Arranging organisms into functional groups aids ecological research by grouping organisms (irrespective of phylogenetic origin) that interact with environmental factors in similar ways. Planktonic protists traditionally have been split between photoautotrophic "phytoplankton" and phagotrophic "microzooplankton". However, there is a growing recognition of the importance of mixotrophy in euphotic aquatic systems, where many protists often combine photoautotrophic and phagotrophic modes of nutrition. Such organisms do not align with the traditional dichotomy of phytoplankton and microzooplankton. To reflect this understanding, we propose a new functional grouping of planktonic protists in an ecophysiological context: (i) phagoheterotrophs lacking phototrophic capacity, (ii) photoautotrophs lacking phagotrophic capacity, (iii) constitutive mixotrophs (CMs) as phagotrophs with an inherent capacity for phototrophy, and (iv) non-constitutive mixotrophs (NCMs) that acquire their phototrophic capacity by ingesting specific (SNCM) or general non-specific (GNCM) prey. For the first time, we incorporate these functional groups within a foodweb structure and show, using model outputs, that there is scope for significant changes in trophic dynamics depending on the protist functional type description. Accordingly, to better reflect the role of mixotrophy, we recommend that as important tools for explanatory and predictive research, aquatic food-web and biogeochemical models need to redefine the protist groups within their frameworks. (C) 2016 The Authors. Published by Elsevier GmbH.

Mixotrophy is found in almost all classes of phytoplankton in a wide range of aquatic habitats ranging from oligotrophic to eutrophic marine and freshwater systems. Few studies have addressed how the nutritional status of the predator and/or the prey affects mixotrophic metabolism despite the realization that mixotrophy is important ecologically. Laboratory experiments were conducted to examine changes in growth rates and physiological states of the toxic haptophyte Prymnesium parvum when fed Rhodomonas sauna of varying nutritional status. Haemolytic activity of P. parvum and prey mortality of R. sauna were also measured. P. parvum cultures grown to be comparatively low in nitrogen (low-N), phosphorus (low-P) or low in both nutrients (low-NP) were mixed with low-NP, low-N, and low-P R. saline in all possible combinations, i.e., a 3 x 3 factorial design. N deficiency was obtained in the low-N cultures, while true P deficiency may not have been obtained in the low-P cultures. Mortality rates of R. salina (both due to ingestion and/or cell rupture as a function of grazing or toxic effects) were higher when R. sauna cells were low-P, N-rich, regardless of the nutritional state of P. parvum. Mortality rates were, however, directly related to the initial prey:predator cell ratios. On the other hand, growth of the predator was a function of nutritional status and a significant positive correlation was observed between growth rates of P. parvum and cell-specific depletion rates of N, whereas no such relationship was found between P. parvum growth rates and depletion rates of P. In addition, the greatest changes in chlorophyll content and stoichiometric ratios of P. parvum were observed in high N:P conditions. Therefore, P. parvum may show enhanced success under conditions of higher inorganic N:P, which are likely favored in the future due to increases in eutrophication and altered nutrient stoichiometry driven by anthropogenic nutrient loads that are increasingly enriched in N relative to P. (C) 2016 Elsevier B.V. All rights reserved.

The toxic haptophyte Prymnesium parvum regularly forms fish-killing blooms in inland brackish water bodies in the south-central USA. Along the Texas coast smaller blooms have occurred in isolated areas. There appears to be an increasing risk that harmful P. parvum blooms will propagate into open coastal waters with implementation of future water plans. These plans will include increased interbasin water transfers from the Brazos River, regularly impacted by P. parvum blooms, to the San Jacinto-Brazos Coastal Basin, which ultimately flows into Galveston Bay (GB). Persisting source populations of P. parvum in inland waters elevates this risk. Thus, there is a need for an increased understanding of how P. parvum might perform in coastal waters, such as those found in GB. Here, two in-field experiments were conducted to investigate the influence of various plankton size-fractions of GB water on inoculated P. parvum during fall and winter, periods when blooms are typically initiating and developing inland. Stationary- and log-growth phase P. parvum were used to represent high and low toxicity initial conditions. Results revealed that P. parvum could grow in GB waters and cause acute mortality to silverside minnows (Menidia beryllina). Depending on season and growth phase, however, P. parvum growth and toxicity varied in different size fractions. During the fall, P. parvum inoculated from stationary-, but not log-growth phase culture, was negatively affected by bacteria-sized particles. During the winter, bacteria and nanoplankton together had a negative effect on P. parvum inoculated from stationary- and, to a lesser degree, log-growth phase cultures. Intermediate- and large-sized grazers when combined with bacteria and nanoplankton had complex relationships with inoculated P. parvum, sometimes stimulating and sometimes suppressing population growth. Toxicity to fish occurred in almost all plankton size fractions. The inclusion of progressively larger sized plankton fractions resulted in trends of decreased toxicity in treatments inoculated with stationary-, but not log-growth phase P. parvum in the fall. In the winter, however, inclusion of larger sized plankton fractions resulted in trends of increased toxicity to fish in treatments inoculated with both stationary- and log-growth phase P. parvum. This study indicates that understanding P. parvum population dynamics in open waters of estuaries and bays will be challenging, as there appears to be complex relationships with naturally occurring components of the plankton. The observations that P. parvum is able to grow to high population density and produce fish-killing levels of toxins underscores the need for advanced risk assessment studies, especially in light of water use plans that will result in P. parvum invasions of greater size. (C) 2015 Elsevier B.V. All rights reserved.

