Palynological records of red tide-producing species in Canada: past trends and implications for the future

Abstract

Increases in ‘red tides’ and other harmful algal blooms (HABs) during the past 50 yr on both the Atlantic and Pacific coasts of Canada suggest that global-scale factors, such as climate change and increased international shipping trade, are the driving forces. Because the historical record is too short to understand the long-term dynamics of HABs, the Holocene history of harmful phytoplankton species was examined using ultra-high resolution studies (annual to decadal scale) of dinoflagellate cysts as a proxy for ‘red tide’ production. Paleoecological transfer functions were then applied to the cyst assemblage data to determine correlations with changes in sea surface temperature and salinity. Pacific records were obtained from varved marine sediments in Saanich Inlet, British Columbia, and in the Santa Barbara Basin off California. Comparison of annual cyst production and historical plankton data provides cyst:theca ratios from which the magnitude of past bloom size can be evaluated. The 10 500-yr Pacific record clearly shows that the largest blooms correspond to cysts of Protoceratium reticulatum and Gonyaulax spinifera during the early Holocene. Sporadic blooms of potentially toxic Alexandrium spp., Lingulodinium polyedrum and Gymnodinium spp. also occurred from ca. 7000 to 5000 yr BP and more frequently from ca. 2000 to 3000 yr BP. Individual Holocene blooms were up to 10 times larger than in the historical record but they almost always occurred sequentially, without overlap of ‘red tide’ species. In pre-historical times, the bloom cycles started and stopped abruptly in 10 yr or less, and they persisted for about 100–1000 yr. In contrast, the variability and near-concurrence of species blooms in the modern (past 60 yr) record is unmatched in the past, and suggests disequilibrium of the natural ecosystem structure. Atlantic records from La Have and Emerald basins on the Scotian Shelf are less precise because bioturbation limits time resolution to ca. 25 yr. However, cyst abundances of cf. Alexandrium, Protoceratium reticulatum, Gonyaulax spinifera and Lingulodinium polyedrum were also an order of magnitude larger in the early Holocene, where they correspond to a summer sea surface warming of 2–5°C. Lesser peaks also appear during the past 2000 yr where they apparently correspond to a slight increase in winter temperature. There is no evidence of much lower salinity that would support stronger stratification. The similarity of pre-industrial age cyst records of ‘red tide’ histories in the oceanographically different Pacific and Atlantic regions of Canada indicates that climate change (including surface temperature and storminess) is the main driving force stimulating blooms. This implies that HABs will reoccur regardless of efforts to limit spreading. However, reduced pollution may decrease the HAB diversity, thereby stabilizing the population dynamics and increasing the predictability of bloom occurrences.