Diel Light Cycle Research Articles

Transcript patterns of chloroplast-encoded genes in cultured Symbiodinium spp. (Dinophyceae): testing the influence of a light shift and diel periodicity.

Microalgae possess numerous cellular mechanisms specifically employed for acclimating the photosynthetic pathways to changes in the physical environment. Despite the importance of coral-dinoflagellate symbioses, little focus has been given as to how the symbiotic algae (Symbiodinium spp.) regulate the expression of their photosynthetic genes. This study used real-time PCR to investigate the transcript abundance of the plastid-encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein in photosystem I), within the cultured Symbiodinium ITS-2 (internal transcribed spacer region) types A20 and A13. Transcript abundance was monitored during a low to high-light shift, as well as over a full diel light cycle. In addition, psaA was characterized in three isolates (A20, A13, and D4-5) and noted as another example of a dinoflagellate plastid gene encoded on a minicircle. In general, the overall incongruence of transcript patterns for both psbA and psaA between the Symbiodinium isolates and other models of transcriptionally controlled chloroplast gene expression (e.g., Pisum sativum [pea], Sinapis alba [mustard seedling], and Synechocystis sp. PCC 6803 [cyanobacteria]) suggests that Symbiodinium is reliant on posttranscriptional mechanisms for homeostatic regulation of its photosynthetic proteins.

A mass-balanced pelagic ecosystem model with size-structured behaviourally adaptive zooplankton and fish

Mesozooplankton is a highly diverse group of organisms ranging from very small to large herbivorous, omnivorous or predatory forms. However, most aquatic ecosystem models typically contain only one or two state variables which represent all mesozooplankton forms and specify their role in marine food webs. We have extended an existing mass-balanced marine ecosystem model to include a wide range of mesozooplankton size-classes and species growing from small to large, and maturing at different size. The model includes a dynamic pool of fish with a fixed mortality rate as a closure term of the model, and mechanistic expressions for fish predation. The zooplankton consumes phytoplankton and smaller zooplankton, and responds adaptively to the instantaneous local rates of growth and predation by migrating towards more profitable habitats. We run the model for long time in a stable and repetitive diel light cycle, and explore the emerging ecosystem structure and complexity. In the stable environment the presence of fish has strong structuring effects over the size-structure of mesozooplankton, but little influence on phytoplankton because the total biomass of mesozooplankton remains relatively stable over the fish cycles. The inclusion of adaptive and flexible behaviour leads to emergent effects of multiple predators; the removal of intra-guild predation among zooplankton result in low fish abundance because zooplankton spend more time in deeper habitats. The model reveals persistent spatial and cascading behavioural interactions and is a step towards a mechanistic and adaptive representation of the upper trophic levels in ecosystem models.

Seasonal and diel vertical migration of zooplankton in the High Arctic during the autumn midnight sun of 2008

The diel vertical migration (DVM) of Calanus (Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus) and Metridia longa was investigated in August 2008 at six locations to the north and northwest of Svalbard (Rijpfjorden, Ice, Marginal Ice Zone, Shelf break, Shelf and Kongsfjorden). Despite midnight sun conditions, a diel light cycle was clearly observed at all stations. We collected data on zooplankton vertical distribution using a Multi Plankton Sampler (200-μm mesh size) and an EK60 echosounder system (38, 120 and 200 kHz). These were supplemented by environmental data collected using a standard conductivity, temperature and depth (CTD) profiler. The sea ice had recently opened in Rijpfjorden, Ice and Shelf stations, and these stations exhibited phytoplankton bloom conditions with pronounced fluorescence maxima at approximately 30 m. In contrast, Kongsfjorden was more representative of autumn conditions, with the Arctic bloom having culminated 2–3 months prior to sampling. All three Calanus species were found shallower than 50 m on average at Rijpfjorden and the Ice station, while C. glacialis and C. hyperboreus were found deeper than 200 m on average at Kongsfjorden. Shallow water DVM behaviour (<50 m) was observed at Rijpfjorden and the Shelf station, especially among the C. finmarchicus CI-CIII population, which was particularly abundant at the Shelf (>5,000 individuals/m3). A bimodal depth distribution was observed among C. finmarchicus at the Shelf break station, with CI-CIII copepodites dominating at depths shallower than 100 m and CIV-adult stages dominating at depths exceeding 600 m. Statistical analyses revealed significant differences between the day and night 200-kHz data, particularly at specified depth strata (25–50 m) where backscatter intensity was higher during the day, especially in Rijpfjorden and at the Ice station. We conclude that DVM signals exist in the Arctic during late summer/autumn, when a need to feed and an abundant food source exists, and these signals are primarily due to mesozooplankton.

