A method to choose water depths for zooplankton samples in lakes

Publication history: Accepted - 13 October 2021; Published online - 23 November 2021.As methods in the literature to sample zooplankton in lakes mostly offered general guidance on the sample depths, a new one was developed. Using the principle of volume-weighted sampling of the lake volume and an empirical function for the hypsometric curve, formulae for the volumes and areas of five equal sections of the lake were derived, which were then used to calculate section mean depths. Vertical net hauls taken at the mean depths are combined using a relation between their mean depths to produce one unbiased composite sample of the zooplankton. While generic formulae were derived, starting values for the depths that divide the lake volume into five equal sections are needed in order to apply the method, which then optimizes the depths; the method is implemented in a spreadsheet. The method was applied to four hypothetical lakes of maximum depth 12 m that cover a wide variation of lake form and how the sample depths vary with form was described; as lake form becomes more convex, the sample depths decrease, reflecting that more of the lake volume is at shallower depth. The method was used to estimate the whole-lake abundance of zooplankton in 51 lakes and no practical difficulties were encountered. It can be used in lakes up to a few tens of km2 in area.The INTERREG IVA Programme of the European Union’s European Regional Development Fund, managed by the Special European Union Programmes Body, funded this work. It formed part of the cross-border DOLMANT project (reference number 002862)

Distribution and dynamics of iron deposition in streams and its effects on aquatic ecology

Availability of iron in an aquatic system is governed by the concentrations present within the catchment and prevailing environmental conditions. Iron is characterised by low solubility at circumneutral pH. Conversion of ferric iron, into soluble ferrous iron, requires a reduction in pH and redox potential. Research to date has tended to focus upon acid mine drainage, where anthropogenic activity initiates extreme acidity and the liberation of mobile iron. In instances of naturally occurring deposition, the acidity is derived from ecological processes, such as organic decay, respiration and natural acids. Once in the soluble form, iron is vulnerable to rainfall induced transport into receiving waterbodies. Precipitation of iron, from the water onto the streambed, takes place when pH increases downstream and conditions become increasingly oxidised. Interaction between iron-water and deposit concentrations was explored across the Antrim Plateau and Sperrin Mountains, in Northern Ireland. Basalt and brown earths dominate the Antrim Plateau, whereas the Sperrin Mountain soils are composed of peat underlain by schist. Significant relationships existed between water and deposit concentrations across both localities. Concentrations of deposit iron were consistently less in the Antrim Plateau, due to lower levels of iron in the water, as a consequence of variance in soil and rock type. In the Sperrin Mountains, eight streams were sampled monthly for one year, over a range of high to low iron sites. Seasonal variation in rainfall influenced iron concentrations in the water. The solubility of iron within streams in the Sperrin Mountains is predominantly controlled by pH. Dissolved oxygen was less influential owing to the hydrodynamics of upland stream networks, which are generally fast flowing and well oxygenated. The occurrence of iron deposition is therefore dependent upon the concentration of iron in the water and the pH of the waterbody. Composition of deposit material present on stone and tile substrates was analysed. Metal concentrations increased with increasing deposit density. A strong association also existed between deposit iron and organic matter concentration. Chlorophyll a concentrations increased with increasing deposit density, up to a critical point, after which they began to decline as deposit density continued to rise. Autotrophic index values behaved negatively with chlorophyll a concentration and positively with deposit organic matter and iron. The density of deposit material present within streams has a strong influence upon both the abundance of algae and invertebrate community structure. Deposit iron concentrations in excess of 1.0 mg cm-2 had a deleterious effect upon chlorophyll a concentration. Invertebrate numbers were also detrimentally affected by elevated concentrations of iron. Species composition was principally controlled by the density of deposit iron, which influenced algal abundance and controlled community structure in relation to feeding habit. The accumulation of deposit iron alters stream ecosystem functioning and facilitates the transition from autotrophy to heterotrophy. Key attributes of stream ecology are, therefore, altered by iron deposition as a result of food web simplification.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Field and Laboratory Tests of Flow-Proportional Passive Samplers for Determining Average Phosphorus and Nitrogen Concentration in Rivers

Flow responsive passive samplers offer considerable potential in nutrient monitoring in catchments; bridging the gap between the intermittency of grab sampling and the high cost of automated monitoring systems. A commercially available passive sampler was evaluated in a number of river systems encapsulating a gradient in storm response, combinations of diffuse and point source pressures, and levels of phosphorus and nitrogen concentrations. Phosphorus and nitrogen are sequestered to a resin matrix in a permeable cartridge positioned in line with streamflow. A salt tracer dissolves in proportion to advective flow through the cartridge. Multiple deployments of different cartridge types were undertaken and the recovery of P and N compared with the flow-weighted mean concentration (FWMC) from high-resolution bank-side analysers at each site. Results from the passive samplers were variable and largely underestimated the FWMC derived from the bank-side analysers. Laboratory tests using ambient river samples indicated good replication of advective throughflow using pumped water, although this appeared not to be a good analogue of river conditions where flow divergence was possible. Laboratory tests also showed good nutrient retention but not elution and these issues appeared to combine to limit the utility in ambient river systems at the small catchment scale

Managing diffuse phosphorus at the source versus at the sink

Judicious phosphorus (P) management is a global grand challenge and critical to achieving and maintaining water quality objectives while maintaining food production. The management of point sources has been successful in lowering P inputs to aquatic environments, but more difficult is reducing P discharges associated with diffuse sources, such as nonpoint runoff from agriculture and urban landscapes, as well as P accumulated in soils and sediments. Strategies for effective diffuse-P management are imperative. Many options are currently available, and the most cost-effective and practical choice depends on the local situation. This critical review describes how the metrics of P quantity in kg ha–1 yr–1 and P form can influence decision-making and implementation of diffuse-P management strategies. Quantifying the total available pool of P, and its form, in a system is necessary to inform effective decision-making. The review draws upon a number of “current practice” case studies that span agriculture, cities, and aquatic sectors. These diverse examples from around the world highlight different diffuse-P management approaches, delivered at the source in the catchment watershed or at the aquatic sink. They underscore workable options for achieving water quality improvement and wider P sustainability. The diffuse-P management options discussed in this critical review are transferable to other jurisdictions at the global scale. We demonstrate that P quantity is typically highest and most concentrated at the source, particularly at farm scale. The most cost-effective and practically implementable diffuse-P management options are, therefore, to reduce P use, conserve P, and mitigate P loss at the source. Sequestering and removing P from aquatic sinks involves increasing cost, but is sometimes the most effective choice. Recovery of diffuse-P, while expensive, offers opportunity for the circular economy