Nutrient input and the competition between Phaeocystis pouchetii and diatoms in Massachusetts Bay spring bloom Mingshun Jiang a, ⁎, David G. Borkman b , P. Scott Libby c , David W. Townsend d , Meng Zhou a a School for the Environment, University of Massachusetts Boston, 100 Morrissey Blvd., Boston, MA 02125, USA b Graduate School of Oceanography, University of Rhode Islands, 215 South Ferry Road, Narragansett, RI 02882, USA c Battelle Memorial Institute, 397 Washington St., Duxbury, MA 02332, USA d School of Marine Sciences, 5706 Aubert Hall, University of Maine, Orono, ME 04469, USA abstract article info Article history: Received 12 October 2013 Received in revised form 19 February 2014 Accepted 20 February 2014 Available online 1 March 2014 Keywords: Nutrients Diatoms P. pouchetii Massachusetts Bay Gulf of Maine Spring bloom Winter storm River discharges North Atlantic Oscillations Ecosystem model The phytoplankton community in Massachusetts Bay has displayed significant inter-annual variability and pos- sible trends over the last two decades, with increasing frequency and magnitude of strong Phaeocystis pouchetii blooms and generally opposite fluctuations in diatom abundances. An analysis of historical data suggests that changes in winter nitrate and silicate concentrations (both their absolute and relative values) may play a critical role in the competition between diatoms and P. pouchetii. We developed a new ecosystem model to simulate Phaeocystis dynamics and to test the significance of variable winter nutrient levels. Idealized simulations for the years 1992–2009 generally reproduced the observed inter-annual variability of P. pouchetii and diatoms during the spring blooms, with modeled peaks in biomass of diatoms and P. pouchetii significantly being correlat- ed with their observed mean abundances. Moreover, modeled peak biomass ratio and observed mean abundance ratio between diatoms and P. pouchetii during the spring blooms were similarly depending on both the winter nitrate and residual nitrate (nitrate minus silicate) concentrations. These results are consistent with resource competition theory in which relatively low winter nutrient concentrations would favor species with faster growth rate (diatoms, in this case). With sufficiently high winter nutrient concentrations, however, P. pouchetii was able to grow before nitrate being depleted by diatoms, even though winter Si N N. Our observations further indicate that inter-annual nutrient variability and consequently spring bloom phytoplank- ton variability in Massachusetts Bay are likely driven by changes in winter nutrient fluxes from Gulf of Maine rivers and winter convective mixing. These fluxes may have been modulated by large-scale processes such as the North Atlantic Oscillations and Arctic melting through the river discharges, winter-storm activities (and hence winter mixing and nutrient supply), and the deep waters inflow into the Gulf of Maine. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Massachusetts Bay (MB) is a coastal embayment in the western Gulf of Maine (GOM) (Fig. 1). Recent studies indicated that the phytoplank- ton community in the bay displayed strong inter-annual variability and possible trends over the last two decades (Hunt et al., 2010). In particu- lar, both the frequency and magnitude of Phaeocystis pouchetii blooms have increased with diatom and P. pouchetii abundances largely fluctu- ating in opposite phase. Yet, the dynamic mechanisms leading to such changes remain unclear. Given the different nutrient requirements of diatoms and P. pouchetii, one potential mechanism is the nutrient competition between these two species (e.g. Egge and Aksnes, 1992; Lancelot and Rousseau, 1994; Officer and Ryther, 1980; Wilson et al., 2007). Circulation in MB is driven by local forcing (winds and runoff) and the GOM intruding current around Cape Ann (Fig. 1), which is a branch of the western Maine Coastal Current (WMCC), a buoyancy- and wind– driven current that extends from the eastern Maine Coastal Current (EMCC) (Bigelow, 1927; Brooks, 1985; Geyer et al., 1992, 2004; Lynch et al., 1997; Pettigrew et al., 2005). The WMCC also includes contribu- tions from a coastal freshwater plume driven by river discharges. In spring, strong river runoff and downwelling favorable winds produce a narrower and stronger coastal jet and an enhanced intruding flow (Churchill et al., 2005; Geyer et al., 2004). The jet may separate from the coastline to form meso-scale eddies in the North Passage (Jiang et al., 2011). During winter and spring, the intruding current from the GOM tends to extend southward along the MB coast, and sometimes can penetrate deeply into Cape Cod Bay. Thus, MB is a semi-enclosed system with strong input from the GOM through the North Passage. Journal of Marine Systems 134 (2014) 29–44 ⁎ Corresponding author at: Harbor Branch Oceanographic Institute, Florida Atlantic University, 5600 US 1 North, Ft. Pierce, FL 34946, USA. Tel.: +1 772 242 2254; fax: +1 772 242 2412. E-mail address: jiangm@fau.edu (M. Jiang). http://dx.doi.org/10.1016/j.jmarsys.2014.02.011 0924-7963/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: www.elsevier.com/locate/jmarsys