Modeling profitability for the smallest marine endotherms: auklets foraging within pelagic prey patches

Auklets (Alcidae) can be very abundant in north-temperate to arctic seas. Their numbers and trophic impacts in a given area depend on their ability to forage profitably as a function of the dis- persion, depth, and density of prey patches. Thus, modeling these relationships is important when pre- dicting the auklets' response to environmental change. This paper presents a simulation model of the foraging costs and intake rates of Cassin's auklets Ptychoramphus aleuticus (~170 g) and least auklets Aethia pusilla (~80 g) once they have located a patch of zooplankton prey. In the model, water temper- ature and dive depth (max. 20 m) have important effects on dive costs, mainly by affecting the duration and magnitude of costs during passive ascent. Within a prey patch, modeled intake rates are limited at relatively low prey densities by pursuit and handling time after a prey item is detected. Because intake rate is limited by capture time and not prey visibility, the model indicates that changes in light condi- tions over these depths have little direct effect on intake rates of zooplankton prey. However, vertical migration of prey in response to diel light cycles can strongly affect profitability (energy gain minus cost) by altering the depth of dives to prey patches. Because pursuit and handling time limit modeled intake rate, profitability cannot be increased further by finding patches of higher density, but rather by extending time in patches by swimming farther or slower. The model suggests that auklet dispersion should be insensitive to variations in patch density above a threshold that is relatively low compared to the very high densities that can occur. However, auklets may be attracted to higher-density patches because the patches themselves are more visible or predictable, or because other predators—from seabirds to whales—may gather in such patches and increase their visibility.

Peptide hydrolysis and the uptake of dipeptides by phytoplankton

Rates of peptide hydrolysis (using the fluorescent substrate, lucifer yellow anhydride‐labeled tetra‐alanine) and dipeptide uptake (using dually labeled, 15N and 13C, dialanine) were measured in phytoplankton cultures and in natural populations during algal blooms dominated by one or two taxa. During most sampling events, both peptide hydrolysis and dipeptide uptake were greatest in the size fraction containing the dominant phytoplankter, suggesting that phytoplankton contribute substantially to or may even dominate observed extracellular peptide hydrolysis and dipeptide uptake in the environment. These are the first data suggesting that dipeptides may be taken up directly by phytoplankton and this may represent a previously unaccounted‐for nitrogen source in aquatic systems. Like many other processes in phytoplankton, peptide hydrolysis appears sensitive to the diel light cycle and the nutrient environment, with rates varying depending on the dominant N source, but with no clear pattern. Uptake of dialanine, the dominant product of the hydrolysis of the peptide tetra‐alanine, also varied depending on the dominant taxa and the nutrient regime. Most of the time, it appeared that low production of dialanine by tetra‐alanine hydrolysis limited the uptake of the dipeptide. Close coupling between peptide hydrolysis and dipeptide uptake may also help explain the absence of correlations between rates of peptide hydrolysis and the concentration and composition of the free amino acid pool.