Prymnesium parvum is a harmful algal bloom species present in many inland water bodies of the southcentral USA, but does not form fish-killing blooms in all of them. The present study tested the hypothesis that rotifer grazing of P. parvum might influence the incidence of blooms. Three-day in-lake experiments, which focused on the size fraction of zooplankton dominated by rotifers and natural phytoplankton assemblages inoculated with P. parvum, were conducted during the time of bloom development in 2 reservoirs of the southcentral USA: Lakes Somerville and Whitney, where the latter experiences P. parvum blooms and the former does not. Toxicity at a level lethal to fish was only occasionally observed during these experiments, so our experimental treatments are considered to be at a low-toxicity level. As a whole, rotifers in Lakes Somerville and Whitney selectively grazed P. parvum. Rotifers in Lake Somerville appeared to benefit from this selective grazing, while rotifers in Lake Whitney did not. The differences between rotifer communities from these lakes might be because rotifers from Lake Somerville historically have only been exposed to low levels of toxins produced by P. parvum and were able to develop resistance to these toxins, thus enabling them to persist and perhaps contribute to the suppression of blooms there. The opportunity for this type of microevolutionary adaptation may not occur in lakes where P. parvum blooms and waters reach high toxicity levels, such as those which have occurred historically in Lake Whitney.

The effect of various nitrogen (N) sources, including riverine dissolved organic matter (DOM), nitrate, ammonium, and urea, on the growth and physiology of the dinoflagellate Prorocentrum minimum was compared in a batch culture experiment. P. minimum grew equally well in the presence of identical amounts of nitrate, ammonium, and urea. Approximately 18 to 20% of organic N bound to the DOM was bioavailable. Although the available N added in the DOM treatment was only 1/3 of the amount of any other N sources, the cell densities of P. minimum in the DOM treatment increased to 61 ~ 65% of those in the nitrate, ammonium or urea treatment. The maximum specific growth rates did not differ significantly between the treatments with the highest in the ammonium treatment (0.55 ± 0.13 d^- 1) and the lowest in the urea treatment (0.39 ± 0.04 d^- 1). P. minimum assimilated the available DOM-bound N in a short period (fewer than 5 days), which was faster than utilizing urea. The increase in the cellular N:P ratios of P. minimum showed the alleviation of N stress in all the treatments after the addition of various N forms. The densities and cellular compositions of P. minimum stabilizing in all the treatments for the whole stationary phase indicated that P. minimum has adaptive physiology under sub-optimal conditions and is a competitive bloom species. We suggest that P. minimum cells utilize DOM-bound N for their growth, and the efficiency in utilizing the available DOM-bound N for growth is comparable to when P. minimum utilizes nitrate, ammonium or urea.

We investigated the effects of aeration on growth and toxicity of the haptophyte Prymnesium parvum in the presence and absence of the algal prey Rhodomonas salina. Batch monocultures of P-limited P. parvum and N and P sufficient R. salina and mixed cultures of the two microalgae were grown with no, low (20) and high (100) ml min¹ aeration for 18 days. Cell growth of P. parvum and R. salina and cell toxicity of P.parvum were studied over the experimental period. The highest specific growth rates of P. parvum were found at low aeration rates. R. salina in monocultures showed typical growth patterns, while R. salina numbers declined rapidly in the mixed cultures. Of the initial cell densities, 98–100% of the R. salina cells were lysed or ingested within 24 h of mixing with P. parvum cells. The maxima P. parvum biomasses were significantly higher in the mixed cultures than in the monocultures. Cell toxicity of P. parvum increased significantly in response to aeration rates and the highest levels were found in the high aeration condition. Availability of prey and resupply of inorganic nutrients decreased P. parvum cell toxicity. Our study suggests that P. parvum is tolerant and is able to grow over a broad range of aeration and associated turbulence effects though low aeration represents an optimal condition for growth. As P. parvum toxicity was higher in the high aeration treatment we suggest that the higher concentrations of oxygen cause more toxins to be produced, as these are oxygen rich compounds. We suggest that oxygenation and turbulence of surface waters caused by mixing may be involved in promoting high toxic P. parvum blooms in shallow lakes and coastal waters.

