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Dinophyta Characterise Nitrogen Scarcity More Strongly Than Cyanobacteria in Moderately Deep Lakes

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Dinophyta Characterise Nitrogen Scarcity More Strongly Than Cyanobacteria in Moderately Deep Lakes Acta Protozool. (2013) 52: 203–216 http://www.eko.uj.edu.pl/ap ActA doi:10.4467/16890027AP.13.0018.1115 protozoologica Special issue: Protists as Bioindicators of Past and Present Environmental Conditions Dinophyta Characterise Nitrogen Scarcity More Strongly than Cyanobacteria in Moderately Deep Lakes Jane FISHER1,2, Cassandra S. JAMES1,3, Victoria L. MOORE1 and Brian MOSS1 1School of Environmental Sciences, University of Liverpool, Liverpool, UK; 2School of Natural Sciences and Psychology, John Moores University, Liverpool, UK; 3Australian Rivers Institute, Nathan Campus, Griffith University, Nathan, QLD, Australia Abstract. A survey of the summer phytoplankton communities of thirty-six moderately-deep north temperate lowland lakes showed that the proportions of Dinophyta and non-heterocyst-bearing cyanobacteria taxa, measured as biovolume, were inversely related to the total nitrogen:total phosphorus (TN:TP) ratio and that these groups were predominant in lakes where available nitrogen fell to undetectable concentrations. The proportion of heterocyst-bearing cyanobacteria was positively correlated to the TN:TP ratio and nitrate. Dinophyta and/or non-heterocystous cyanobacteria were prevalent in lakes with the highest epilimnion nutrient concentrations, whilst heterocystous cyanobacteria predominated in lakes with moderate nutrient concentrations. It is argued that the ability of Dinophyta to migrate vertically and to supplement their nutrient requirements though heterotrophy may enable them to be at least as successful as cyanobacteria in high nutrient lakes and in overcoming nitrogen-scarcity. Our findings provide evidence that Dinophyta can be used as indicators of water quality. Key words: Lakes, Shropshire and Cheshire Meres, phytoplankton, seasonality, nutrients, vertical movement. INTRODUCTION anobacteria (Tilman et al. 1982, Harper 1992, Levine and Schindler 1999, Dokulil and Treubner 2000). Red- field (1958) noted that algae required N:P in a ratio Recent studies into the frequency of occurrence of around 7:1 (by weight). At ratios lower than this, algae nitrogen and phosphorous scarcity in lakes suggest that that are capable of fixing their own nitrogen, (many cy- nitrogen limitation might be as common as phosphorus anobacteria and others having cyanobacterial symbi- limitation (Elser 2007, Lewis and Wurtsbaugh 2008). onts) are advantaged. Large populations of cyanobac- Eutrophication is often attributed to increased phos- teria are sometimes a nuisance and can result in toxic phorus loading, leading to a reduction in the ratio of blooms that pose a health risk to recreational users and nitrogen:phosphorus (N:P), further promoting nitrogen animals drinking the water (Howard et al. 1996). (N) limitation, a decrease in phytoplankton diversity, The proposed links among eutrophication, low N:P and a phytoplankton assemblage predominantly of cy- ratios and cyanobacterial dominance were emphasised Address for correspondence: Jane Fisher, School of Natural Sci- by Schindler (1974) who showed that addition of phos- ences and Psychology, John Moores University, Liverpool, L3 3AF, phorus (P) to one lobe of a lake, separated by a nylon UK; E-mail: [email protected] curtain from the rest of the lake, resulted in increased 204 J. Fisher et al. chlorophyll concentrations and a phytoplankton popula- logical tests of nutrient limitation. The “Redfield ratio” tion dominated by N-fixing cyanobacteria. Whilst many (Redfield 1958) suggests that N deficiency is likely to field and laboratory studies have shown cyanobacterial occur at N:P ratios < 7:1 (by weight). Absolute concen- domination to be greater in waters with low N:P ratios, trations are also important, as if the concentrations of there is growing evidence that such dominance is more both bio-available nutrients are high, the ratio becomes highly correlated with other indicators of trophic sta- irrelevant (Reynolds 1999) and ratios are incalculable tus (Prepas and Trimbee 1988, Reynolds and Petersen if concentrations are below laboratory detection levels. 