D.V. Leach

Major ion concentrations and the inorganic carbon chemistry of the Humber rivers

Measurements of major ion concentrations in the main rivers draining into the Humber estuary show two dominant spatial patterns, related to anthropogenic sources from catchments draining urban/industrial areas and background weathering sources from the rural catchments. Most major ions exhibit dilution effects with flow, with higher concentrations at baseflow compared with stormflow conditions. This suggests a predominance of point (effluent) and/or groundwater (weathering) sources of major ions. An exception is NO3−, which exhibits higher concentrations under stormflow conditions in certain rural catchments, suggesting diffuse (catchment) sources, possibly derived from agricultural runoff. Inter-ion relationships were used for endmember mixing analysis to identify whether or not components were exhibiting conservative behaviour within the river. The dominant relationship between major ions is a straight line between baseflow and stormflow endmembers, indicative of conservative mixing processes. The chemical mixing patterns appear to be controlled by hydrology and the existence of distinct inter-ion ratios between baseflow and stormflow endmembers. In terms of the inorganic carbon system, HCO3− is the major component of dissolved inorganic carbon (DIC), with dissolved CO2 (H2CO30 and CO2(aq)) representing approximately 10% and CO32− less than 1% of DIC. All river waters are oversaturated with respect to CO2, with excess partial pressures of carbon dioxide (EpCO2) typically between 5 and 20 times atmospheric partial pressure. The elevated levels of EpCO2 in the river water are sufficient to decouple the relationship between pH and alkalinity. EpCO2 is related to microbial production of CO2 by breakdown of organic carbon and the availability of nutrients (particularly NO3−) and is generally highest in the rivers draining urban and industrial catchments, together with the Derwent which receives high rates of agricultural runoff. EpCO2 and pH do not exhibit simple mixing relationships and both determinands are inherently non-conservative in behaviour. These results provide important background information on which to base more detailed process studies and modelling work. The results imply that most of the major ions can be modelled using simple mixing relationships. In contrast the non-conservative behaviour of EpCO2 and pH may require more thorough process evaluation for modelling the in-stream carbon system.

The LOIS river monitoring network: strategy and implementation

Abstract The large-scale, highly integrated, multidisciplinary nature of the LOIS rivers programme mean that certain measurements were common to many LOIS research projects (including water discharge, suspended sediment concentrations, pH, conductivity) or depended upon routine weekly or flood-related sampling (eg. for nutrients, metals and organics). This paper describes the underlying strategy which led to the design of the major observational programme in LOIS rivers and provides information on siting of sample points, field sampling methods, instrumentation and associated chemical analyses. There was a need for field measurements to reflect the full range of river flows and to identify critical points for measurement of flux from, and within, the large-scale study river basins. Tests of manual sampling runs against the time series of river flows indicate good representation of both low to moderate flows and extreme flood events in the LOIS rivers field data. Field and analytical methods for sampling the wide range of chemical determinands (major and minor elements, trace metals, nutrients, inorganic and organic carbon and micro-organic pollutants) are outlined. The first LOIS chemical harmonisation assessment, designed to investigate the comparability and accuracy of the chemical and analytical techniques used by the LOIS community is also reported.

Micro-organic compounds in the Humber rivers

The concentrations of a range of micro-organic compounds in the Humber rivers have been measured at weekly intervals over a period of 1 year. The compounds include the triazine herbicides (simazine, atrazine, propazine, desmetryn and prometryn), selected organophosphorus insecticides (fenitrothion, malathion and parathion), phenylurea (chlorotoluron, diuron, isoproturon and linuron) and phenoxyacid herbicides (2,4-D, MCPA, MCPB and mecoprop), phenol derivatives (phenol, 2-ethylphenol, 2-chlorophenol, 4-nitrophenol, 2-methylphenol, 4-ethylphenol and 2,4-dichlorophenol), organochlorine insecticides (HEOD, DDT, TDE, DDE, HCB, α-BHC and lindane), PCBs and some synthetic pyrethroid insecticides (cis/trans-permethrin, fenvalerate and cypermethrin). The results indicate the high frequency of occurrence of many compounds in the southern Humber rivers Aire, Calder, Trent and Don compared with the more occasional concentrations found in the upland rivers with catchments dominated by low intensity agriculture. The more water soluble herbicides, atrazine, simazine, isoproturon and diuron are detected frequently in the southern rivers with the highest concentrations and abundance in the rivers Aire, Calder and Trent. The most abundant phenolic compound is 2,4-dichlorophenol usually occurring at concentrations <1 μg/1. The organochlorine insecticides and PCBs are generally at concentrations < 0.01 μg/l. cis/trans-permethrin are the only synthetic pyrethroids detected and these are found in the rivers Aire and Calder at about equal concentrations of between 0.01 and 0.11 μg/1. The results of the occurrence of simazine and atrazine in the rivers Trent, Don and Aire illustrate peaks in concentration in the spring and then later in the year during the early autumn coinciding with the first major storm after the summer. In the rivers Trent and Don, the annual exports (March 1994–1995) of atrazine are lower than for simazine whereas in the R. Calder the yield of atrazine is higher than for simazine suggesting differences in use in these catchments. The maximum triazine concentration observed, i.e. 8 μg/l of atrazine in the R. Calder, could have inhibiting effects on the phytoplankton and algal growth, although because of the transient nature of the peaks, recovery is expected to be rapid. The only other compounds measured at concentrations likely to produce detrimental ecotoxicological effects are cis/trans permethrin in the rivers Aire and Calder.

