Soluble Nutrient Concentrations Research Articles

Water availability and seasonality shape elemental stoichiometry across space and time.

The interaction of climate change and increasing anthropogenic water withdrawals is anticipated to alter surface water availability and the transport of carbon (C), nitrogen (N), and phosphorus (P) in river networks. But how changes to river flow will alter the balance, or stoichiometry, of these fluxes is unknown. The Lower Flint River Basin (LFRB) is part of an interstate watershed relied upon by several million people for diverse ecosystem services, including seasonal crop irrigation, municipal drinking water access, and public recreation. Recently, increased water demand compounded with intensified droughts have caused historically perennial streams in the LFRB to cease flowing, increasing ecosystem vulnerability. Our objectives were to quantify how riverine dissolved C: N: P varies spatially and seasonally and determine how monthly stoichiometric fluxes varied with overall water availability in a major tributary of LFRB. We used a long-term record (21-29 years) of solute water chemistry (dissolved organic carbon, nitrate/nitrite, ammonia, and soluble reactive phosphorus) paired with long-term stream discharge data across six sites within a single LFRB watershed. We found spatial and seasonal differences in soluble nutrient concentrations and stoichiometry attributable to groundwater connections, the presence of a major floodplain wetland, and flow conditions. Further, we showed that water availability, as indicated by the Palmer Drought Severity Index (PDSI), strongly predicted stoichiometry with generally lower C:N and C:P and higher N:P fluxes during periods of low water availability (PDSI < -4). These patterns suggest there may be long term and significant changes to stream ecosystem function as water availability is being dramatically altered by human demand with consequential impacts to solute transport, in-stream processing, and stoichiometric ratios. This article is protected by copyright. All rights reserved.

Earthworm-Collembola interactions affecting water-soluble nutrients, fauna and physiochemistry in a mesocosm manure-straw composting experiment

A mesocosm fermentation experiment was undertaken to investigate interactions between Eisenia fetida and Collembola affecting composting processes. Earthworms, Collembola, respiration, water soluble nutrients and compost characteristics (near infrared spectra – NIRS) were monitored on four occasions over 136 days. Earthworms were the main drivers of early changes in composts, increasing the general abundance of Collembola, although responses varied with species. Earthworms accelerated substrate mineralisation and release of soluble nutrients whilst also changing compost characteristics. Collembola alone had little direct effect on soluble nutrient concentrations or respiration; they did however alter compost characteristics (NIR spectra). Earthworm-Collembola interactions affecting respiration and soluble nutrients were mainly antagonistic in the early stages of composting but synergistic in later stages. In the later stages of composting, the higher abundance of Collembola when combined with earthworms resulted in greater concentrations of soluble nitrate and phosphate. These findings emphasise the importance in vermicomposting practice of different invertebrate groups having access to feedstock at appropriate stages of the process. The high concentrations of soluble nutrients released during vermicomposting indicate the need for control measures to avoid off-site pollution and loss of this resource.

Colonial nesting waterbirds as vectors of nutrients to Lake Lesser Prespa (Greece)

ABSTRACT The nutrients imported by breeding waterbirds should be considered when identifying the main sources of nutrient input to lakes. Lake Lesser Prespa (Greece), including the adjacent Vromolimni pond, hosts numerous protected waterbirds that nest in densely populated colonies across the reedbeds. The accelerated eutrophication of the lake in recent years has been of increasing concern. In addition to likely large sources of nutrients (i.e., anthropogenic activities, especially agriculture), nutrient input via waterbird excrement may further trigger eutrophication. We estimated the annual phosphorus (P) and nitrogen (N) input by the most abundant colonial-nesting waterbirds (great white pelican, Dalmatian pelican, great cormorant, and pygmy cormorant) into the lake and investigated their influence on water and sediment quality. Near the waterbird colonies, soluble nutrient concentrations in the lake sediments were higher, and chlorophyll measurements indicated higher algal growth near these sites in summer. Stable isotope analysis suggests that near the colonies, waterbirds are responsible for nutrient loadings that affect the lake sediment. The estimated N and P nutrient input into the lake by both pelican and cormorant species is at least 1243 and 1649 kg/yr, respectively. On a landscape scale, this level of loading could be of minor importance for the lake because N and P can reach 32.8 (SD 9.3) and 38.9 (5.8) mg/m2 per year, respectively. Locally, however, this level of loading might induce cyanobacterial blooms, illustrated by the analysis of isolated Vromolimni pond near the lake. Our findings emphasize the likely importance of nutrient loading by waterbirds for the lake system.

