Coidingham Loch, S.E. Scotland

Abstract

SUMMARY. 1. Coidingham Loch (Berwickshire. Scotland) (area. 8.4 ha, mean depth 2.9 m, max. depth 12.3 m) belongs to Hakansson's convex shape category. It lies in a basin of Silurian Greywackes rock within 0.25 km of coastal sea cliffs (c. 133 m a.s.l.). The theoretical hydraulic replacement time is 3.17 years. 2. The loch stratifies intermittently in summer. Fluctuations in oxygen concentration generally correspond to spells of mixing and stratification; low values of 10% saturation occur at the bottom. 3. The sum of the concentrations of major cations (Na+, K+, Ca 2+ , Mg 2+ ) is in accordance with measured conductivities ranging between 380 μS cm −1 and 420 μS cm −1 (k. 25 ). The ratios (by equivalents) of Na + /Cl − (0.04) are similar to those in sea‐water, whilst values for Ca+ 2+ /Cl − (0.85–1.01) and Mg 2+ /Cl − (0.79–0.88) reflect the bedrock. 4. Nitrate concentrations were lowest (<0.05 mg N1 −1 ) in summer following losses from the column of 107 mg N m −2 1 −1 , a rate corresponding well with published figures on microbial nitrate reduction. Nitrate increased at a rate of 8μg N I −1 d −1 to a winter maximum of 1.55 mg N I −1 . Mass balance calculations show that if this rise is attributed to run‐off from surrounding land, a loss rate of 11.1 kg N ha −1 yr −1 would be required; this value is also commensurate with published figures. 5. Changes in phosphorus and factors controlling them contrast markedly with those of nitrate. The minimum concentration of 55 μg total P l −1 (mainly in soluble reactive form) occurs in spring. An increase to the maximum of c. 300 μg l −1 in summer is sustained mainly by release from the sediments at a regular rate of 3 μg P l −1 d −1 (8.7 mg m −2 d −1 ). Adsorption by the sediments is considered to be the major process accounting for autumnal losses of phosphorus of 2.6 mg P M −2 d −1 . 6. Silica showed a less regular seasonal pattern, but varied some 45‐fold with a maximum of 2.25 mg SiO 2 , l −1 in August.