Theodore J. Smayda

Normal and accelerated sinking of phytoplankton in the sea

Abstract The sinking and deposition of phytoplankton remains lead to sediment varving, the formation of sedimentary oozes, and the transport of partially decomposed and viable, autotrophic cells to great depths. Some specific examples are discussed, and the associated settling rates, depositional characteristics and individual species responses are examined in relationship to experimentally determined sinking rates of both living and dead, intact phytoplankton. The data include field and experimental observations on the sinking behavior for the same species. The in vitro rates are too low, and a mechanism or mechanisms causing accelerated rates of phytoplankton sinking in situ seems necessary to account adequately for some of the observed transport of phytoplankton remains to great depths and to the sea floor. Various potential mechanisms of heightened diatom, dinoflagellate and coccolithophorid sinking rates are then outlined and examined quantitatively. These include density inversion plumes, downwelling, and fecal pellet sinking. Sinking rates of naturally occurring fecal pellets and those produced by cultured zooplankton are presented. The importance of fecal pellet settling as a means of accelerated sinking of phytoplankton remains to depth, and their subsequent contribution to the fossil record and organic content of ocean floor sediments are then discussed.

Turbulence, watermass stratification and harmful algal blooms: an alternative view and frontal zones as “pelagic seed banks”

Abstract Watermass stratification has been considered the essential physical condition that dinoflagellates require to bloom because of their relative inability, unlike diatoms, to tolerate the elevated shear-stress associated with water-column mixing, turbulence and high velocity, coastal currents. The swimming speeds of 71 flagellate taxa, with a focus on dinoflagellates, are compared to the turbulence fields and vertical velocities that develop during representative wind conditions, upwelling and at frontal zones. The results suggest that the classical stratification–dinoflagellate bloom paradigm needs revision. Tolerance of turbulence, growth within well-mixed watermasses and survival and dispersal while entrained within current systems are well developed capacities among dinoflagellates. Their secretion of mucous, often copious during blooms, is suggested to be an environmental engineering strategy to dampen turbulence. Biophysical tolerance of turbulence by dinoflagellates is often accompanied by high swimming speeds. Motility speeds of many species exceed in situ vertical current velocities; this also allows diel migrational patterns and other motility-based behavior to persist. Species belonging to “mixing-drift” life-form assemblages can increase their swimming speeds through chain formation, which helps to compensate for the increased turbulence and vertical water-column velocities of their habitats. The ability of dinoflagellate species to tolerate the vertical velocities of offshore, frontal zones, where abundant populations often develop, suggests that fronts may serve as “pelagic seed banks”, occurring as pelagic analogues of nearshore seed beds, from which seed stock is dispersed. The different ecologies associated with the hypothesized, “pelagic seed banks” of vegetative cells and the “seed beds” of resting stage cells deposited onto sediments are discussed. There is a contradiction in the stratification–HAB paradigm: the quiescent conditions of a stratified watermass, with its characteristic nutrient-poor conditions are expected to promote stasis of the population, rather than growth and blooms. The analyses suggest that dinoflagellate blooms do not preponderate in stratified watermasses because the bloom species are biophysically intolerant of the higher velocities and turbulence of more mixed watermasses. The watermass stratification that often accompanies flagellate blooms is probably a secondary, parallel event and less essential than some other factor(s) in triggering the observed bloom.

Strategies of marine dinoflagellate survival and some rules of assembly

Dinoflagellate ecology is based on multiple adaptive strategies and species having diverse habitat preferences. Nine types of mixing-irradiance-nutrient habitats selecting for specific marine dinoflagellate life-form types are recognised, with five rules of assembly proposed to govern bloom-species selection and community organisation within these habitats. Assembly is moulded around an abiotic template of light energy, nutrient supply and physical mixing in permutative combinations. Species selected will have one of three basic (C-, S-, R-) strategies: colonist species (C-) which predominate in chemically disturbed habitats; nutrient stress tolerant species (S-), and species (R-) tolerant of shear/stress forces in physically disturbed water masses. This organisational plan of three major habitat variables and three major adaptive strategies is termed the 3-3 plan. The bloom behaviour and habitat specialisation of dinoflagellates and diatoms are compared. Dinoflagellates behave as annual species, bloom soloists, are ecophysiologically diverse, and habitat specialists whose blooms tend to be monospecific. Diatoms behave as perennial species, guild members, are habitat cosmopolites, have a relatively uniform bloom strategy based on species-rich pools and exhibit limited habitat specialisation. Dinoflagellate bloom-species selection follows a taxonomic hierarchical pathway which progresses from phylogenetic to generic to species selection, and in that sequence. Each hierarchical taxonomic level has its own adaptive requirements subject to rules of assembly. Dinoflagellates would appear to be well suited to exploit marine habitats and to be competitive with other phylogenetic groups, yet fail to do so.