Burn Severity Effects on Sediment and Nutrient Exports from Southeastern Forests Using Simulated Rainfall

Abstract Burn severity, commonly assessed as the amount of fuel consumed during fire, is an indicator of postfire sediment yield and erosion. This study examined the effect of burn severity on sediment and nutrient response in three different fire-adapted forest types of the Southeast. Soil and litter samples were experimentally burned to achieve increasing levels of fuel consumption. Simulated rainfall was applied to burned litter samples collected from pine, hardwood, and mixed hardwood-pine forests in the Clemson Experimental Forest. Runoff and leachate samples were collected and analyzed for sediment yield (kilograms per hectare) and total suspended solids (grams per liter); both runoff and leachate samples were analyzed for ammonium (NH4–), nitrate (NO3–), and orthophosphate (PO43–). Sediment yield and total suspended solids increased at only the highest burn severity treatment in all three forest types, with pine litter samples yielding significantly greater sediment in surface runoff than both mixed and hardwood samples. Burn treatment did not readily affect soluble nutrient concentrations in either runoff or leachate, but the data suggest that high-severity burning increases the availability of PO43– bound to sediment. This study suggests that high-severity burn patches function as sediment sources, but overall sediment and nutrient response to burning was minimal. Study Implications This study suggests that low-to-moderate burn severity, in terms of litter removal, is not an important indicator of sediment and nutrient exports from southeastern forests; precipitation in the immediate aftermath of fire (&amp;lt;48 hours) is not an effective transport mechanism for biologically available macronutrients; and that even at the highest burn severity treatment, sediment yield was relatively low.

Effect of inoculants combining Lactobacillus buchneri strain LN40177 in different strata of the silo

The objective of this work was to evaluate the chemical composition and rumen disappearance rate of dry matter of corn silages with inoculants combining L. buchneri strain LN40177 in different strata of the silo. The experimental design was a 3x2 randomized complete block design, with three treatments: Control: corn silage without inoculant; 11CFT: corn silage with inoculant which combines L. buchneri strain LN40177 (1.1 x 1011 CFU g-1) with L. casei (1.1 x 1011 CFU g-1); and 11C33: corn silage with inoculant which combines L. buchneri strain LN40177 (1.1 x 1011 CFU g-1) with L. plantarum (1.1 x 1011 CFU g-1) and Enterococcus faecium (1 x 1010 CFU g-1), associated with two strata of the silo (lower and upper). The silage inoculated with 11C33 presented higher contents of crude protein and NDF and lower hemicellulose content in relation to the control treatment and 11CFT. The use of both inoculants resulted in silages with higher concentrations of soluble nutrients. Lower stratum silage had a higher rumen disappearance rate of dry matter compared to the upper stratum. In general, the combinations of L. buchneri promoted nutritional improvements in corn silage, but in presence of L. casei, there were more outstanding improvements.

ĐÁNH GIÁ ĐẶC ĐIỂM THỦY HÓA TRONG MÔ HÌNH TÔM SINH THÁI TẠI XÃ TAM GIANG, HUYỆN NĂM CĂN, TỈNH CÀ MAU

This paper clarifies the hydrochemical characteristics of the organic shrimp model certified by Naturland at Tam Giang commune, Nam Can district, Ca Mau province. Pond water was sampled in 8 ponds at 3 points of time of distinct precipitation fluctuations during the year (March, July and November 2015) and hydrochemical parameters related to shrimp were analysed. Results show that salinity, total alkalinity and total hardness are largely dependent on the seasonal precipitation and evapotranspiration, highest in the middle of the dry season (March) and gradually reduced until the wet season-dry season transition (November). Most of the parameters fluctuate within the sampling times but still remain suitable for shrimp. The major constraints include oxygen supersaturation during daylight hours (7.48 ± 0.45 mg/l), low concentrations of soluble nutrients (NH4-N 0.21 ± 0.05 mg/l, NO3-N 0.06 ± 0.02 mg/l, PO4-P 0.02 ± 0.01 mg/l), and high iron contents (Fe2+ 0.08 ± 0.01 mg/l, Fe3+ 0.64 ± 0.14 mg/l) as compared to limits for shrimp aquaculture. The total shrimp yields are very low (355.4 kg/ha water surface/year in which wild shrimp species have accounted for 55%), positively correlated with water depth and temperature but negatively correlated with Fe2+. To improve pond water quality and shrimp yield, it is recommended to increase water depth, nutrient concentrations and restrict the effects of runoff at the beginning of the wet season.