Organic carbon in the Humber rivers

Abstract Dissolved organic carbon (DOC), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophyll- a , and alkalinity were determined weekly or more frequently in samples from 11 rivers in the Humber catchment, between September 1993 and February 1995. [DOC] varied overall from 1 to 15 mg l −1 , [POC] from 0.2 to 67 mg l −1 , [LOC] from 0.01 to 7.6 mg l −1 and [DIC] from 1 to 52 mg l −1 . For the rivers with predominantly rural catchments, the complex dependence of [DOC] on season and discharge can be interpreted in terms of soil humification and hydrological processes, whereas the inverse dependence of [DOC] on discharge in the more polluted industrial rivers indicates the dominance of point-source effluents. Concentrations of POC depend significantly upon discharge and on concentrations of suspended particulate matter, suggestive of particle mobilisation when physical thresholds are exceeded. During summer months, the ‘living organic carbon’ of algae accounts for much of the POC in all but the most polluted rivers. Fluxes of organic carbon were estimated by combining daily concentrations (measured or interpolated) with discharge data. The contribution of DOC to the annual flux of organic carbon in the different rivers varies from 51 to 80%, the overall contribution being 63%. For both DOC and POC, the main transport to the estuary takes place during the autumn-winter period. For the study rivers, the total flux of organic carbon during the period September 1993 to September 1994 was 73 × 10 9 g a −1 . However, these rivers represent only about two-thirds of the total catchment area of the Humber estuary, and so the true total flux is probably closer to 100 × 10 9 g a −1 .

Nutrient transport in the Humber rivers

Abstract The results of the weekly and storm sample measurement of the nutrient concentrations in ten Humber rivers over one annual cycle are presented. The nutrients include soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), total phosphorus (TP), nitrate and silicon (soluble reactive silicon). These data are combined with the river discharge data to calculate the nutrient loads discharged into the intertidal zones. The loads are expressed as the nutrient exports normalised with respect to the catchment areas. The results clearly demonstrate that the majority of the nutrient load is transported in autumn/winter storms. The rivers in the south of the region, i.e. Trent, Don, Aire and Calder, have high nutrient exports compared with the rivers in the north of the region such as the Wharfe, Swale, Nidd and Ure. These differences are attributable to the relative importance of effluent discharges and differences in the land use and extent of urbanisation in the regions. With the exception of the R. Derwent, those rivers with a high nitrate export also have the highest SRP export. The results for the R. Swale, the only river having two monitoring sites, highlight the importance of urban and agricultural impacts in the lowland region of the catchment. The export of silicon is least variable, although a sharp spring minimum in concentration, caused by phytoplankton uptake, is evident for the larger rivers such as the R. Trent. The fractionated forms of phosphorus also varied between the rivers but each of the fractions: SRP, (TDP-SRP), (i.e. mainly organophosphorus and inorganic polyphosphates hydrolysed in digestion procedure), and particulate phosphorus, significantly contributed to the load in all the rivers. For example the phosphorus load in the R. Trent is dominated by the SRP fraction, whereas the R. Swale export is highly influenced by the transport of suspended solids, most of which originates from the lowland region between Catterick and the confluence with the R. Ure. The results of intensive hourly sampling through a storm event on the R. Swale demonstrate the importance of the tributaries in the downstream 54 km section on the river. Although the majority (85%) of the water at the downstream site originates from the upland region north of Catterick, the majority of the nitrate (74%) and SRP (78%) originates from the rivers Wiske and Cod Beck as well as minor tributaries in the southern region of the catchment. A detailed mass balance of the section indicates net losses of nitrate, silicon, SRP and TDP from the water column.

Major, minor, trace element and suspended sediment variations in the River Tweed: results from the LOIS core monitoring programme

Abstract A wide range of major, minor and trace element data is presented for three sites in the Tweed basin. These are located at the downstream freshwater limit, the upland-lowland transition point and a major tributary, the Teviot. In comparison to the agricultural, industrial and urban impacted rivers of the UK, water quality in the study region is good with low levels of dissolved nitrogen, phosphorus, many trace elements, micro-organic compounds, and carbon dioxide. Sodium, chloride and sulphate, mainly derived from atmospheric sources, are positively correlated. These components show increasing concentrations downstream due to the influence of evapotranspiration and sewage effluent. Base cation concentrations, alkalinity and some trace elements such as barium, strontium, lithium and uranium are enriched under baseflow conditions, exhibiting an inverse relationship with flow. These variations are associated with chemical weathering reactions of the bedrock. The particulate trace metals show high correlations in line with an aluminium and iron oxide/hydroxide soil source. In the case of copper and chromium, two linear relationships are observed because of an additional industrial source. Particulate nitrogen, phosphorus and carbon are highly correlated due to soil derived organic matter. However, the dissolved forms of nitrogen, phosphorus, organic carbon and silicon show complex patterns between concentration and flow due to the interplay of hydrological and biological processes. There is a large biological reactivity within the rivers. This is reflected by variable but often low partial pressures of carbon dioxide throughout the year and dissolved silicon concentration minima during the summer months when algal activity is maximal.

