Dissolved organic matter in Cumbrian lakes and streams

SUMMARY. 1. A survey of dissolved organic carbon concentration [DOC] and optical absorbance at 340 nm (A340) was carried out for thirteen lakes and twenty‐three streams in Cumbria over the period May 1986 to April 1987.2. Values of [DOC] were in the range 0‐6.2 mg 1‐1‐1, no water having a mean [DOC] greater than 3.6mgI‐1. Streamwaters showed no gross seasonal dependence in [DOC], but there was a peak in [DOC] associated with the first major rain event of autumn 1986. Summer values of [DOC] in lakes were greater than winter values, probably because of the production of DOC by phytoplankton.3. In streams, [DOC] was weakly positively correlated (r2 <0.3) with discharge (as estimated from rainfall); A340 was more strongly correlated (0.4 < r2 < 0.6). Linear regression of mean A340 against mean [DOC] for streams gave a slope consistent with the absorptivities of aquatic humic substances, and a positive intercept on the abscissa that suggests the presence of, on average 0.6 mg 1‐1 of DOC that does not absorb at 340 nm. Lake samples taken in winter gave a similar A.340/[DOC] relationship to that for streams, but in summer A340/[DOC] ratios were lower, suggesting that phytoplankton‐derived [DOC] absorbs weakly or not at all at 340nm.