Embryonic motility and hatching success of Ambystoma maculatum are influenced by a symbiotic alga

To augment O2supply through the jelly mass and egg capsule, embryonic yellow-spotted salamanders ( Ambystoma maculatum (Shaw, 1802)) take advantage of a unicellular alga, Oophila ambystomatis . Convective currents from surface cilia, however, may also enhance O2transport, whereas muscular contractions could either enhance delivery or contribute to O2consumption. Embryonic motion is, therefore, potentially vital to salamander development. We examined embryonic motility across multiple developmental stages, survivorship, and hatching timing in response to different algal levels by rearing salamander egg masses under three different diel light cycles: 24 h dark, 12 h light, and 24 h light per day. Embryos raised in continuous light hatched synchronously and at slightly earlier developmental stages than embryos raised in the dark or in 12 h light per day. We removed eggs at multiple stages to examine embryonic rotation and muscular contraction rates under 180 min periods of both light and dark. Rotational movements occurred more frequently in alga-free than in algae-inhabited eggs, and more frequently in algae-inhabited eggs in the dark than in light. At later developmental stages, muscular contractions were more frequent in embryos from algae-inhabited egg masses in light than those in the dark; thus embryos with less O2reduced muscular activity, thereby reducing energy consumption when O2availability was compromised.

Formation of very young colonies by Phaeocystis pouchetii from western Norway

The marine phytoplankton species Phaeocystis pouchetii often forms large blooms of gelatinous colonies during spring in north temperate, boreal, and Arctic waters. The macroscopic colonies in such blooms develop originally from free-swimming solitary cells that shed their flagella, secrete a gelatinous membrane, and undergo repeated cell divisions within an enlarging colony membrane. However, the essential first steps in this process have not been described in situ, and hence the factors controlling initiation of colony blooms are poorly understood. Studies were con- ducted in mesocosms in a western Norway fjord to document the development of small young colonies under simulated and ambient conditions, resulting in 2 novel perspectives: (1) The critical early stages of colony bloom formation may occur in a relatively brief window in time. The rate of for- mation of young colonies from solitary cells was faster in nutrient-fertilized compared to unfertilized mesocosms, but this only happened after a period of diatom growth in the mesocosms and subse- quent increases in P. pouchetii colony abundance. Then, small percentages of solitary cells were mathematically necessary to account for observed colony numbers. (2) Initial formation of new young colonies occurred during the dark period of the ambient springtime diel light cycle, and patterns of colony accumulation implied rapid multiple cell divisions within colonies during the dark period, a process termed phased ultradian growth. Such an evolutionary strategy of rapid change from single cells to multi-cell colonies would be advantageous as a means of minimizing grazing losses during the critical period of size transition from microscopic free-swimming cells that are susceptible to microzooplankton and viruses, to larger colonies that are susceptible to metazoan zooplankton but relatively impervious to viruses and microzooplankton. The combined results offer a new insight into colony proliferation: colony blooms do not necessarily require a long period of constant conversion of solitary cells into colonies, but rather may be the product of punctuated, rapid life cycle transforma- tions and ultradian growth of young colonies.

EXAMINATION OF DIEL CHANGES IN GLOBAL TRANSCRIPT ACCUMULATION IN SYNECHOCYSTIS (CYANOBACTERIA)1

Phytoplankton in nature must acclimate to a wide range of light conditions resulting from diel light cycles, ocean circulation and mixing, cloud cover, and the variable bio‐optical characteristics of the water column. In this study, we used whole‐genome cDNA microarrays to investigate the effects of a gradually fluctuating daily light cycle on gene expression in the cyanobacterium Synechocystis sp. strain PCC6803. From these data, we developed a conceptual framework depicting the diel regulation of metabolic pathways in the cell. The framework is focused on potential photoacclimation responses, including the regulation of the photosystems, cell division, and DNA replication. The mRNA abundance of genes involved in many metabolic pathways, and particularly those encoding proteins that function in photosynthesis and DNA replication, changed markedly over the course of the day. The levels of mRNA encoding polypeptides important for the formation of the light‐harvesting apparatus, photosystems I and II, and cell division were found in high concentrations during the day. The transcript levels of many genes encoding enzymes involved in anabolic processes also increased considerably during the day. In contrast, transposon transcripts and mRNAs encoding proteins involved in DNA replication, cell wall synthesis, and respiratory activity were not found in high concentrations during the day. Although gradually varying light exposure induced significant changes in transcript accumulation within Synechocystis, the direction of these changes differed between our study and previous studies in which there was an abrupt transition between irradiances.