Using Spatial Analysis of ANR1 Gene Transcription Rates for Detecting Nitrate Irregularities in Cherry Tomatoes (Solanum lycopersicum var. cerasiforme) Organic Greenhouses

Organic fertilizers differ from the chemical ones by high inconsistency in their mineralization rates in the soil. This intrinsic property occasionally results in formation of distinct ‘soil patches’ different from one another with regard to the concentrations of soluble nutrients. This feature together with the different nutritional requirements during the crop’s physiological development complicates the designing of appropriate organic fertilization scheme. To overcome these difficulties a methodology was designed and tested based on analysis of molecular indicators. As a case study we tested cherry tomato (Solanum lycopersicum var. cerasiforme) organically grown in greenhouse located in Netzer Hazani village, Northern Negev, Israel at 2001. As a model nutrient we chose the soluble nitrate, due to its importance for the crop growth and development. We found that the best indicator of nitrate content were the ANR1 gene transcription rates which values were best correlated to the measured soluble nitrate in soil and the crop’s needs during development. Practically, the spatial analysis helped identifying the surpluses and deficits of soluble nitrate in soil patches which were subsequently treated in a quick and precise manner. Implementing this methodology on other crops and nutrients will allow constructing accurate and economical fertilization scheme which decrease the damages to ecosystems.

Remediation of Flow‐Through Trout Raceway Effluent via Aquaponics

AbstractProduction and nutrient removal were assessed for three vegetable crops (kohlrabi, lettuce, and Swiss chard) and two edible flowers (calendula and nasturtium) grown entirely on effluent from a flow‐through trout raceway. Soluble nutrient concentrations in the effluent averaged 0.56 and 0.19 mg/L for total nitrogen and soluble reactive phosphate, respectively. Seeds were sown into Styrofoam trays filled with vermiculite and placed directly into the plant growing channels. Lettuce was harvested at 6, 9, and 12 weeks for a total mean harvest biomass of 4.5 kg/m2. Kohlrabi and Swiss chard were harvested at 12 weeks, with a mean harvest biomass of 15.4 and 7.5 kg/m2, respectively. Harvest biomass of kohlrabi was significantly greater than that of lettuce or Swiss chard. Nasturtium flowers were harvested at 9 and 12 weeks, while calendula did not begin to produce flowers until 12 weeks. There were no significant differences in percent removal of total ammonia nitrogen (TAN), nitrate, or phosphate at 6 weeks. However, at 9 weeks, kohlrabi, Swiss chard, and nasturtium removed significantly more TAN and nitrate than lettuce and calendula. Total ammonia nitrogen removal decreased at 12 weeks while nitrate removal increased for kohlrabi, Swiss chard, and calendula. Phosphate removal was low for all cultivars and may be the result of high phosphate concentrations in the source water. Careful crop selection will be necessary to maximize the benefits of aquaponics as crops varied in growth rates, overall nutrient removal capacity, and timing of nutrient removal.

Vegetation cover and rainfall seasonality impact nutrient loss via runoff and erosion in the Colombian Andes

Mountain ecosystems provide key services to a large portion of the population in the tropics. However, they are particularly vulnerable to regional environmental changes such as soil degradation, via soil erosion and associated nutrient loss, both dissolved in runoff and suspended in sediment. Current trends in land use conversion combined with projections of intensified hydrological extremes potentially amplify these threats. We analyze the interactive effects of rainfall characteristics (at three time scales) and vegetation cover on the runoff–erosion–nutrient loss progression for a group of vegetation cover types that represent different land use conversion stages. After a year of observations we found, as expected, that natural forests have the highest potential for regulating precipitation–runoff–erosion–nutrient loss. The highest amounts of runoff occurred in pasturelands, and croplands had the highest erosion losses. Croplands showed the highest concentrations of soluble nutrients in runoff and in sediment. However, due to higher runoff amounts, pasturelands had the greatest loss of dissolved nutrients. Precipitation seasonality significantly influenced both erosion and nutrient loss. This is particularly critical in managed agricultural and pasture systems where increased runoff and erosion rates, combined with unsustainable management practices, may lead to alterations in soil and water quality. Our results indicate how agricultural practices need to adapt fertilization scheduling to rainfall seasonality to minimize potential environmental impacts. Collectively, our results highlight a fundamental management need in tropical mountains where the combination of rapid land use change and altered climate threatens ecosystem integrity and ecosystem services.

