Comparative assessment of two agriculturally-influenced estuaries: Similar pressure, different response Daniel A. Lemley a, ⁎, Janine B. Adams a , Susan Taljaard a,b a Botany Department, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa b Council for Scientific and Industrial Research, PO Box 320, Stellenbosch 7599, South Africa abstract article info Article history: Received 26 October 2016 Received in revised form 19 January 2017 Accepted 25 January 2017 Available online xxxx This study compared the spatio-temporal dynamics in two agriculturally-influenced South African estuaries - Gamtoos and Sundays - to investigate how contrasting hydrological alterations influence physical, chemical and biological responses. With the Gamtoos Estuary experiencing regular high flow conditions, a key difference between the two systems is the propensity for natural flushing events to occur; a mechanism largely eliminated from the highly-regulated Sundays Catchment. Phytoplankton blooms (N 20 Chl-a μgl -1 ) were persistent and seasonal in the Sundays, inducing summer bottom-water hypoxia (b 2 mg l -1 ), whilst those in the Gamtoos were episodic and flow-dependent. Of concern in the Sundays Estuary, was the magnitude (N 550 μgl -1 ) and re- current nature of two harmful algal bloom (HAB) species. This study provides the first account of HAB persistence and seasonal hypoxia in a South African estuary, demonstrating the possible consequences of shifting an ecosys- tem into a new stable state. © 2017 Elsevier Ltd. All rights reserved. Keywords: Phytoplankton Heterosigma akashiwo Heterocapsa rotundata Hypoxia Eutrophication Ecohydrodynamics 1. Introduction Over recent history the intensification of catchment activities has in- creased rapidly, leading to transformation from natural to heavily-de- veloped landscapes. Anthropogenic manipulation of freshwater, such as that induced by agricultural practices, affects the physical and biogeo- chemical balance of estuaries by altering the timing, magnitude, and na- ture of inputs (e.g. freshwater, inorganic nutrients and organic matter) (Hopkinson and Vallino, 1995; Buzzelli et al., 2007). Such changes can result in undesirable disturbances to the ecological functioning of coast- al ecosystems, thus increasing the likelihood of eutrophication (Tett et al., 2007). Food production, particularly via agricultural activities, is one of the key contributors to nutrient enrichment ‐ particularly nitrogen (N) and phosphorus (P) ‐ of coastal ecosystems. The high nutrient inputs to ag- ricultural lands, related to synthetic fertilizer application (i.e. level, type, method and timing) and high livestock numbers, can potentially lead to elevated nutrient concentrations in adjacent waterbodies by way of leaching or runoff (Pearce and Schumann, 2001; Abdus Sattar et al., 2014; Jickells et al., 2014; Sharpley and Wang, 2014; Lawniczak et al., 2016; Reed et al., 2016). An early study by Hopkinson and Vallino (1995) provides credence for this by demonstrating the strong linear relationship between water column inorganic nutrients and the degree to which adjacent land has been cleared for agricultural practices, the consequences of which have both localised and far-field effects. For ex- ample, alterations to the nutrient stoichiometry of coastal systems due to increased river nutrient loading have been associated with impacts such as the formation of harmful algal blooms (HAB), hypoxia and per- sistent eutrophication in estuarine and marine ecosystems (Statham, 2012; Jickells et al., 2014; Reed et al., 2016). In South Africa, this has been observed in numerous estuaries (Snow et al., 2000a; Pearce and Schumann, 2001; Kotsedi et al., 2012; Kaselowski and Adams, 2013), where nutrient-enriched runoff from catchments used extensively for agriculture have been identified as the key driver of eutrophication. Fur- ther, phytoplankton community composition and abundance are close- ly linked to changing nutrient environments (Statham, 2012). The primary objective of this study was to assess the spatial and temporal dynamics of two permanently open estuaries ‐ both subjected to agricultural nutrient inputs ‐ by investigating selected physical, chemical and biological parameters. The characterisation of spatial and temporal dynamics in these estuaries aims to facilitate a greater under- standing of the consequences of agricultural practices on estuarine eco- systems. We hypothesise that an estuary with significant agricultural inputs and highly-regulated flow will result in more persistent and prevalent phytoplankton bloom conditions, and associated hypoxia, as compared to an estuary with a similar agricultural pressure but which still maintains an element of natural flow variability. Such studies are Marine Pollution Bulletin xxx (2017) xxx–xxx ⁎ Corresponding author. E-mail address: lemleydaniel7@gmail.com (D.A. Lemley). MPB-08359; No of Pages 12 http://dx.doi.org/10.1016/j.marpolbul.2017.01.059 0025-326X/© 2017 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Please cite this article as: Lemley, D.A., et al., Comparative assessment of two agriculturally-influenced estuaries: Similar pressure, different response, Marine Pollution Bulletin (2017), http://dx.doi.org/10.1016/j.marpolbul.2017.01.059