Diatom production responses to the development of early agriculture in a boreal forest lake-catchment (Kassjon, northern Sweden)

1 Diatom analysis of varved sediments from Kassjon (Northern Sweden) were used to determine the productivity response of planktonic and benthic diatoms to the start of agriculture. The development of agriculture was inferred from pollen stratigraphy, which indicated that continuous agriculture in the catchment started in the 13th century. 2 Prior to AD 1200, the diatom assemblages were dominated by benthic Aulacoseira spp., commonly found in boreal lakes, Fragilaria spp. and a variety of small Achnanthes and Navicula spp. From AD 1230 Asterionella formosa percentages increased dramatically, and after 1350 Tabellaria flocculosa cf. var. asterionelloides increased from zero to > 40%. These two planktonic diatoms dominated the diatom assemblages for the next approx 600 years. Between 1900 and 1980 there were a number of changes consistent with changed nutrient resource ratios, notably an increase in Synedra spp. These changes are assumed to relate to the decline in arable agriculture (and a shift to pasture and reforestation) in the post-1950 period, which resulted in a reduced nutrient load to the lake. 3 Finer resolution pollen analysis between AD 1000 and 1500 suggests that the initial opening of the catchment resulted in the increase in Asterionella. The slightly later establishment of Tabellaria matches with the start of continuous arable farming and, presumably, higher nutrient concentrations in the lake. There were significant relationships between changes in the diatom assemblages and three pollen types (Salix, Juniperus and Rumex), as determined by constrained ordination (a partial-redundancy analysis) and Monte Carlo permutation tests. 4 Total diatom accumulation rates increased from 2 x 10(6) frustules cm-2 year-1 prior to AD 1000 to > 10 x 10(6) frustules cm-2 year-1 in the late 19th century. The application of focusing correction factors, to account for the nonuniform pattern of microfossil deposition within the lake, altered the inferred-contribution of the littoral and planktonic diatoms to the palaeoproductivity of the lake. Prior to the start of agriculture the diatom productivity was dominated by benthic diatoms (2-3 x 10(6) frustules cm-2 year-1 corrected values). Planktonic diatom accumulation rates increased significantly after AD 1300 and then continued to increase steadily until approximately 1860 (approximately 3 x 10(6) frustules cm-2 year-1) after which they declined. Despite these increased planktonic diatom accumulation rates, benthic diatoms continued to dominate the productivity of the lake with approximately 5 x 10(6) frustules cm-2 year-1 in the 19th century. 5 Because of differences in cell size between taxa, planktonic diatoms were corrected by estimates of their mean biovolume and both individual and total planktonic biovolume accumulation rates calculated. Prior to AD C. 1350, planktonic biovolume accumulation rates averaged around 250 x 10(6) cubic micrometers cm-2 year-1. From this date, they increased steadily, peaking at > 4000 x 10(6) cubic micrometers cm-2 year-1 in the mid- to late-19th century. There was a statistically significant relationship between the increasing biovolume accumulation rates and the increasing area of fields in the catchment for the period AD 1607-1854.