Structures of Microbial Communities in Alpine Soils: Seasonal and Elevational Effects.

Microbial communities in alpine environments are exposed to several environmental factors related to elevation and local site conditions and to extreme seasonal variations. However, little is known on the combined impact of such factors on microbial community structure. We assessed the effects of seasonal variations on soil fungal and bacterial communities along an elevational gradient (from alpine meadows to a glacier forefield, 1930–2519 m a.s.l.) over 14 months. Samples were taken during all four seasons, even under the winter snowpack and at snowmelt. Microbial community structures and abundances were investigated using Terminal Restriction Fragment Length Polymorphism (T-RFLP) and quantitative PCR (qPCR) of the 16S and 18S rRNA genes. Illumina sequencing was performed to identify key bacterial groups in selected samples. We found that the soil properties varied significantly with the seasons and along the elevational gradient. For example, concentrations of soluble nutrients (e.g., , , ) significantly increased in October but decreased drastically under the winter snowpack. At all times, the alpine meadows showed higher soluble nutrient concentrations than the glacier forefield. Microbial community structures at the different sites were strongly affected by seasonal variations. Under winter snowpack, bacterial communities were dominated by ubiquitous groups (i.e., beta-Proteobacteria, which made up to 25.7% of the total reads in the glacier forefield). In the snow-free seasons, other groups (i.e., Cyanobacteria) became more abundant (from 1% under winter snow in the glacier forefield samples to 8.1% in summer). In summary, elevation had a significant effect on soil properties, whereas season influenced soil properties as well as microbial community structure. Vegetation had a minor impact on microbial communities. At every elevation analyzed, bacterial, and fungal community structures exhibited a pronounced annual cycle.

Trophic state mediates the effects of a large colonial cyanobacterium on phytoplankton dynamics

Although cyanobacterial blooms are typically found in eutrophic lakes, where they are able to exert inhibitory effects on other plankton, they are also reported from oligotrophic and mesotrophic lakes. Here, we explored whether trophic state mediates the effects of Gloeotrichia echinulata blooms in freshwater ecosystems. This taxon is a large, colonial cyanobacterium that may be increasing in low-nutrient lakes in northeastern North America. We manipulated Gloeotrichia presence in mesotrophic and eutrophic mesocosms and measured its ef- fects on phosphorus, nitrogen, phytoplankton growth in two size fractions (< 30 µm, and total fraction), and zoo- plankton. In mesotrophic mesocosms, Gloeotrichia stimulated the growth of smaller-sized phytoplankton, poten- tially through significantly higher total nitrogen and phosphorus concentrations than in non-Gloeotrichia controls, although nearly all measured soluble nutrient concentrations were below method detection limits. In contrast, the growth of smaller-sized phytoplankton was inhibited in eutrophic mesocosms, where concentrations of total nitrogen and phosphorus were significantly lower in the presence of Gloeotrichia in comparison to controls. The Gloeotrichia colonies likely inhibited phytoplankton growth in the eutrophic mesocosms by creating scums that decreased light availability, although other mechanisms may be involved. The positive or negative effect of Gloe- otrichia did not cascade to higher trophic levels: zooplankton biomass was significantly higher in the eutrophic than mesotrophic mesocosms, but not affected by Gloeotrichia presence. In summary, trophic state determined if the effects of Gloeotrichia on smaller-sized phytoplankton were stimulatory or inhibitory, likely due to several interacting mechanisms.

