High resolution monitoring of episodic stratification events in an enclosed marine system Timothy Sullivan a, * , Stefanie Broszeit b , Keith P.A. O’Sullivan c , Rob McAllen b , John Davenport b , Fiona Regan a a Marine and Environmental Sensing Technology Hub, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Glasnevin D9, Ireland b School of Biological, Earth and Environmental Sciences, University College Cork, Distillery Fields, North Mall, Cork, Ireland c Hydraulics and Maritime Research Centre, University College Cork, Cork, Ireland article info Article history: Received 26 November 2012 Accepted 21 February 2013 Available online 4 March 2013 Keywords: hypoxia anoxia monitoring organic matter water chemistry biogeochemical cycle coastal waters abstract While hypoxic and anoxic environments have existed throughout geological time, their frequency of occurrence in shallow coastal and estuarine areas appears to be increasing. However, few data are available on the physicochemical conditions at the boundary between anoxic and normoxic layers, including the conditions required for both formation and dissipation of stratification. Advances in autonomous environmental sensing technology have produced robust sensors capable of detailed measurements under inhospitable conditions created in such environments. In this study, an autono- mous sensor approach was used to compare water column properties above and below the stratification before during and after dissipation of the stratification. Further, an investigation into the effect of the stratification on sedimentation rates of organic and inorganic matter and current speeds is reported here. Lough Hyne, a seasonally stratified temperate marine lake provided favourable conditions for this study. It was shown that temperatures dropped rapidly above the oxy-thermocline while increasing rapidly below the stratification, leading to a mixing of the complete water column. This was reflected in oxygen measurements below the stratification, which rose from anoxia to normoxic conditions over the same time period. During summer, the thermocline formed a barrier to organic matter sedimentation, reducing it significantly when present, while inorganic matter sedimentation was unaffected by the presence of thermocline. It also caused a reduction in current speeds below the thermocline. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Marine water column stratification and the conditions involved in the formation and dissipation of zones of low oxygen in the coastal oceans have been of increasing research effort in recent decades (Diaz, 2001; Neretin et al., 2001; Rabalais et al., 2002; Canfield et al., 2010; Hofmann et al., 2011). This is driven by interest in the effects of coastal eutrophication, climate change, greenhouse gas cycling, and evidence that the occurrence of low oxygen (hypoxic/anoxic) zones has increased globally since the 1960s (Diaz and Rosenberg, 2008). The biogeochemistry of oxygen minimum zones has been increasingly investigated to understand the role of such conditions in ocean nutrient cycling, sediment biogeochem- ical processes, and ultimately influences on ecosystem dynamics (Middelburg and Levin, 2009). Of particular interest are the boundary conditions and the steep gradient of physicochemical conditions occurring in the water column between hypoxic and normoxic waters, profoundly affecting biological and physical processes within that water column. According to Diaz, conditions of hypoxia occur when dissolved oxygen (DO) concentrations decrease below 2.8 mg L 1 (Diaz and Rosenberg, 2008). Such DO concentrations affect the behaviour of benthic fauna, culminating in mass mortality for species unable to avoid locations and times when DO levels are reduced below 0.5 mg L 1 . Therefore, the study and understanding of these zones is important from a biological perspective and is of increasing importance in ecosystem man- agement in coastal waters. Although marine coastal hypoxic zones are now relatively common globally, few are readily accessible to detailed analysis using current autonomous sensors. Lough Hyne Marine Reserve (LHMR), on the SW coast of Ireland has features typical of coastal seasonally hypoxic zones caused by restricted circulation. It is a small marine Lough (area w0.5 km 2 ) that is highly sheltered and relatively shallow (maximum depth w48 m). Records of anoxic * Corresponding author. E-mail addresses: tim.sullivan@dcu.ie, timothy.sullivan2@mail.dcu.ie (T. Sullivan). Contents lists available at SciVerse ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.02.012 Estuarine, Coastal and Shelf Science 123 (2013) 26e33