Diel vertical migration of the marine copepod Calanopia americana. I. Twilight DVM and its relationship to the diel light cycle

Marine copepods commonly exhibit vertical movements in the water column over the diel cycle, termed diel vertical migration (DVM), with the most common pattern being an ascent in the water column to minimum depth around sunset and descent to maximum depth around sunrise. The present study characterized the DVM pattern of the pontellid copepod Calanopia americana Dahl in the Newport River estuary (North Carolina, USA, in July 2003). The estuary is shallow and well-mixed, and the study site (34°43′N; 76°40′W), 1.5 km inside the estuary entrance, is unusual in lying within a gyre where tidal currents are always in the seaward direction. Changes in C. americana vertical abundance were related to spectrally relevant changes in light throughout the diel cycle. Simultaneous measurements of light and zooplankton abundance near the surface (0.5 m depth) and near the bottom (0.5 m above bottom) were made over one 4-h period and two 3-day periods during different phases of the tide. These observations suggest that C. americana undertook twilight DVM in the Newport River estuary; an ascent to the surface occurred at sunset, followed by a descent to near the bottom around midnight, with a second ascent to the surface and then descent to near bottom at sunrise. DVM in C. americana was independent of the tidal cycle, with the initial ascent in the water column at sunset possibly associated with relative rates of irradiance change. Copepod vertical movements were consistent with a night-active endogenous rhythm, and appeared independent of the abundance of predatory chaetognaths, Sagitta spp. In DVM studies with migrators like C. americana that are broadly sensitive to visible wavelengths of light, measuring photosynthetically active radiation may be a reasonable alternative to measuring light in a spectrally relevant photometric unit.

Low-temperature incubation of walleye pollock (Theragra chalcogramma) eggs from the southeast Bering Sea shelf and Shelikof Strait, Gulf of Alaska

To determine the effect of low water temperature on development, walleye pollock (Theragra chalcogramma) eggs from the Bering Sea were reared at � 0.61C, 0.41C, 2.01C, and 3.81C. One group of eggs was reared at 3.91C under a diel light cycle (14 h light, 10 h dark) to observe the effect of light on development and hatching. Development was normal for all temperatures except � 0.61C; abnormal development of the tail and lack of development of eyes occurred in some embryos. Time to 50% hatch was 820, 620, and 424 h at 0.41C, 2.01C, and 3.81C. Eggs incubated in diel light at 3.91C developed at the same rate as eggs incubated in constant dark at 3.81C, but required an additional 72 h to reach 50% hatch. A piece-wise regression model was generated to predict egg age for incubation temperatures of � 0.61C to 3.81C. For temperatures recorded in the southeastern Bering Sea 1995–1998, the model predicted incubation periods for walleye pollock eggs that varied by 13 days between the warmest and coldest years. Walleye pollock eggs from Shelikof Strait, Alaska, were incubated at 0.21C, 1.81C, and 2.81C. Development was normal for all temperatures. A piece-wise regression model (as above) was generated for incubation temperatures 0.2–2.81C. When the regression models were compared, Bering Sea eggs (1.4–1.7 mm in diameter), required more time for development prior to hatch than Shelikof Strait eggs (1.2–1.3 mm in diameter) at 1.81C and 2.81C. However, for temperatures 0.2–2.01C, Bering Sea walleye pollock began hatching earlier and at a developmentally younger age than Shelikof Strait walleye pollock. Published by Elsevier Science Ltd.

