Coidingham Loch, S.E. Scotland

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.