Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Spatiotemporal patterns of phytoplankton composition and abundance in the Maryland Coastal Bays: The influence of freshwater discharge and anthropogenic activities Ozuem F. Oseji, Paulinus Chigbu, Efeturi Oghenekaro, Yan Waguespack, Nianhong Chen * Department of Natural Sciences, University of Maryland Eastern Shore, Princess Anne, MD, 21853, United States ARTICLE INFO Keywords: Maryland Coastal Bays Phytoplankton pigments Chlorophyll a Freshwater discharge HPLC ABSTRACT The spatial and temporal variations in phytoplankton abundance and community structure in the northern and southern parts of the Maryland Coastal Bays (MCBs) that differ in anthropogenic activities and hydrological characteristics were studied in 2012 and 2013 using photosynthetic pigments as biomarkers. Phytoplankton pigment biomass and diversity were generally higher in the northern bays that receive high nutrient input from St. Martin River, than in the southern bays where nutrient levels were comparatively low. Sites close to the mouths of tributaries in northern and southern bays had higher nutrient levels, which favored the development of dinoflagellates, and nano- and picophytoplankton, than sites closer to the inlets. The microplankton domi- nated the phytoplankton community in spring (> 90%) and decreased in relative abundance into fall (< 60%) whereas nanoplankton peaked in summer or fall. Picoplankton relative abundance increased from late spring (< 10%, March 2012 & 2013) to summer (40%, July 2012 and August 2013) and was correlated positively with NH 4 + and negatively with salinity. The observed spatial and seasonal patterns of phytoplankton relative abundance and diversity are likely due to changes in nutrient concentrations and ratios, driven by variations in freshwater discharge, and selective grazing of phytoplankton. Water quality management in the MCBs should continue to focus on reducing nutrient inputs into the bays. 1. Introduction Phytoplankton play a key role in aquatic ecosystems, hence un- derstanding the structure, dynamics and composition of phytoplankton is essential in ecological studies and environmental monitoring pro- grams (Henriksen et al., 2002; Rodrigues et al., 2014). Changes in phytoplankton community structure may occur in response to en- vironmental variables (e.g. light, nutrients, temperature, and salinity) and/or upper trophic level changes, micro-versus mesozooplankton grazing (Miller et al., 1995; Glibert, 1998; Lewitus et al., 1999; Noble et al., 2003). In coastal oceans, changes in nutrients and water quality due to temporal variations in precipitation and freshwater discharge, coupled with anthropogenic activities can cause changes in phyto- plankton species composition, and lead to increases in the occurrence and severity of harmful phytoplankton blooms (Glibert and Burkholder, 2006; Zhu et al., 2010; Gobler and Sunda, 2012). Coastal lagoons are characterized by shallow water depth, limited exchange with the adjacent oceans, and high primary productivity (Lankford, 1977; Boynton et al., 1996). Photosynthesis in surface water is often augmented by production from macroalgae and seagrasses (Lee and Olsen, 1985). Although they are well-mixed, hypoxia has been reported in some localized areas (Maryland DNR, 2002; Hall and Wazniak, 2005). In the Maryland Coastal Bays (MCBs), very few studies have ad- dressed the ecology of phytoplankton (Boynton et al., 1996), and have focused on the occurrence of brown tide (Aureococcus anophagefferens), the factors influencing its abundance (Glibert et al., 2001; Trice at al. 2004; Deonarine et al., 2006; Minor et al., 2006; Glibert et al., 2007), and its impacts on hard clams, Mercenaria mercenaria (Wazniak et al., 2007). The occurrence of other potentially harmful algae species in the MCBs has also been documented (Tango et al., 2004). Glibert et al. (2014) described the eutrophication process in the MCBs and the as- sociated seasonal, spatial and inter-annual variations in four major phytoplankton groups, based on their photosynthetic biomarker pig- ments. Specifically, they reported increasing trends in the concentra- tions of zeaxanthin (cyanobacteria), peridinin (dinoflagellates), and fucoxanthin (diatoms), and the fact that NH 4 + or DON dominated the nitrogen in the water column causing a shift in the phytoplankton community towards picoplankton. Additionally, using freshwater dis- charge data from Nassawango Creek, which is part of the Chesapeake https://doi.org/10.1016/j.ecss.2018.04.004 Received 20 September 2017; Received in revised form 21 February 2018; Accepted 2 April 2018 * Corresponding author. E-mail address: nchen@umes.edu (N. Chen). Estuarine, Coastal and Shelf Science 207 (2018) 119–131 Available online 04 April 2018 0272-7714/ © 2018 Published by Elsevier Ltd. T