Parameters in the photosynthesis-irradiance (P-E) relationship of phytoplankton were measured at weekly to bi-weekly intervals for 20 yr at 6 stations on the Rhode River, Maryland (USA). Variability in the light-saturated photosynthetic rate, PBmax, was partitioned into interannual, seasonal, and spatial components. The seasonal component of the variance was greatest, followed by interannual and then spatial. Physiological models of PBmax based on balanced growth or photoacclimation predicted the overall mean and most of the range, but not individual observations, and failed to capture important features of the seasonal and interannual variability. PBmax correlated most strongly with temperature and the concentration of dissolved inorganic carbon (IC), with lesser correlations with chlorophyll a, diffuse attenuation coefficient, and a principal component of the species composition. In statistical models, temperature and IC correlated best with the seasonal pattern, but temperature peaked in late July, out of phase with PBmax, which peaked in September, coincident with the maximum in monthly averaged IC concentration. In contrast with the seasonal pattern, temperature did not contribute to interannual variation, which instead was governed by IC and the additional lesser correlates. Spatial variation was relatively weak and uncorrelated with ancillary measurements. The results demonstrate that both the overall distribution of PBmax and its relationship with environmental correlates may vary from year to year. Coefficients in empirical statistical models became stable after including 7 to 10 yr of data. The main correlates of PBmax are amenable to automated monitoring, so that future estimates of primary production might be made without labor-intensive incubations.

Methods: Water Sampling: Samples for depth-averaged chlorophyll concentration were collected by slowly lowering and raising a Labline Teflon sampler in less time than required to fill. Samples for P-E parameters, discrete chlorophyll a, and species composition were collected by filling the sampler at the Secchi depth. In the journal article, stations are referenced by their approximate distance from the mouth: Station 1, -1.4 km; Station 2, 0.0 km; Station 3, 2.0 km; Station 4, 3.8 km; Station 5, 4.3 km; Station 6, 5.2 km; Station 7, not referenced. Mean total water depths at the stations were: Station 1, 4 m; Station 2, 3.5 m; Station 3, 3 m; Station 4, 1.94 m; Station 5, 1 m; Station 6, 1 m; Station 7, 0.7 m.Phytoplankton P-E parameters: Phytoplankton photosynthesis was measured as 14C uptake using the small volume 'photosynthetron' method of Lewis & Smith (1983, doi:10.3354/meps013099). A 50 ml subsample was inoculated with 0.5 to 2.5 µCi ml-1 NaH14C, depending on phytoplankton biomass. Subsamples of 1 ml were dispensed into 24 lighted and 2 dark 7-ml glass scintillation vials, which were placed in an aluminum block drilled with holes for lighting from below. Samples from 3 stations at a time were incubated for 1 h at a range of light intensities supplied by a Westinghouse 400 W metal halide lamp. Incubations were terminated by the addition of 250 µl of 1 N HCl and shaking 1 h to drive of unincorporated 14C. Rates of 14C fixation were calculated by the equations of Strickland & Parsons (1972, http://www.dfo-mpo.gc.ca/Library/1507.pdf), using an isotope discrimination factor of 1.06. Various instruments were used to measure the concentration of dissolved inorganic carbon (DIC): a Shimadzu TOC5000 (1990-1997); a Capni-Con 5 Total CO2 Analyzer (Cameron Instruments, Port Aransas, Texas USA) (1998-2006); and a Li-Cor 7000 (Li-Cor, Lincoln, Nebraska USA) (2007-2009). Instruments were calibrated to known solutions before each cruise. DIC concentrations were determined by titration (Strickland & Parsons, 1972, http://www.dfo-mpo.gc.ca/Library/1507.pdf) whenever instruments were being repaired.Chlorophyll a: Samples for chlorophyll a analyses were filtered onto Whatman GF/F glass fiber filters immediately upon returning to the laboratory. Filters were extracted in 10 ml of 90% acetone either immediately or after freezing for <2 weeks. Extracted chlorophyll was estimated spectrophotometrically using the wavelengths and equations given by Jeffrey & Humphrey (1975).Calculation of P-E parameters: In the absence of photoinhibition, the P-E relationship was represented by the hyperbolic tangent equation (Jassby & Platt, 1976) where PB is the 14C photosynthesis rate normalized to Chlorophyll a, E is the photon flux density measured in the incubation vial, PBmax is the maximal photosynthesis rate at light saturation, alpha-B is the slope of the linear portion of the P-E relationship at low light intensities, and the intercept, Rb, is allowed to be positive or negative to avoid bias in the calculation of alpha-B. This is designated as Model No. 1. When photoinhibition was observed I used the three-parameter equation of Platt et al. 1980, where PSB controls the vertical scaling, and beta-B controls the magnitude of photoinhibition. This is designated as Model No. 2. When using the photoinhibition equation, the light saturated rate of photosynthesis is calculated as given in Platt et al. 1980. Parameters were estimated by minimization of squared residuals using the Solver routine in Microsoft Excel, except in 2007 when, due to funding limitations, PBmax was estimated from measurements of 14C uptake in 5 vials incubated at light intensities previously found to be light-saturating. Approximate standard deviations of the parameters were calculated from the diagonal elements of the first term of a Taylor's series expansion of the error propagation for the selected model (Bevington, 1969). Typical magnitudes for the coefficients of variation of the parameters were 10% for PBmax 20% for alpha-B and 100% for Rb.