Short period of oxygenation releases latch on peat decomposition

Extreme summer droughts are expected to occur more often in the future in NW Europe due to climate change. These droughts might accelerate the rate of peat oxidation in drained peat areas, with impacts on soil subsidence, GHG emission and water quality. This study aimed at providing more insight in the oxidation of deep peat layers that had not previously been exposed to air, the so-called secondary decomposition.We incubated two types of peat (eutrophic and oligotrophic), sampled from permanently anoxic peat layers from nature reserves and agricultural peat meadows. Peat samples were incubated for thirteen weeks under anoxic conditions, but were exposed to air for one to eight weeks. The production of CO2 and CH4 was quantified as a proxy for decomposition; concentrations of soluble nutrients and phenolic compounds were also measured.The results showed that oxygenation led to a steep increase in the rate of decomposition, indicated by higher carbon loss rates during and after oxygenation compared to non-oxygenated samples. Carbon loss rates increased more for eutrophic peat (agricultural area: 352%, nature reserve: 182%) than for oligotrophic peat (83% and 159% respectively). Most peat samples investigated showed higher post-oxygenation CO2 and/or CH4 production compared to the anoxic pre-oxygenation period. This indicates that oxygenation stimulates decomposition, even after anoxic conditions have returned. Contrary to the enzymic latch theory, no effects of oxygenation on the concentrations of soluble or condensed phenolic compounds were detected. Soluble nutrient concentrations did not change due to oxygenation either. Noteworthy is the occurrence of pyrite mineralization and associated acidification in eutrophic peat. Thus, low summer water levels, for example due to climate change, should be avoided in order to limit exceptionally high decomposition rates and associated problems such as increasing subsidence rates, greenhouse gas emission, sulfate release and acidification.

Estimates of atmospheric dry deposition and associated input of nutrients to Gulf of Aqaba seawater

Dry deposition rates and associated inputs of soluble inorganic nitrogen (N) phosphorus (P) and iron (Fe) were calculated for the north coast of the Gulf of Aqaba in Eilat, Israel, between August 2003 and September 2005. The main inorganic N compounds in the water soluble fraction of aerosol particles were nitrate (60%) and ammonium (38%), with mean concentrations of 35 and 22 nmol m−3 of air, respectively. Soluble phosphate ranged between 0.09 and 2 nmol m−3 of air with a mean value of 0.4 nmol m−3 of air. The soluble inorganic nitrogen and soluble phosphate account for approximately 86% and 69% of total soluble nitrogen and total soluble phosphorus, respectively; the difference is assigned to organic N and P compounds. The mean concentration of soluble aerosol Fe was 0.3 nmol m−3 of air. Our measurements of the soluble nutrient concentrations are comparable to data previously reported for the eastern Mediterranean area. Dry deposition fluxes of nutrients were estimated for each sampling date using a size‐dependent deposition model. The estimated fluxes were highly variable over the sampling period with the mean fluxes of 38, 0.2 and 0.02 μmol m−2 d−1 for seawater soluble inorganic N, P and Fe, respectively. The soluble phosphate flux shows a seasonal pattern with higher input during the winter (September to December) than in other seasons. The inorganic N/P molar ratios in the seawater‐soluble fraction of the dry deposition (ranging from 32 to 541) were well above the Redfield ratio (N/P = 16), suggesting that atmospheric inputs of nutrients increase the likelihood for P limitation in the Gulf. Atmospheric deposition could contribute a substantial fraction (35%) of dissolved inorganic N to the euphotic zone during the stratification period (April to October), and the N flux from this source could support over 10% of surface primary production and possibly all of the new production during the summer; however, these estimates have a relatively large uncertainty due to error associated with deposition flux calculation and the temporal variability in dust flux. Atmospheric input of seawater‐soluble Fe is in large excess compared to that required for the phytoplankton growth driven by the N deposition.

Effects of an invasive cattail species (Typha×glauca) on sediment nitrogen and microbial community composition in a freshwater wetland

Sediments from Cheboygan Marsh, a coastal freshwater wetland on Lake Huron that has been invaded by an emergent exotic plant, Typhaxglauca, were examined to assess the effects of invasion on wetland nutrient levels and sediment microbial communities. Comparison of invaded and uninvaded zones of the marsh indicated that the invaded zone showed significantly lower plant diversity, as well as significantly higher aboveground plant biomass and soil organic matter. The sediments in the invaded zone also showed dramatically higher concentrations of soluble nutrients, including greater than 10-fold higher soluble ammonium, nitrate, and phosphate, which suggests that Typhaxglauca invasion may be impacting the wetland's ability to remove nutrients. Terminal restriction fragment length polymorphism analyses revealed significant differences in the composition of total bacterial communities (based on 16S-rRNA genes) and denitrifier communities (based on nirS genes) between invaded and uninvaded zones. This shift in denitrifiers in the sediments may be ecologically significant due to the critical role that denitrifying bacteria play in removal of nitrogen by wetlands.