Effect of the diel light cycle on the diving behaviour of two bottom feeding marine birds:the blue-eyed shag Phalacrocorax atricepsand the European shag P. aristotelis

Consistent diel patterns in foraging depths have been recorded in several avian predators which feed on pelagic prey, but nothing is known about such effects in bottom feeding seabirds. In the underwater environment, illumination is influenced primarily by water depth. Thus a predator's ability to exploit feeding areas located in deep water could, theoretically, be constrained by light availability. We investigated the effects of the diel light cycle on the diving behaviour of 2 species of bottom feeding seabird: the blue-eyed shag Phalacrocorax atriceps and the European shag P. aristotelis. Time-based data loggers were attached to 22 blue-eyed shags and 21 European shags and used to obtain detailed and concurrent records of foraging conditions in terms of depth and illumination during the chick-rearing period. Both species were exclusively diurnal feeders. There were marked differences in mean foraging depth and foraging illumination between individual birds (blue-eyed shag: ranges 3.2 to 73.3 m and -0.3 to 2.0 log10 lx; European shag: 8.5 to 34.6 m and 0.9 to 2.0 log10 lx). Superimposed on this variation was a consistent effect whereby individuals significantly modified their diving behaviour in response to the diel light cycle, in accord with the prediction that foraging depths would be maximised when ambient illumination was highest. However, it appeared that individuals did not fully exploit the potential of this diel effect since, although foraging depths in the middle of the day were generally deeper, the associated illumination was higher than that experienced during shallower dives made earlier or later in the day. Nevertheless, we believe that diel effects can play an important role in shaping the foraging behaviour of bottom feeding, avian predators.

Feeding behaviour of cod,Gadus morhua: activity rhythm and chemically mediated food search

I studied the feeding behaviour of cod in late spring in a fjord in northern Norway by means of a stationary positioning system. The activity rhythm of eight cod tagged with acoustic transmitters was observed continuously for 3–8 days in terms of swimming speed and size of area occupied. I studied responses to food odour by setting a line with mackerel baits in the experimental area. The cod had a diurnal rhythm of swimming activity with a slightly lower swimming speed during a 5-h period at night, indicating that they searched more actively for food during the day. A shift towards a larger proportion of active prey taken in the day, accompanied by a change in hunting strategy where vision became more important, may explain this rhythmicity in feeding behaviour. The diurnal rhythm in the range of area occupied varied more than the swimming activity rhythm and there was no significant difference in range between day and night indicating that factors other than the diel light cycle also influenced periodicity in activity. A higher proportion of fish encountered by the bait odour plume located baits than that of fish that were out of range of the odour plume, indicating the importance of chemically mediated food searching in this species. The cod detected a food odour source from a distance of several hundred metres, and responded immediately to the odour plume by carrying out a rheotactic search at a moderately higher swimming speed.

Patterns and occurrence of diel vertical migration of zooplankton biomass in the Mid-Atlantic Bight described by an acoustic Doppler current profiler

The seasonal occurrence, timing, and pattern of diel vertical migration of acoustically estimated zooplankton biomass at the edge of the continental shelf off the eastern United States is described based on the SEEP II (Shelf Edge Exchange Processes in the Southern Middle Atlantic Bight) data set obtained between February 1988 and May 1989. Zooplankton biomass was estimated using a bottom-moored, upward-looking 300 kHz acoustic Doppler current profiler deployed at the 90 m isobath. Vertical distributions of biomass were represented by the median depths of the acoustically estimated biomass which had been normalized relative to the maximum and minimum depths for each day and time standardized such that sunrise and sunset occurred at 0600 and 1800, respectively. Periods of diel vertical migration during the 15 month record were identified by determining daily least-squares fits of hourly vertical velocities to an ideal curve of migrating biomass velocities such that maximum upwards and downwards velocities of zooplankton biomass coincided with sunset and sunrise, respectively. Diel vertical migration occurred for 35, 15, and 20% of the days ( p<0.05) during each of the spring, summer, and fall/winter mooring deployments, respectively. The occurrence of diel vertical migration at the site appeared to be affected primarily by changes in the community composition of the zooplankton which was strongly influenced by the advection of different water types and associated zooplankton communities over the site. Little correlation was observed between the occurrence or magnitude of vertical migration and environmental parameters such as chlorophyll a concentration and temperature. Daily vertical migrations were timed to the diel light cycle.