Adaptive Ecology, Growth Strategies and the Global Bloom Expansion of Dinoflagellates

Dinoflagellates exhibit unique differences from diatoms in their adaptive ecologies that may be favoring their increasingly successful exploitation of coastal waters and global bloom expansion. Dinoflagellates behave as annual species, bloom soloists, are ecophysiologically diverse and habitat specialists, whereas diatoms behave as perennial species, guild members and are habitat cosmopolites. Diatoms have a relatively uniform bloom strategy based on species-rich pools and exhibit limited habitat specialization. Dinoflagellates have multiple life-form strategies consistent with their diverse habitat specializations, but rely on impoverished bloom species pools. Niche structure and dinoflagellate competition for niche space are considered. The “open niche period” formulated originally for Narragansett Bay is extrapolated as a general bloom paradigm. It is suggested that successful niche occupancy leading to blooms involves adaptive strategies at three heirarchic taxonomic elements: phylogenetic, generic and species-specific, and in that sequence. Transoceanic expatriation of emigrant species leading to indigenous status and blooms requires completion of a three-stage colonization process. Anthropogenic seedings are not, in themselves, bloom stimulation events; they are only the first phase of a multiple-step process. The organismal and niche features required for a hidden flora member to become a bloom species are considered, and the interplay between niche structure, habitat carrying capacity, colonization requirements and stochasticity as factors in the changing global bloom behavior of dinoflagellates discussed. The question is posed whether traditional perspectives of phytoplankton behavior apply completely to dinoflagellates.

EXPERIMENTAL OBSERVATIONS ON THE INFLUENCE OF TEMPERATURE, LIGHT, AND SALINITY ON CELL DIVISION OF THE MARINE DIATOM, DETONULA CONFERVACEA (CLEVE) GRAN

The influence of 116 combinations of temperature (2, 7, 12, 16 C), salinity (5–35‰ at 5‰ intervals) and light (5 levels) on the mean daily cell division rate (K) of the Narragansett Bay clone of Detonula confervacea was examined following appropriate preconditioning. Growth did not occur at 16 C, but was excellent (K = 1.2–1.5) under certain combinations of light and salinity at 2, 7, and 12 C, being somewhat better at the 2 highest temperature levels. At 32%, and 1100–1200 ft‐c, K increased approximately 2.5 fold from 0.6 to 1.5 between 2 and 12 C.

Status, trends and the future of the marine pelagic ecosystem

SUMMARY Globally, humans impact environments and ecosystems faster than they become aware of their effects. The marine pelagic ecosystem includes a tremendously large and diverse environment, which might accordingly be considered to be resilient to externally forced changes, whether from humans or climate. This review considers that general hypothesis by pursuing two objectives. The first is to document the current status of and recent anthropogenic impacts on the marine pelagic ecosystem, with emphasis on the epipelagic zone (0‐200 m) where organisms are concentrated and human impacts have been greatest. It shows that humans have proven capable of assuming the role of top carnivore in pelagic ecosystems where living resources are attractive and financially amenable to exploitation, and that overexploitation is the rule under such circumstances. Other anthropogenic activities associated with changes in various marine pelagic ecosystems, such as increased diseases, mortalities, extinctions, habitat invasions, and species replacements, function as sentinels and indicate that portions of the pelagic ecosystem are under considerable stress. It is argued that, without attention, these problems can be expected to worsen up to the year 2025 and beyond. In addition to a comprehensive evaluation of status and trends relating to conservation of the marine pelagic ecosystem, a second major objective is to evaluate whether current paradigms of ecosystem function are sufficient to improve the ability of the scientific community to predict future changes and to recommend relevant management strategies. This review differs from previous ones by proposing that current conceptual models have failed to provide the basis for accurately predicting patterns and features of pelagic communities, notably why specific organisms occur where and when they do. It is argued that predation pressure is shaped by natural selection in the sea as on land, and that it influences organism behaviour, life history strategy and morphology, all of which determine marine pelagic ecosystem structure, and therefore should be used to interpret function. From this perspective, attempting to understand present patterns and predict the future of marine pelagic ecosystems, without understanding the intertwined roles of evolution and predation in forging contemporary pelagic communities, is a hopeless endeavour. It is proposed that both perspectives, resource availability and predation pressure, be incorporated into a revised paradigm of pelagic ecosystem structure and function, a necessity if policies are to predict anthropogenic impacts and environmental conservation is to be effective.

