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In situ measurements confirm the seasonal dominance of benthic algae over phytoplankton in nearshore primary production of a large lake
- MALKIN, SAIRAH Y., BOCANIOV, SERGHEI A., SMITH, RALPH E., GUILDFORD, STEPHANIE J., HECKY, ROBERT E.
- Freshwater biology 2010 v.55 no.12 pp. 2468-2483
- Cladophora, alkalinity, biomass, carbon, carbonic acid, dissociation, growing season, lakes, models, monitoring, photosynthesis, phytoplankton, primary productivity, regrowth, spring, summer, Lake Ontario
- 1. Despite the recognition of its importance, benthic primary production is seldom reported, especially for large lakes. We measured in situ benthic net primary production by monitoring flux in dissolved inorganic carbon (DIC) concentration in benthic incubation chambers, based on continuous measurements of CO₂₍aq₎ flux, alkalinity, and the temperature‐dependent dissociation constants of carbonic acid (K₁ and K₂). This methodology has the advantages of monitoring net primary production directly as change in carbon, maintaining continuous water recirculation, and having sufficient precision to detect change in DIC over short (i.e. 15 min) incubations, even in alkaline waters. 2. Benthic primary production on Cladophora‐dominated rocky substrata in western Lake Ontario was measured biweekly. Maximum biomass‐specific net photosynthetic rates were highest in the spring (2.39 mgC g Dry Mass⁻¹ h⁻¹), decreased to negative rates by early summer (−0.76 mgC g DM⁻¹ h⁻¹), and exhibited a regrowth in late summer (1.98 mgC g DM⁻¹ h⁻¹). 3. A Cladophora growth model (CGM), previously validated to predict Cladophora biomass accrual in Lake Ontario, successfully simulated the seasonality and magnitude of biomass‐specific primary production during the first cohort of Cladophora growth. Averaged over this growing season (May-Aug), mean areal net benthic production at the estimated depth of peak biomass (2 m) was 405 mg C m⁻² d⁻¹. 4. We measured planktonic primary production in proximity to the benthic study and constructed a depth‐resolved model of planktonic production. Using the CGM, benthic primary production was compared with planktonic primary production for the period May-Aug. Net benthic production from the shoreline to the 12 m contour (1-2 km offshore) equalled planktonic production. Closer to shore, benthic primary production exceeded planktonic primary production. Failure to account for benthic primary production, at least during abundant Cladophora growth, will lead to large underestimates in carbon and nutrient flows in the nearshore zone of this Great Lake.