Sympatric kelp species share a large portion of their surface bacterial communities.

Kelp forest ecosystems are biodiversity hotspots, providing habitat for dense assemblages of marine organisms and nutrients for marine and terrestrial food webs. The surfaces of kelps support diverse microbial communities that facilitate the transfer of carbon from algal primary production to higher trophic levels. We quantified the diversity of bacteria on the surfaces of eight sympatric kelp species from four sites in British Columbia. Kelp-associated bacterial communities are significantly different from their environment, even though 86% of their bacterial taxa are shared with seawater and 97% are shared with rocky substrate. This differentiation is driven by differences in relative abundance of the bacterial taxa present. Similarly, a large portion of bacterial taxa (37%) is shared among all eight kelp species, yet differential abundance of bacterial taxa underlies differences in community structure among species. Kelp-associated bacterial diversity does not track host phylogeny; instead bacterial community composition is correlated with the life-history strategy of the host, with annual and perennial kelps supporting divergent bacterial communities. These data provide the first community-scale investigation of kelp forest-associated bacterial diversity. More broadly, this study provides insight into mechanisms that may structure bacterial communities among closely related sympatric host species.

Spatiotemporal patterns of mercury accumulation in lake sediments of western North America.

For the Western North America Mercury Synthesis, we compiled mercury records from 165 dated sediment cores from 138 natural lakes across western North America. Lake sediments are accepted as faithful recorders of historical mercury accumulation rates, and regional and sub-regional temporal and spatial trends were analyzed with descriptive and inferential statistics. Mercury accumulation rates in sediments have increased, on average, four times (4×) from 1850 to 2000 and continue to increase by approximately 0.2µg/m(2) per year. Lakes with the greatest increases were influenced by the Flin Flon smelter, followed by lakes directly affected by mining and wastewater discharges. Of lakes not directly affected by point sources, there is a clear separation in mercury accumulation rates between lakes with no/little watershed development and lakes with extensive watershed development for agricultural and/or residential purposes. Lakes in the latter group exhibited a sharp increase in mercury accumulation rates with human settlement, stabilizing after 1950 at five times (5×) 1850 rates. Mercury accumulation rates in lakes with no/little watershed development were controlled primarily by relative watershed size prior to 1850, and since have exhibited modest increases (in absolute terms and compared to that described above) associated with (regional and global) industrialization. A sub-regional analysis highlighted that in the ecoregion Northwestern Forest Mountains, <1% of mercury deposited to watersheds is delivered to lakes. Research is warranted to understand whether mountainous watersheds act as permanent sinks for mercury or if export of "legacy" mercury (deposited in years past) will delay recovery when/if emissions reductions are achieved.