlated with seston concentrations. The correlation with seston is a result of increases in particle-associated bacteria at high seston concentrations. The proportion of bacterioplankton attached to particles varied from 0 to 70%. Bacterial numbers and production in the sediments were significantly correlated with the organic content of the sediment rather than with the pH of the overlying water. Thus, reduced abundance and activity of bacteria as a result of acidification could be detected only for the relatively active community on rock surfaces; this community was exposed to the low pH because of the unbuffered nature of its environment.     

Selective foraging on terrestrial invertebrates by rainbow trout in a forested headwater stream in northern Japan

The important contribution of terrestrial invertebrates to the energy budget of drift-foraging fishes has been well documented in many forested headwater streams. However, relatively little attention has been focused on the behavioral mechanisms behind such intensive exploitation. We tested for the hypothesis that active prey selection by fishes would be an important determinant of terrestrial invertebrates contribution to fish diets in a forested headwater stream in northern Japan. Rainbow trout, Oncorhynchus mykiss, were estimated to consume 57.12 mg m^–2 day^–1 (dry mass) terrestrial invertebrates, 77% of their total input (73.89 mg m^–2 day^–1), there being high selectivity for the former from stream drift. Both the falling input and drift of terrestrial invertebrates peaked at around dusk, decreasing dramatically toward midnight. In contrast, both aquatic insect adults and benthic invertebrates showed pronounced nocturnal drift. Because the prey consumption rates of rainbow trout were high at dawn and dusk, decreasing around midnight, the greater contribution of terrestrial invertebrates to trout diet was regarded as being partly influenced by the difference in diel periodicity of availability among prey categories. In addition, selectivity also depended upon differences in individual prey size among aquatic insect adults, and benthic and terrestrial invertebrates, the last category being largest in both the stream drift and the trout diets. We concluded that differences in both the timing of supplies and prey size among the three prey categories were the primary factors behind the selective foraging on terrestrial invertebrates by rainbow trout.     

The importance of terrestrial resource subsidies for shredders in open‐canopy streams revealed by stable isotope analysis

1. Allochthonous detritus is the major source of energy in forested streams, but less is known of the importance of terrestrial subsidies to open‐canopy streams. Here, we used stable isotope analysis to assess the importance of allochthonous versus autochthonous energy sources to invertebrate shredders in four open‐canopy streams in Sweden. Shredders and potential food sources were analysed at both open sites and those with deciduous trees in the riparian zone. 2. Mixing models showed that allochthonous coarse particulate organic matter was the most important energy source to shredders at both the open and wooded sites, suggesting that terrestrial subsidies may be an important process in open‐canopy streams, just as they are in forested streams. 3. However, shredders at open sites had a larger proportion of biofilm in their diet than at wooded sites, indicating an ability of shredders to adjust their diet with food availability. 4. We also used the carbon and nitrogen stable isotope signatures to assess the size of the feeding niche of shredders. Feeding on both allochthonous and autochthonous sources at open sites was reflected in a larger feeding niche than at wooded sites for one of the three species analysed. There was substantial overlap of the feeding niche among shredder species, indicating a high functional redundancy within this guild.     

Phosphorus and nitrogen retention in five Precambrian shield wetlands

Phosphorus and nitrogen mass balances of five wetlands (two beaver ponds, two conifer-Sphagnum swamps and one sedge fen) situated in three catchments in central Ontario, Canada, were measured. Monthly and annual input-output budgets of total phosphorus (TP), total nitrogen (TN), total organic nitrogen (TON), total inorganic nitrogen (TIN), ammonium ion (NH₄ ⁺ -N), nitrate (NO ₃ ^– -N) and dissolved organic carbon (DOC) were estimated for the five wetlands during the 1982–83 and 1983–84 water years. Except for the deepest beaver pond (3.2 m) which had annual TP retention of –44% (–0.030 ± 0.015 g m^–2 yr^–1), the wetlands retained < 0.001 to 0.015 g M^–2 yr^–1 ; however, this wasless than 20% of the inputs and the estimated budget uncertainties were equal to or greater than the retention rates. Annual TN retentions ranged from –0.44 to 0.56 g m^–2 yr^–1 (–12 to 4%) but were not significantly different from zero. The wetlands transformed nitrogen by retaining TIN (16 to 80% RT) and exporting an equivalent amount as TON (–7 to 102% RT). The beaver ponds, however, retained NO ₃ ^– while NH ₄ ⁺ was passed through or the outputs exceeded the inputs. In contrast, the conifer swamps retained both NH ₄ ⁺ and NO ₃ ^– . DOC fluxes into and out of the beaver ponds were equal (–18 and 4% RT) but output from the conifer swamps exceeded input by > 90%. Marked seasonal trends in nutrient retention were observed. Nutrient retention coincided with low stream flow, increased evapotranspiration and biotic uptake during the summer. Net nutrient export occurred during the winter and spring when stream flows were highest and biotic uptake was low.     

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.     

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

Although the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundations long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha^–1 y^–1 in the urban and suburban watersheds compared with less than 1 kg N ha^–1 y^–1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha^–1 y^–1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha^–1 y^–1) and atmospheric deposition (11.2 kg N ha^–1 y^–1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.