Nodularia spumigena was exposed directly and indirectly (grazer cages) to increasing densities of Acartiacf. bifilosa to investigate if the presence of copepods influenced the morphology and/or the toxicity of thecyanobacterium. Monocultures with only N. spumigena and mixed cultures, containing N. spumigena andthe non-toxic Dunaliella tertiolecta, were included in each experiment. Following 6 days of incubation,the morphology and toxicity in grazer treatments were compared with grazer-free controls. Weobserved no effects of A. cf. bifilosa on either morphology or toxicity of N. spumigena. The lack of grazerinduced nodularin production and morphological alterations suggest that these two potential defensestrategies either has evolved as constitutive defenses or never evolved as grazer defenses. The mortalityof copepods was higher in the monoculture than in the mixed culture treatments. Gut contentobservations indicated a low level of grazing in monoculture treatments and a higher level of grazing inmixed culture treatments. This higher level of grazing most likely occurred on the alternative food D.tertiolecta. Given the indications of low grazing and the concentrations of dissolved nodularin observed,we postulate that the higher mortality was not related to toxic effects, but to starvation. This in turn mayhave resulted from bad taste, production of unknown grazer deterrents or morphological constraints;although the size of the filaments would not have imposed an absolute limit for ingestion by A. cf. bifilosa.The higher copepod mortality observed on monocultures of N. spumigena may contribute to the successand maintenance of N. spumigena blooms.

We investigated whether or not the presence of copepods and different light conditions induced cyst ­formation in dinoflagellates. Scrippsiella trochoidea was exposed to Acartia tonsa directly and indirectly (grazer filtrate), in high light and low light conditions. The ingestion, faecal ­production and egg production of A. tonsa were compared between diets of S. trochoidea vegetative cells and temporary cysts. We found no effect of direct or indirect exposure to A. tonsa on S. ­trochoidea cyst formation in either high light or low light conditions. Controls and A. tonsa treatments kept in light displayed around 20% temporary cysts, whereas controls and A. tonsa treatments in low light were shown to have 50 to 80% temporary cysts. Thus, low light conditions had a strong effect on ­temporary cyst formation. No hypnocysts were observed in any experiment, which is probably related to the longer incubation times needed for their observation. Feeding on diets dominated by temporary cysts compared to vegetative cells decreased ingestion by a factor of 2.7, while faecal and egg production decreased by a factor of 2.2 and 2.9, respectively, suggesting that induction of temporary cysts in response to A. tonsa could be a survival strategy. However, S. trochoidea does not ­possess any grazer-induced defence in terms of temporary cyst formation, as it did not produce ­temporary cysts when exposed to A. tonsa. Rather, induction of temporary cysts seems to be controlled by decreased light intensity, which is a favorable trait for this species when driven to water depths where light is scarce.

Many phytoplankton species have evolved a variety of different defenses to decrease losses from grazing; morphological features, changes in life-history/behavior, and production of toxins. These defenses may be associated with costs. Therefore, some phytoplankton only express the defense when needed, i.e. when grazers are present.The defense can be induced by direct contact with the grazer, or infochemicals released during grazing activities may function as reliable signals of grazer presence. Morphological defenses were studied in the colony forming prymnesiophyte Phaeocystis globosa, in combination with varying nutrient status, such as nitrogen(N) and phosphorus (P) sufficiency, N deficiency and P deficiency. NP sufficient P. globosa remained mainly as solitary cells in response to infochemicals. The responses were more complex in the nutrient deficient experiments, due to the increased mortality of copepods observed, which may have resulted from lower food quality in nutrient stressed cells. This could affect both grazers and the infochemicals released, which could have been to weak to affect P. globosa. Morphological defenses include formation of digestion resistant gelatinous sheaths, which were examined in the chlorophyte Oocystis submarina. Direct, not indirect, exposure to copepods and cladocerans caused a shift towards cells and colonies with gelatinous sheaths. Thus, infochemicals played no role in these responses. The cyanobacterium Nodularia spumigena has two potential defense mechanisms; morphology (filament size/structure), and toxicity. These defenses are not induced by the direct or indirect presence of copepod grazers. However, N. spumigena increased the mortality of copepods, which was probably related to starvation. This may contribute to the success of N. spumigena blooms, as there could be a shift ingrazing towards other phytoplankton species. The combined effects of direct/indirect copepod exposure and low light conditions on the dinoflagellate Scrippsiella trochoidea life-history (e.g. temporary cyst formation) were examined. Induction of temporary cysts occurred in response to decreased light intensity, but not in response to copepods despite the fact that copepods showed decreased ingestion on temporary cysts. In low light situations, temporary cyst formation can be an effective tool to minimize population losses.The results presented here contribute to the complex understanding of factors influencing phytoplankton-zooplankton interactions.

We examined the combined effects of grazer infochemicals and nutrient status on colony development ofPhaeocystis globosa cultures grown under nitrogen and phosphorus (NP)–sufficient, P-deficient, and N-deficientconditions exposed to high and low Acartia spp. density filtrates. Changes in colony development relative tocontrols receiving no grazer signals were estimated. P. globosa colony development responded to grazerinfochemicals regardless of nutrient status, although the expression of the response varied between nutrients.Significant colony suppression (in terms of percent of cells allocated to colonies) occurred in both NP-sufficientand P-deficient experiments, with the response being dependent on the density of grazers for NP-sufficient cells.The percent of cells in colonial form in N-deficient P. globosa decreased in response to low grazer density filtratesbut increased in response to high grazer density filtrates. These opposite results for the N-deficient experiment arerelated to a high mortality of Acartia in the high grazer density filtrate treatment, which may affect theinfochemicals released from such grazers