Lake warming favours small-sized planktonic diatom species

Diatoms contribute to a substantial portion of primary production in the oceans and many lakes. Owing to their relatively heavy cell walls and high nutrient requirements, planktonic diatoms are expected to decrease with climate warming because of reduced nutrient redistribution and increasing sinking velocities. Using a historical dataset, this study shows that diatoms were able to maintain their biovolume with increasing stratification in Lake Tahoe over the last decades; however, the diatom community structure changed. Increased stratification and reduced nitrogen to phosphorus ratios selected for small-celled diatoms, particularly within the Cyclotella genus. An empirical model showed that a shift in phytoplankton species composition and cell size was consistent within different depth strata, indicating that altered nutrient concentrations were not responsible for the change. The increase in small-celled species was sufficient to decrease the average diatom size and thus sinking velocity, which strongly influences energy transfer through the food web and carbon cycling. Our results show that within the diverse group of diatoms, small-sized species with a high surface area to volume ratio were able to adapt to a decrease in mixing intensity, supporting the hypotheses that abiotic drivers affect the size structure of planktonic communities and that warmer climate favours small-sized diatom cells

Marine diatoms grown in chemostats under silicate or ammonium limitation. III. Cellular chemical composition and morphology of Chaetoceros debilis, Skeletonema costatum , and Thalassiosira gravida

Three marine diatoms, Skeletonema costatum, Chaetoceros debilis , and Thalassiosira gravida were grown under no limitation and ammonium or silicate limitation or starvation. Changes in cell morphology were documented with photomicrographs of ammonium and silicate-limited and non-limited cells, and correlated with observed changes in chemical composition. Cultures grown under silicate starvation or limitation showed an increase in particulate carbon, nitrogen and phosporus and chlorophyll a per unit cell volume compared to non-limited cells; particulate silica per cell volume decreased. Si-starved cells were different from Si-limited cells in that the former contained more particulate carbon and silica per cell volume. The most sensitive indicator of silicate limitation or starvation was the ratio C:Si, being 3 to 5 times higher than the values for non-limited cells. The ratios Si:chlorophyll a and S:P were lower and N:Si was higher than non-limited cells by a factor of 2 to 3. The other ratios, C:N, C:P, C:chlorophyll a , N:chlorophyll a , P:chlorophyll a and N:P were considered not to be sensitive indicators of silicate limitation or starvation. Chlorophyll a , and particulate nitrogen per unit cell volume decreased under ammonium limitation and starvation. NH 4 -starved cells contained more chlorophyll a , carbon, nitrogen, silica, and phosphorus per cell volume than NH 4 -limited cells. N:Si was the most sensitive ratio to ammonium limitation or starvation, being 2 to 3 times lower than non-limited cells. Si:chlorophyll a , P:chlorophyll a and N:P were less sensitive, while the ratios C:N, C:chlorophyll a , N:chlorophyll a , C:Si, C:P and Si:P were the least sensitive. Limited cells had less of the limiting nutrient per unit cell volume than starved cells and more of the non-limiting nutrients (i.e., silica and phosphorus for NH 4 -limited cells). This suggests that nutrient-limited cells rather than nutrient-starved cells should be used along with non-limited cells to measure the full range of potential change in cellular chemical composition for one species under nutrient limitation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46631/1/227_2004_Article_BF00392568.pd

Seasonal variations in the phytoplankton biomass and productivity of a warm-core Gulf Stream ring

Phytoplankton biomass, as chlorophyll a (Chl a) and ATP, and carbon productivity rates were determined in three cruises to a warm-core Gulf Stream ring. Ring 82B was formed in late February 1982 with observations made in April to May, June and August. Vertical profiles of Chl a, ATP and particulate organic carbon (POC) in April to May showed little vertical structure during a period when the ring mixed layer extended from the surface to >350 m . Daily productivity rates, Chl a and ATP biomass estimates of the euphotic zone were similar to those in the source waters, the Sargasso Sea, but lower than those in contiguous Slope Waters. Despite the absence of a stratified surface layer in the ring, the phytoplankton productivity rates, assimilation numbers, and carbon-specific growth rates were relatively high. In June, a pycnocline existed at 25 m across the ring with biomass maxima of Chl a, ATP, and POC occurring near the seasonal pycnocline. Although the range in productivity in June (0.26 to 0.98 gC m −2 day −1) was similar to that in April, the carbon biomass estimated from ATP increased from 1.42 to 4.77 gC m −2 between the two cruises. The increase was partially attributed to an increased heterotrophic biomass. Carbon-specific specific doubling times in June were, most likely, influenced by the presence of a large heterotrophic ATP component. In August the surface layer of the ring was displaced by intrusions, or overwash, of Slope and Gulf Stream waters. During the 6 month lifespan of 82B total primary production was estimated at 126 gC m −2, a value similar to productivity estimates for the contiguous Slope Waters