........................................................................................................................................................................................... Phytoplankton, physical and chemical microscale variations in three brackish rock pools Kerstin Häggqvist* and Tore Lindholm Faculty of Science and Engineering, Environmental and Marine Biology, Åbo Akademi University, Turku, Finland ........................................................................................ SUMMARY Variations at the microscale in phytoplankton distribution with respect to physical and chemical variables in three brackish rock pools of different depth, size and exposure were studied during the growth season. Three hypotheses were made about the microspatial characteristics of the rock pool habitat: (i) there are distinct microscale variations in physical and chemical variables, (ii) the microscale distribution of phyto- plankton is related to characteristic physical and/or chemical variables, and (iii) microscale variations are more pronounced in deep rock pools. Variations were studied at a 10 cm scale by close interval siphon sampling. Physical and chemical var- iations were small irrespective of rock pool. Prevailing weather, as well as the similar round basin shape of the pools contributed to the similarities. Variations in phytoplankton microscale distributions were clear and consistent throughout the study period, and less variable in the deepest pool com- pared to the shallower ones. The distribution of dominant phy- toplankton species correlated with microscale variations in temperature, dissolved oxygen and pH, often during seasonal minima or maxima. The microscale correlations implied that short reaction times of phytoplankton enabled them to exploit, or be influenced by, brief, local variations. The results demon- strated the importance of studies at fine scales to disentangle processes even in shallow weather-influenced ecosystems. Key words: Baltic Sea, dinoflagellates, high pH, microspatial variation, oxygen saturation, shallow, temperature. ........................................................................................ INTRODUCTION Shallow aquatic systems are generally perceived as well mixed, but some are better described as frequently stratifying rather than non-stratifying (Padisák & Reynolds 2003). They can even be strongly and persistently stratified (Abis & Mara 2006; Song et al. 2013). In shallow ponds and pools, vertical temperature differences often develop during calm and sunny weather when the surface temperature rises (Ganning 1971). An emerging temperature stratification may be amplified by turbid water or high densities of phytoplankton at the surface, which both restrict light penetration to subsurface layers (Fairchild et al. 2005; McEnroe et al. 2013). The short fetch of small ponds and pools in combination with a sheltered location reduce wind mixing at the surface (Abis & Mara 2006; Song et al. 2013). In coastal pools influenced by saline water, substantial stratification may develop due to ver- tical salinity differences when an inflow of freshwater remains on top of dense saline water (McGregor 1965). However, less than 1 m deep urban ponds are likely to be fully mixed (Song et al. 2013) and in very shallow rock pools, winds, or even organism movements, quite easily destabilize the water col- umn (Ganning 1971). Stratified water columns with pro- nounced physical and chemical variations may have profound impacts on the phytoplankton distribution. Moreover, small spatial variations also create microhabitats (Siegel 1998) with a wide range of impacts on phytoplankton, from photons hit- ting the individual cells to aggregations of grazers, leading to distinct distributions of the phytoplankton (Reynolds 2006). Globally, rock pools are largely unexplored and studies on ecological processes in these often temporary habitats have usu- ally considered the faunal component (Jocque et al. 2010). In the Baltic Sea area rock pools are especially abundant on the over 10 000 islands in the Finnish archipelago, where they are included in the national environmental protection plan (Raunio et al. 2008). Eutrophication, pollution and climate change pose real threats to these systems (Raunio et al. 2008). Conse- quently, there is a need for fundamental research of species and processes in them. Rock pool microalgae in the south-western archipelago of Finland have fascinated researchers for over 100 years, which has resulted in numerous taxonomic studies, including species descriptions (Levander 1900; Järnefelt 1939; Droop 1953; Uherkovich 1968,1973; Hällfors 1984; Hällfors & Niemi 1989; Lindholm 1996; Häggqvist & Lindholm 2015). However, ecological processes related to phytoplankton and the spatial distributions of phytoplankton within rock pools in this area have been overlooked. In this study, microscale variations in phytoplankton distri- butions with respect to physical and chemical variables were investigated in three rock pools of different size and depth. The hypotheses of the study were: (i) there are distinct physi- cal and chemical microscale variations in different rock pools, (ii) the microscale distribution of phytoplankton is related to characteristic physical and chemical variables in each rock pool, and (iii) microscale variations are more pronounced in deep rock pools. MATERIALS AND METHODS The studied rock pools, Ådgrundet, Storgrund and Ådskär (islands A, S and Ad, respectively in Häggqvist & Lindholm 2015), are located on three islands in the outer north-western part of Åland Islands, southwest Finland (Table 1). They differ ......................................................................................... *To whom correspondence should be addressed. Email: kerstin.haggqvist@abo.fi Communicating Editor: Mitsunobu Kamiya Received 22 October 2015; accepted 11 May 2016. © 2016 Japanese Society of Phycology Phycological Research 2016; 64: 241–250 doi: 10.1111/pre.12136