Changes in water quality of the River Frome (UK) from 1965 to 2009: is phosphorus mitigation finally working?

The water quality of the River Frome, Dorset, southern England, was monitored at weekly intervals from 1965 until 2009. Determinands included phosphorus, nitrogen, silicon, potassium, calcium, sodium, magnesium, pH, alkalinity and temperature. Nitrate-N concentrations increased from an annual average of 2.4 mg l⁻¹ in the mid to late 1960s to 6.0 mg l⁻¹ in 2008-2009, but the rate of increase was beginning to slow. Annual soluble reactive phosphorus (SRP) concentrations increased from 101 μg l⁻¹ in the mid 1960s to a maximum of 190 μg l⁻¹ in 1989. In 2002, there was a step reduction in SRP concentration (average=88 μg l⁻¹ in 2002-2005), with further improvement in 2007-2009 (average=49 μg l⁻¹), due to the introduction of phosphorus stripping at sewage treatment works. Phosphorus and nitrate concentrations showed clear annual cycles, related to the timing of inputs from the catchment, and within-stream bioaccumulation and release. Annual depressions in silicon concentration each spring (due to diatom proliferation) reached a maximum between 1980 and 1991, (the period of maximum SRP concentration) indicating that algal biomass had increased within the river. The timing of these silicon depressions was closely related to temperature. Excess carbon dioxide partial pressures (EpCO₂) of 60 times atmospheric CO₂ were also observed through the winter periods from 1980 to 1992, when phosphorus concentration was greatest, indicating very high respiration rates due to microbial decomposition of this enhanced biomass. Declining phosphorus concentrations since 2002 reduced productivity and algal biomass in the summer, and EpCO₂ through the winter, indicating that sewage treatment improvements had improved riverine ecology. Algal blooms were limited by phosphorus, rather than silicon concentration. The value of long-term water quality data sets is discussed. The data from this monitoring programme are made freely available to the wider science community through the CEH data portal (http://gateway.ceh.ac.uk/).

Concentrations of suspended chlorophyll a in the Humber rivers

Abstract Concentrations of chlorophyll a have been determined at weekly intervals for all of the major rivers feeding the Humber Estuary since September 1993, as part of the LOIS monitoring programme. Highest concentrations generally occurred in the two southern rivers, the R. Trent and the R. Don, and also in the larger of the northern rivers, the R. Ouse and its main feeder, the R. Ure. High concentrations also developed on occasions in some of the other northern rivers, especially during the summer of 1995 when the unusually dry conditions led to low flows and extended retention times. Heavy pollution levels in the R. Aire and the R. Calder, a major tributary of the Aire, were probably responsible for the consistently low concentrations recorded in these rivers.

Temporal and spatial variation in zooplankton populations in the River Great Ouse : An ephemeral food resource for larval and juvenile fish

The spatial and temporal dynamics of zooplankton populations were investigated in a highly engineered and regulated section of the River Great Ouse from 1989 to 1993. In addition to main river sites some secondary channels (back channels) and a marina were also included in the study. At main river sites Rotifera were strongly dominant, with peak numbers of 2000 1−1 to more than 18000 1−1, corresponding closely with spring and early summer maximum concentrations of chlorophyll a. Copepoda (mostly nauplii) were recorded at consistently lower densities (maxima, 20–125 1−1) whereas Cladocera were rarely recorded and never exceed 10 1−1. Most Cladocera belonged to species generally associated with macrophytes rather than truly planktonic species. The situation in main river and back channel situations contrasted markedly with that found in marinas, where densities of Rotifera, although extremely variable spatially, were generally higher than those in the adjacent main river. Copepoda and Cladocera were also much more abundant than at the river sites with taxa such as Bosmina and other typically planktonic taxa predominant. As rotifers are the main food for newly hatched cyprinids, spatial and temporal variation in their abundance relative to the spawning locations and hatching times of cypinid eggs may have important consequences for the growth and survival of young fish.

TEMPORAL AND SPATIAL VARIATIONS IN THE DIET OF 0 GROUP ROACH (RUTILUS RUTILUS) LARVAE AND JUVENILES IN THE RIVER GREAT OUSE IN RELATION TO PREY AVAILABILITY

0 group roach, (Rutilus rutilus) in the River Great Ouse showed marked ontogenetic changes in their diet. Initial prey were diatoms and Rotifera, followed by Cladocera and other small aquatic invertebrates. From late June onwards, roach in the main river channels switched to feeding on detrital aufwuchs and associated meiofauna on the surface of submerged aquatic plants. This switch did not occur in a marina habitat, where planktonic Cladocera dominated the diet. Between-year differences were observed in the numbers of prey eaten per fish, which largely reflected variations in the abundance of the prey organisms in the water column, or associated with submerged macrophytes.