Environmental factors influencing the zonation and activity patterns of a population of Periophthalmus sobrinus Eggert in a Kenyan mangrove

The activity rhythms and zonation of a population of Periophthalmus sobrinus were studied along the muddy mangrove banks of the Tana river delta (Kenya). A transect, perpendicular to the water channel, was subdivided into sections 1 m in width parallel to the river bank. The number of surface-active mudskippers and their zonation was recorded over several tidal cycles. Fishes found resting on their nests were also recorded. The observations were carried out over two periods of 48 consecutive tidal hours corresponding to a spring and a neap tide. Environmental variables such as tidal level, air temperature and relative humidity were also recorded. Periophthalmus sobrinus was active both diurnally and nocturnally and during spring tide all the observed mudskippers were active on the mud surface. At neap tide more than half of the total number of animals sighted were found resting on their nests. During this synodic phase, daytime sightings decreased drastically because the mudskippers took refuge inside the nests. Since P. sobrinus normally tends to avoid water, its mean hourly zonation at spring tide was more variable than at neap tide. Differences in mean hourly zonation were also found between night and day of the spring tide. Activity patterns and zonation of the mudskippers were directly influenced by the synodic and tidal cycles and depended more on environmental factors such as air temperature and relative humidity than on the diel light cycle.

The chlorophyll‐labeling method: Measuring specific rates of chlorophyll a synthesis in cultures and in the open ocean

We tested the assumptions underlying the Chl‐labeling method of measuring phytoplankton growth rates. The observed 14C‐labeling patterns of Chl a are consistent with the presence of three kinetically important pools, the macrocycle, the phytol sidechain, and at least one precursor pool. The turnover rates of these pools were determined from a kinetic analysis of their labeling patterns. Turnover rates of Chl a or its two subunits were not significantly different from zero. The turnover rate of the precursor of the Chl a macrocycle was a constant multiple of the specific rate of pigment synthesis (µChl). An equation was derived that predicts µChl a from the 14C labeling of Chl a. Rates of 14C incorporation into Chl a and rates of C fixation are severely unbalanced when algal cultures photoadapt. It is likely that µChl a as determined by the Chl‐labeling method equals µC, the specific rate of carbon synthesis, only when growth is balanced. We demonstrated that the growth of natural phytoplankton populations can be severely unbalanced when the algae grow under a natural photocycle or when the algae are subjected to shifts in light intensity; we also observed that Chl a was synthesized in the dark. Thus, with the Chl‐labeling method it is not possible to determine carbon‐based growth rates when algae photoadapt or when the duration of the incubation covers only part of the diel light cycle. Chl‐labeling experiments should therefore be based on 24‐h incubations so that the measurements integrate over the diel photocycle.

Annual and diel light cycle as a predictive factor in deep-water fisheries for the prawn Aristeus antennatus Risso, 1816

Using daily catch data on the rose prawn ( Aristeus antennatus) an attempt was made to construct a model relating catch per unit effort (CPUE) and a luminosity factor proportional to luminosity levels at the bottom on the prawn fishing grounds. A close relationship between this luminosity factor (defined in the text) and catchability was observed. Daily catch rates were highest during the 2 h immediately after sunrise, associated with a certain optimum light threshold. Within this pattern catch rates varied with longitude according to season and trawling depth. Based on these findings, novel alternatives for the future management of this resource are presented.