Taxonomic richness of stream benthic algae: Effects of flood disturbance and nutrients

We sampled benthic algae monthly for 15 months in 12 New Zealand gravel‐bed streams to investigate amongstream differences in algal taxonomic richness and how this might relate to among‐stream differences in flood disturbance and nutrient resource regimes. The mean number of benthic algal taxa per month ranged from 9.4 to 21.3 among streams. There were moderate month‐to‐month fluctuations in richness, partly in response to flood disturbances. Flow perturbations generally reduced richness in streams with unarmored bed sediments but had little or no effect on richness in streams with armored sediments. Richness was moderate within days of floods in most streams but then did not vary significantly for periods of up to 50 d. However, in streams with prolonged periods without bed‐moving floods, richness slowly increased after ~100 d, and, in two of these streams, it peaked at 200–300 d before declining. Unexpectedly, there was not a significant pattern in mean monthly richness among the streams as a function of annual flood frequency. Mean richness was strongly negatively related to soluble nutrient concentrations (particularly with soluble inorganic nitrogen [SIN]; R2 = 0.511, P = 0.018). However, this was not a direct relationship, because nitrogen‐fixing taxa were common in the streams with low SIN, resulting in intermediate to high mat‐scale nitrogen concentrations. This possibly shifted these communities to P‐limited growth. A combination of disturbance frequency and mat P gave the most parsimonious model of among‐stream variations in benthic algal richness (R2 = 0.635, P = 0.037). The highest richness occurred in streams with low to intermediate frequencies of flood disturbance (up to 10 bed‐moving events yr-1) and intermediate to high concentrations of mat P (&gt;0.6% P). We could not define an upper nutrient concentration that negatively affected benthic algal richness (perhaps because none of our streams were highly enriched). We discuss our results in relation to three contemporary models of biodiversity.

Modeling diffuse pollution with a distributed approach

The transferability of parameters for non-point source pollution models to other watersheds, especially those in remote areas without enough data for calibration, is a major problem in diffuse pollution modeling. A water quality component was developed for WATFLOOD (a flood forecast hydrological model) to deal with sediment and nutrient transport. The model uses a distributed group response unit approach for water quantity and quality modeling. Runoff, sediment yield and soluble nutrient concentrations are calculated separately for each land cover class, weighted by area and then routed downstream. The distributed approach for the water quality model for diffuse pollution in agricultural watersheds is described in this paper. Integrating the model with data extracted using GIS technology (Geographical Information Systems) for a local watershed, the model is calibrated for the hydrologic response and validated for the water quality component. With the connection to GIS and the group response unit approach used in this paper, model portability increases substantially, which will improve non-point source modeling at the watershed scale level.

NUTRIENT LIMITATION OF PHYTOPLANKTON IN A SEASONALLY OPEN BAR‐BUILT ESTUARY: WILSON INLET, WESTERN AUSTRALIA

The potential for nutrient limitation to affect the phytoplankton community was assessed from 1997 to 1998 in Wilson Inlet, Western Australia. Samples of the endemic phytoplankton community were assayed by giving them nutrient mixes deficient in one of the following: nitrogen, phosphate, silicate, iron, trace metals, or vitamins. When ranked from those treatments with the most potential to limit phytoplankton biomass to those with less potential, the bioassay results suggested that N&gt;P&gt;&gt;Si&gt;Fe, and trace metals and vitamins were never potentially limiting. In summer and autumn, the bioassay data suggested that nitrogen and phosphorus were the nutrients with greatest potential to constrain phytoplankton biomass. Though the concentrations of soluble nutrients were high in winter, bioassays indicated that the phytoplankton community was potentially nutrient limited. Physical conditions such as high flow rate, greater turbidity, lower temperature, and greater light attenuation contributed to lower phytoplankton growth rates during winter. The bioassay data indicated that the phytoplankton biomass was least likely to be constrained by dissolved inorganic nutrients in spring than any other time of the year. In spring the endemic phytoplankton community responded to salinity‐stratification induced sediment‐nutrient release with a marked increase of biomass. Nitrogen was the nutrient with greatest potential to limit the phytoplankton biomass during spring. The bioassay data conflicted with the dissolved inorganic nitrogen to phosphorus molar ratios, which suggested that P was more likely to limit the phytoplankton biomass during spring. The discrepancy between the two data sets was probably caused by inaccuracies in measuring the concentration of dissolved inorganic phosphorus in spring. Therefore, the results from the current study suggest that the Redfield paradigm, based on the ratio of dissolved inorganic nutrient concentration, provided an inadequate description of phytoplankton nutrient limitation in Wilson Inlet.

