Combining limnology and palaeolimnology to investigate recent regime shifts in a shallow,eutrophic lake

In this study, we demonstrate that an integrated approach, combining palaeolimnological records and limnological monitoring data, can increase our understanding of changing ecological patterns and processes in shallow lakes. We focused on recent regime shifts in shallow Lake Krankesjön, southern Sweden, including the collapse of the clear-water state in 1975 and its subsequent recovery in the late 1980s. We used diatom, hydrocarbon and biogenic silica sediment records, in concert with limnological data sets on nutrient concentrations, water clarity, chlorophyll-a and water depth, to investigate the shifts. The shift from clear to turbid conditions was abrupt and occurred over 1 to 2 years, whereas recovery of the clear-water state was more gradual, taking 4–5 years. In 1978, shortly after the first regime shift in water clarity, the diatom community underwent a significant shift. It became less diverse, with decreased abundance of epiphytic and planktonic taxa. Despite rising phosphorus concentrations and lower abundance of submerged macrophytes, Lake Krankesjön has remained in the clear-water state over the past 20 years, although this state seems to be increasingly unstable and susceptible to collapse. The complex reactions of the entire lake ecosystem to major changes in lake-water clarity, as shown by the palaeolimnological variables investigated in this study, emphasize the importance of careful lake and catchment management if a stable, clear-water state is desired.     

Stability constants of NaSO4−, MgSO4, MgF+, MgCl+ ion pairs at the ionic strength of seawater by potentiometry

The stability of the ion pair NaSO₄ was determined by measuring the change in sodium activity with medium composition at constant ionic strength, using a sodium-sensitive glass electrode. The stability constants of MgSO₄ and MgCl⁺ were determined indirectly from measurements of the stability of MgF⁺ in different media. All measurements were performed at 1 atm pressure, 25 ± 0.1 °C and 0.7 M formal ionic strength. The stability constants for NaSO₄^?, MgSO₄, MgF⁺ and MgCl⁺ are 1.8 ± 0.1, 6.3 ± 0.1, 22.9 ± 0.1 and 0.34 ± 0.02 M^?1, respectively.     

A remote controlled freeze corer for sampling unconsolidated surface sediments

A new coring device is presented which allows the recovery of loose watery surface sediments and the water/sediment interface byin situ freezing, resulting in well preserved samples. The instrument consists of a tripod with adjustable legs, a hydraulic system, an insulated thermos (with two electrical pumps), and a wedge-shaped freeze box. Alcohol chilled with dry ice is the freezing agent. The corer can be deployed from a boat or a raft and permits recovery of undisturbed surface sediment sections up to 100 cm long in deep waters. The corer is particularly useful for sampling varved sediments.     

Sulphate complexation in seawater: A relation between the stability constants of sodium sulphate and magnesium sulphate in seawater from a determination of the stability constant of hydrogen sulphate and a calculation of the single ion activity coefficient of sulphate

The conditional stability constant of HSO₄^? has been determined at 25°C, 1 atm and a formal ionic strength of 0.7 M in solutions containing sodium, magnesium, chloride and sulphate. This was done spectrophotometrically (UV), using diphenylamine as indicator. The value obtained was 17.0 ± 0.1 (molar scale). Single ion activity coefficients for Na₂SO₄, K₂SO₄ and MgSO₄ have been calculated according to the Bates et al. (1970) model, assuming that the sulphate ion is not hydrated. It was found that the single ion activity coefficient of sulphate changes very little between Na₂SO₄, K₂SO₄ and MgSO₄ when the formal ionic strength is kept constant.These results have been used to obtain relations between the stability constants of NaSO₄^? and MgSO₄ valid for seawater.