Entrainment of the circadian rhythm of food demand by infradian cycles of light-dark alternation inHoplosternum littorale(Teleostei)

Hoplosterum littorale (Hancock, 1828) shows a markcd circadian rhythm of food demand. Feeding activity is mainly nocturnal, with two pcaks, and is synchronized with the diel light cycle. We tested the cffect of infradian (period >28 h) lightldark cycles on this rhythm in fish wilh dcmand-fccdcrs: fish subjected to a 13.5 L122.5 D lightldark cycle for 8 days and a 25.5L/10.5D cycle for a further 8 days showed a strictly nocturnal fccding activity, a response that only passively reflected the cyclic altcrnation of light and dark.

Social Displays of Experimentally Paired Dwarf Geckos (Sphaerodactylus clenchi)

Members of the gekkonid genus Sphaerodactylus are small, voiceless, nocturnal or crepuscular geckos that are found in the West Indies, Central America, and Colombia (Ditmars, 1933). Because of their diminutive size and secretive habits, virtually nothing is known about their social behavior. Sphaerodactylus clenchi is a small (maximum snout-vent length, SVL, = 33 mm), recently described species from Hispaniola (Shreve, 1968). It is found among palm trash in moist habitats (Schwartz and Thomas, 1983). There is a small to moderate amount of sexual dimorphism in dorsal patterning (Schwartz and Thomas, 1983). In this paper we describe and quantify social behavior observed during encounters between experimentally paired S. clenchi. One of us (HYC) collected S. clenchi on Hispaniola during summer, 1984. The geckos were housed individually in cylindrical plastic chambers (115 mm in diameter x 105 mm high) in the Centenary College Animal Room, Shreveport, Louisiana, USA. Water was given ad libitum, and vestigial-winged fruit flies were supplied as food every two days. The temperature of the room was approximately 22 C. The geckos were exposed to a natural diel light cycle. Observations on social behavior were made from October through early December 1984. Most observations occurred from 1930-2200 h Central Standard Time because noctur-

Temporal and spatial variability in phytoplankton ammonium and nitrate uptake in the Delaware Estuary

Phytoplankton NH 4 + and NO 3 − uptake was examined along the longitudinal salinity gradient of the Delaware Estuary over several seasonal cycles using 15N-tracer techniques. Saturated nitrogen uptake rates increased directly with water temperature and reached a maximum of 380 nmol Nl −1h −1 during summer. This temperature dependence was related primarily to changes in the rate of maximum chlorophyll specific uptake, which varied exponentially between 2 and 70 nmol N [μg Chl h] −1 over a temperature range of 2–28°C. Despite these high uptake rates, balanced growth (C:N⋍7:1) could be maintained over the diel light cycle only by highly efficient nitrogen uptake at low light intensities and dark uptake below the photic zone and at night (dark uptake=25% maximum uptake). Ammonium fulfilled 82% of the annual phytoplankton nitrogen demand in the estuary despite dominance of NO 3 − in the ambient dissolved inorganic nitrogen pool. The predominance of NH 4 + uptake occurred because of the general suppression of NO 3 − assimilation at NH 4 + concentrations in excess of 2 μ m. This suppression, however, was not as universal as has been reported for other systems, and it is suggested that the extremely high NO 3 − concentrations found in the estuary contribute to this pattern. Nitrate was a significant source of nitrogen only during periods of high phytoplankton production in summer, and when NH 4 + concentrations were low towards the end of the spring bloom.

The vertical distribution of zooplankton in the western Irish Sea

Zooplankton diel vertical migration is evident on the mixed isothermal side of the western Irish Sea frontal system but is often influenced by large tides and persistent geostrophic currents. On the stratified side of the front, temperature acts as a controlling factor with most of the zooplankton occurring above the thermocline and carrying out pronounced vertical migration when chlorophyll a levels are low and diffuse. At higher chlorophyll levels, when discrete chlorophyll a maxima form, zooplankton vertical movement may be greatly modified with a large number of species and stages concentrating within these maxima at all times of the diel light cycle.