NUTRIENT-PHYTOPLANKTON RELATIONSHIPS IN NARRAGANSETT BAY DURING THE 1974 SUMMER BLOOM

ABSTRACT Nutrient and 14C uptake by natural phytoplankton populations was measured. Concentrated populations were incubated with uptake saturating concentrations of nitrate, silicate and phosphate, and nutrient uptake was then monitored at 30 min intervals over an eight hour period to yield Vmax, V, ρN and ρsi. Changes in the amount of particulate matter and absolute rates of uptake were related to in situ levels of phytoplankton biomass and dissolved nutrients, and replenishment (generation) times (R) estimated. Independent estimates of R were derived from calculations of particulate nitrogen (Np) and silica production (Sip) based on 14C uptake and appropriate elemental ratios. Forty-two percent of the annual carbon production of 308 g m−2 occurred during July and August when in situ nutrient levels were very low. The hourly uptake rates for nitrate (ρN) ranged from 0.118 to 0.136; ρsi was 0.007 to 0.150 μM. The daily N supply for the water column would have to be replenished one to 12 times daily to support Np and 2.5 to 17 days for Sip. It would take from 2 to 5 days for the nitrogen excretion rates of the zooplankton (> 153 μM.) and benthos to supply the daily phytoplankton nitrogen needs.

AUTECOLOGICAL STUDIES ON THE MARINE DIATOM RHIZOSOLENIA FRAGILISSIMA BERGON. I. THE INFLUENCE OF LIGHT, TEMPERATURE, AND SALINITY1

The influence of 113 combinations of temperature (9, 12, 18, 25, 30 C), salinity (5–35 ‰ at 5 ‰ intervals), and light (4 levels) on the mean daily cell division rate (K) of the Narragansett Bay clone of Rhizosolenia fragilissima was examined following appropriate preconditioning. Growth did not occur below 9 C, but was excellent (K =∼1.2) under certain combinations of light and salinity at 12, 18, and 25 C. The optimal salinity of 20–25 ‰ was temperature independent. Growth was not measurable at 5 ‰, although survival occurred. At 20 ‰ and 1200 ft‐c, K increased approximately 1.8‐fold from 0.65 to ∼1.2 between 9 and 18–25 C.

Diel changes in nitrite concentration in the chlorophyll maximum in the Gulf of Mexico

Abstract A diel study of the deep chlorophyll maximum and its associated nitrite maximum showed that the nitrite maximum strengthens during the day and diminishes at night. A one-dimensional vertical model was used to differentiate physical from biological processes affecting the nitrite distribution. The rates of net production (or uptake) of nitrite, nitrate, ammonia, phosphate, and silicate were estimated at local dawn, noon, dusk, and midnight. The nitrite maximum at the base of the euphotic zone is produced mainly by phytoplankton during the day by reduction of nitrate. Daytime release of nitrite by phytoplankton occurs at a rate higher than the capacity of nitrifying microorganisms to oxidize it, and this leads to the nitrite maximum, which diminishes at night by nitrification. Nitrification rates were 0.2 to 0.5 μg-at. N1 −1 h −1 . The rates are one to three orders of magnitude higher than those based on the long-term (24-h) incubations of samples usually used to estimate nitrification rates.

Phased cell division in natural populations of the marine diatom Ditylum brightwelli and the potential significance of diel phytoplankton behavior in the sea

Abstract Diel phased cell division characterized natural populations of Ditylum brightwelli, and possibly Biddulphia mobiliensis, during early upwelling off the coast of Baja California. The general occurrence and types of diel phenomena characterizing in situ phytoplankton populations and their environment are sketched, and the potential significance of this commonplace response the ‘plankton paradox’, to the sampling of and experimentation with natural populations, and to modeling efforts, are briefly considered.