Nonpoint source pollution: a distributed water quality modeling approach

A distributed water quality model for nonpoint source pollution modeling in agricultural watersheds is described in this paper. A water quality component was developed for WATFLOOD (a flood forecast hydrological model) to deal with sediment and nutrient transport. The model uses a distributed group response unit approach for water quantity and quality modeling. Runoff, sediment yield and soluble nutrient concentrations are calculated separately for each land cover class, weighted by area and then routed downstream. With data extracted using Geographical Information Systems (GIS) technology for a local watershed, the model is calibrated for the hydrologic response and validated for the water quality component. The transferability of model parameters to other watersheds, especially those in remote areas without enough data for calibration, is a major problem in diffuse modeling. With the connection to GIS and the group response unit approach used in this paper, model portability increases substantially, which will improve nonpoint source modeling at the watershed-scale level.

Eutrophication of streams and rivers: dissolved nutrient-chlorophyll relationships for benthic algae

AbstractStatistical models for predicting the effects on algal biomass of eutrophication are much better developed for lentic systems than for lotic systems, partly because of the dynamic physical nature of streams as controlled by flood regimes. I analyzed data from 30 sites in 25 runoff-fed streams and rivers to develop statistical models for mean monthly and maximum chlorophyll a as a function of soluble nutrient concentrations and days of accrual (reflecting the frequency of flood disturbance events). Variation in stream-water nutrients explained 12–22.6% of the variation in mean monthly chlorophyll a and 29.5–32.5% of the variation in maximum chlorophyll a among sites. Days of accrual explained 39.7% and 61.8 % of the variation in mean monthly and maximum chlorophyll a, respectively. Multiple regression models combining dissolved nutrient data and days of accrual explained 43.7–48.8% of the variation in mean monthly chlorophyll a and 72.1–74.1% of the variation in maximum chlorophyll a among sites. I...

Contrasting Relationships Between Nutrients, Chlorophyll a and Secchi Transparency in Two Shallow Subtropical Lakes: Lakes Okeechobee and Apopka (Florida, USA)

ABSTRACT Ten years of monthly water quality data were compared from two large shallow lakes in Florida, USA – Lakes Okeechobee and Apopka. Seasonal changes in trophic state index (TSI) values and log-log regression models relating total phosphorus (TP), total nitrogen (TN), chlorophyll a (CHLA), and Secchi transparency (SD) were considered. The objective was to quantify the extent to which empirical models might vary due to the heterogeneous nature of benthic-pelagic coupling that can occur in shallow lakes. In the offshore region of Lake Okeechobee, TP and SD-based TSI values increase dramatically during winter, while CHLA-based TSI declines. These changes coincide with the windy season in south Florida, when average wind velocities can exceed 20 km h−1. Resuspension of bottom sediments occurs and this reduces light penetration to the extent that the growth of phytoplankton is inhibited. During summer, winds are calm, and these conditions occur less often. Log-log regression models of CHLA vs. TP have a negative slope during winter and a positive slope during summer in offshore Lake Okeechobee. The slope of SD vs. CHLA models displays the opposite seasonal pattern. Seasonal changes are considerably muted in the near-shore region, where sediments are more consolidated and where shallower depths reduce the severity of light suppression of phytoplankton growth. The magnitude of seasonal variation in TSI values in Lake Apopka is muted, perhaps because seasonal variation in wind velocity is less than in the region of Lake Okeechobee. Hypereutrophic levels of nutrients, CHLA, and SD occur year-round. The log-log regression models of CHLA vs. TP and CHLA vs. TN are highly significant, and light availability does not appear to limit rates of algal production. Frequent resuspension events may occur, but negative effects on phytoplankton may be mitigated by: (1) shallower depth; (2) a greater presence of viable algae in the surface sediments that might “seed” the water column with phytoplankton; (3) high concentrations of soluble nutrients in sediments that might stimulate algal growth; and (4) lower concentration of light-attenuating inorganic particles than in Lake Okeechobee. The results indicate that a high degree of variability in both the seasonal patterns and relationships among nutrients, CHLA, and transparency can occur in shallow subtropical lakes, even when they share common features (e.g., geographic location, large size, wind patterns). This finding has practical implications regarding the use of regression models in the context of shallow lake management. Models may be quite useful on a lake-by-lake basis (e.g., in Lake Apopka they have successfully been used in eutrophication management), but considerable care should be taken when generalizing models to other lake ecosystems.