Integration of lakes and streams in a landscape perspective: the importance of material processing on spatial patterns and temporal coherence

1. We studied the spatial and temporal patterns of change in a suite of twe nty-one chemical and biological variables in a lake district in arctic Alas ka, U.S.A. The study included fourteen stream sites and ten lake sites, nin e of which were in a direct series of surface drainage. All twenty-four sit es were sampled between one and five times a year from 1991 to 1997. 2. Stream sites tended to have higher values of major anions and cations th an the lake sites, while the lake sites had higher values of particulate ca rbon, nitrogen, phosphorous and chlorophyll a. There were consistent and st atistically significant differences in concentrations of variables measured at the inlet versus the outlet of lakes, and in variables measured at upst ream versus downstream sites in the stream reaches which connect the lakes. In-lake processing tended ro consume alkalinity, conductivity, H+, DIC, Ca 2+, Mg2+, CO2, CH4, and NO3-, and produce K+ and dissolved organic carbon ( DOC). In-stream processing resulted in the opposite trends (e.g. consumptio n of K+ and DOC), and the magnitudes of change were often similar to those measured in the lakes but with the opposite sign. 3. Observed spatial patterns in the study lakes included mean concentration s of variables which increased, decreased or were constant along the lake c hain from high to low altitude in the catchment (stream sites showed no spa tial patterns with any variables). The strongest spatial patterns were of i ncreasing conductivity, Ca2+, Mg2+, alkalinity, dissolved inorganic carbon and pH with lake chain number (high to low altitude in the basin). These pa tterns were partly determined by the effect of increasing catchment area fe eding into lakes further downslope, and partly by the systematic processing of materials in lakes and in the stream segments between lakes. 4. Synchrony (the temporal coherence or correlation of response) of variabl es across all lakes ranged from 0.18 for particulate phosphorus to 0.90 for Mg2+; the average synchrony for all twenty-one variables was 0.50. The syn chronous behaviour of lake pairs was primarily related to the spatial locat ion or proximity of the lakes for all variables taken together and for many individual variables, and secondarily, to the catchment to lake area ratio and the water residence time. 5. These results illustrate that, over small geographic areas, and somewhat independent of lake or stream morphometry, the consistent and directional (downslope) processing of materials helps produce spatial patterns which ar e coherent over time for many limnological variables. We combine concepts f rom stream, lake and landscape ecology, and develop a conceptual view of la ndscape mass balance. This view highlights that the integration of material processing in both lakes and rivers is critical for understanding the stru cture and function of surface waters, especially from a landscape perspecti ve.