Environmental controls on the nitrogen-fixing cyanobacterium Nodularia spumigena in a temperate lagoon system in SE Australia

Abstract

Estuarine blooms of N 2 -fixing cyanobacteria are rare, and the factors controlling them remain poorly understood outside the Baltic Sea. We measured in situ physico-chemical conditions and undertook nutrient-addition bioassays and dilution-based grazing experiments fortnightly over the 2010 to 2011 Austral spring-summer-autumn, to evaluate the role of physical environmental drivers, grazing and macronutrients on the growth of Nodularia spumigena Mertens (hereafter Nodularia) in a temperate lagoon system (Gippsland Lakes, Australia). Nodularia appeared in late summer, following a period of warm weather (water temperature > 22°C), high solar exposure, calm conditions, relatively low salinity (< 22) and low dissolved inorganic nitrogen (~0.4 µmol l ) in control bioassays (no nutrients added), whereas other phytoplankton taxa showed zero or negative growth. N addition caused significantly lower Nodularia growth, while other taxa had higher growth. P addition did not significantly enhance Nodularia growth. This suggests Nodularia was able to obtain P from the water column and from phytoplankton breakdown. A reduction in grazing pressure via dilution led to lower growth rates of Nodularia and higher growth rates of other phytoplankton taxa. This suggests that the grazers selectively consumed taxa other than Nodularia, and that grazing positively influences Nodularia growth in this system. These data indicate that Nodularia growth was limited in situ by physical conditions (primarily solar radiation) and that nutrients and grazing were of second order importance. KEY WORDS: Nodularia spumigena · Nutrient limitation · Grazing · BioassayResale or republication not permitted without written consent of the publisher

Assimilation of Diazotrophic Nitrogen into Pelagic Food Webs

“…2). The negative r G observed for N. spumigena was due to increased growth rates occurring at higher grazer concentrations, suggesting that increased grazing pressure on sympatric diatoms and dinoflagellates reduces nutrient limitation for the ungrazed N. spumigena [11].…”

“…Herbivorous grazing by microzooplankton (c. 20–200 µm body length) and mesozooplankton (c. 200–2000 µm) is the primary process by which phytoplankton are consumed and organic nutrients remineralized; yet, toxic cyanobacteria are often avoided by planktonic herbivores [9], [10], [11]. Despite this, grazing by zooplankton on cyanobacteria has been hypothesized to explain the rapid assimilation of diazotrophic nitrogen (N D ) from a toxic cyanobacterium into zooplankton [12].…”

“…We tested this conceptual model during a bloom of the cyanobacterium Nodularia spumigena in Lake King, Gippsland Lakes, Australia. The filamentous N. spumigena is a toxic diazotrophic cyanobacterium that produces the hepatotoxin nodularin [9] and there is substantial evidence that herbivorous zooplankton preferentially avoid grazing on this species under natural conditions [9], [11], [19]. We hypothesised that palatable phytoplankton taxa would be central to the trophic transfer of N D , be it via uptake of N D exuded by N. spumigena during the bloom or remineralised post bloom.…”

Blooms of cyanobacteria in a temperate Australian lagoon system post and prior to European settlement

“…Previous work has shown that the Gippsland Lakes are generally nitrogen limited outside N. spumigena blooms , and increased inputs of this element have most likely resulted in increased productivity in the Gippsland Lakes, particularly during the winter and spring diatom blooms when most of the nutrient load is delivered. The increased settling of phytodetritus after the collapse of these blooms would have driven increased water column anoxia over the late-spring period, triggering the release of phosphorus stored in the sediment leading to more favourable conditions for N. spumigena blooms (Cook et al, 2010;Scicluna et al, 2015;Holland et al, 2012). We therefore speculate that the recent re-emergence of cyanobacterial blooms is amplified by increased nitrogen loads, which drive an increased internal release of phosphorus through increased bottom water hypoxia and anoxia in late spring through to summer.…”

“…Winter and spring inflows typically lead to blooms of diatoms and dinoflagellates, and since 1987, periodic blooms of Nodularia spumigena have occurred in Lake King during late spring and summer when surface water salinities are ∼ 9-20 . Previous studies have shown that these blooms are phosphorus limited, that they are sustained by high sediment phosphorus release focused in the northern basin of Lake King and that these blooms can fix significant quantities of nitrogen with a δ 15 N of ∼ 0 ‰ Cook et al, 2010;Holland et al, 2012;Woodland and Cook, 2014;Woodland et al, 2013).…”