2000). A survey of 99 lakes (Downing et al. 2001) Furthermore, the Redfield ratio reflects the average nu- showed cyanobacterial population to be correlated with trient requirement of algae while different species may total phosphorus (TP), total nitrogen (TN) and phyto- have different nutritional requirements (Rhee 1978, plankton production, rather than to ratios of N:P. Indeed, Klausmeier et al. 2004). In addition nutrient limitation the proportion of nuisance cyanobacteria, in conjunction is likely to be stoichiometric (Kilham 1990, Kim et al. with total chlorophyll, has been proposed as a means of 2007), and while it has been questioned if two or more monitoring nutrient enrichment in lakes under the Water nutrients can be simultaneously limiting to the same Framework Directive (WFD UKTAG 2008). species (Droop 1973) it is certain that different species The lack of a clear relationship between cyanobac- of algal will be limited by different nutrients, such as teria and measures of N-limitation in some studies may N, P, silica or carbon, at the same time. It is insufficient be a product of the diverse requirements of this group. to rely on annual mean nutrient concentrations to de- Blomquist et al. (1994) argue that it is more appropriate termine limitation, but sustained periods in which the to analyse the links among cyanobacteria, trophic sta- bioavailable form of a nutrient is undetectable indicates tus and low N:P ratio by considering N-fixing and non which nutrient is potentially limiting (Maberly et al. N-fixing species separately. Non N-fixing cyanobacteria 2004), though phytoplankton may be able to take-up will be favoured by the presence of ammonium, while nutrients at concentrations far lower than laboratory de- low concentrations of nitrogen in general will promote tection limits and be able to use luxury uptake to over- those cyanobacteria capable of fixing N (Blomquist et come periods when one or more nutrient may be limit- al. 1994). The ability of some cyanobacteria, such as the ing. This raises the issue of whether algal groups can Nostocales, to fix nitrogen can be recognised by their be used to indicate nutrient limitation, and whether cy- ability to produce heterocysts, (recently called hetero- anobacteria are effective indicators of nitrogen scarcity, cytes, which is etymologically correct, but the original despite strong conventional beliefs in their efficacy. term remains most widely used and understood). N-fixing Cyanobacteria are associated with moderate but not capabilities are not confined to these taxa however, and the highest nutrient concentrations, where green al- some species within the genera of Oscillatoria, Lyngbya gae, particularly Chlorococcales predominate, at least and Phormidium, all of which are non-heterocystous cy- in shallow unstratified lakes (Jensen et al. 1994). The anobacteria, are also capable of N-fixation (Paerl 1988), position in deeper lakes is less clear, but the predomi- though generally under microaerophilic conditions that nance of cyanobacteria in moderately deep, nutrient- are more associated with sediments and metalimnia than rich lakes seems to depend on their ability either to fix surface waters. In open waters, the frequency of het- N or to migrate between deeper, more nutrient-rich wa- erocysts is generally taken to be a reasonable guide to ters and the illuminated surface waters using regulation the potentiality for nitrogen-fixation. Investigations into of their content of gas vesicles (Reynolds and Walsby how to recognise which nutrient limits phytoplankton 1975, Ganf and Oliver 1982). Such migrations are pos- biomass are becoming more important now that there is sible also for flagellate algae. A clear indicator of nitro- increasing evidence that phosphorus limitation may not gen scarcity would be useful for monitoring purposes be so universal as once thought and that nitrogen limita- owing to the high cost of direct investigations and the tion or co-limitation by both N and P may be at least as number of lakes now requiring monitoring under the common (Maberly et al. 2002, James et al. 2003, Elser et European Water Framework Directive. In this study we al. 2007, Lewis and Wurtsbaugh 2008). investigated if other algae capable of vertical migration, Recognising N-limitation from in-lake nutrient con- such as the Dinophyta, are as effective as heterocystous centrations is challenging, though Hameed et al. (1999) cyanobacteria, at indicating nitrogen scarcity in moder- showed that interpretation of in-lake concentrations ately deep lakes. We used extensive correlation among were just as reliable as the results of a series of physio- lakes in the UK that are generally severely eutrophi- Dinophyta Characterise Nitrogen Scarcity 205 cated and lakes in Poland where agriculture has been integrated water samples were taken from the top one metre at the less intensive to investigate this hypothesis. centre of each lake, with the exception of Bomere where samples were taken from a jetty. The Polish lakes were visited once, between July and August 2001, and three replicate water samples were taken from wading depth from the shore, or from a jetty where possible, MATERIALS AND METHODS using a 2 m long extended sampler. Laboratory analysis Study sites One litre samples were filtered through Whatman GF/C glass- Thirty-six moderately deep (Zmax > 3 m) lowland freshwater fibre filters on the day of collection and the filter frozen for chlo- lakes were studied.
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