Photoacclimation of Ulva rotundata (Chlorophyta) under natural irradiance

Two vegetative clones (designated 11/85 and 7/86 in accordance with month/year of collection) of the green macroalga Ulva rotundata were collected in the vicinity of Beaufort, North Carolina, USA. Each was grown in an outdoor continuous-flow system in summer (20°C) of 1986 and late winter (10° to 17°C) of 1987, in irradiances ranging from 9 to 100% of full sunlight, with and without NH ₄ ⁺ enrichment. Continuous enrichment of influent estuarine water (dissolved inorganic nitrogen 2 M, N:P5) to 8–12 M NH ₄ ⁺ had only a slight effect on growth rate. Temperature changes of 2 to 3°C had a much greater effect. Prolonged exposure to a given daily irradiance resulted in acclimation, exposure to a given daily irradiance resulted in acclimation, indicated by faster growth of conditioned plants relative to those transferred from a different irradiance. Most of the difference in growth rates between transferred and control plants was attributed to differences in thallus absorptance. Growth was photoinhibited above 40% sunlight at temperatures below 15°C, but not above 20°C. Following interday irradiance transfers, thallus percent dry weight changed in a manner that suggests different response times for photosynthesis and cell division.     

The common jellyfish Aurelia aurita: standing stock,excretion and nutrient regeneration in the Kiel Bight,Western Baltic

The population dynamics, ammonia and inorganic phosphate excretion, and nutrient regeneration of the common jellyfish Aurelia aurita was investigated from 1982 to 1984 in the Kiel Bight, western Baltic Sea. During summer 1982, medusae abundance ranged between 14 and 23 individuals 100 m^-3, biomass was estimated at about 5 g C 100 m^-3 and the mean final diameter of individuals was 22 cm. Abundance, based on numbers, in 1983 and 1984 was an order of magnitude lower; biomass was less than 2 g C 100 m^-3 and jellyfish grew to 30 cm. During the summers of 1983 and 1984, A. aurita biomass constituted roughly 40% of that of the total zooplankton>200 m. In 1982, for which zooplankton data were lacking, it was assumed that medusae biomass was greater than that of all other zooplankton groups. Total ammonia excretion ranged between 6.5 and 36 mol h^-1 individual^-1, whereas inorganic phosphate release was 1.4 to 5.7 mol h^-1 individual^-1. Allometric equations were calculated and exponents of 0.93 for NH₄–N release and 0.87 for PO₄–P excretion were determined. Nitrogen and phosphorus turnover rates were 5.4 and 14.6% d^-1, respectively. In 1982, the medusae population released 1 100 mol NH₄–N m^-2 d^-1, about 11% of the nitrogen requirements of the phytoplankton. The inorganic phosphate excretion (150 mol m^-2 d^-1) sustained 23% of the nutrient demands of the primary producers. In the other two years the nutrient cycling of the medusae was much less important, and satisfied only 3 to 6% of the nutrient demands. It is suggested that in some years A. aurita is the second most important source of regenerated nutrients in Kiel Bight, next to sediment.     

Interactive effects of nitrogen and dissolved inorganic carbon on photosynthesis,growth, and ammonium uptake of the macroalgae Cladophora vagabunda and Gracilaria tikvahiae

Dissolved inorganic carbon (DIC) is rarely considered limiting for macroalgae, but some research suggests that under conditions of N sufficiency, photosynthetic capacity is enhanced with DIC enrichment. During spring (April–May) and summer (July–August) 1993, we investigated the interactive effects of nitrogen (N) and DIC on photosynthetic capacity, growth, and nutrient uptake rates of the macroalgae, Cladophora vagabunda (L.) van den Hoek and Gracilaria tikvahiae (McLachlan), dominant species in a temperate eutrophic estuary (Cape Cod, Massachusetts, USA). Water-column CO₂ concentrations showed significant diurnal fluctuations, ranging from a morning CO₂ peak (21 M) to an afternoon low (13 M) during summer, probably associated with metabolic activities in a thick algal mat. Results from instantaneous photosynthesis measurements and microcosm experiments indicate that DIC limits photosynthetic capacity and growth rates of C. vagabunda during summer, perhaps related to tissue N sufficiency and low water-column CO₂ concentrations. For example, this species showed enhanced growth (F=8.69, P<0.02) under DIC but not N enrichment. G. tikvahiae showed marginal DIC enhancement of maximum photosynthetic rate, while growth was significantly stimulated by addition of N. Reduced thallus N of this species during the summer further identifies N as the primary factor limiting growth. In addition, G. tikvahiae has the ability to use DIC in its several forms, while C. vagabunda primarily uses dissolved CO₂. DIC enrichment resulted in a depression of NH₄ ⁺ uptake rates for both species, particularly during summer at saturating (60 M) ammonium levels, suggesting competition between NH₄ ⁺ uptake and DIC acquisition under conditions of N sufficiency. Dominance of C. vagabunda and G. tikvahiae in areas undergoing eutrophication has been attributed to their successful procurement and storage of N as well as to high growth rates. The present study revealed that under conditions of N sufficiency during summer, DIC may control rates of production of these opportunistic macroalgae.     

Ultrastructure of the gonad and gametogenesis in the eastern oyster,Crassostrea virginica. II. Testis and spermatogenesis

The pelagic harpacticoid copepod, Macrosetella gracilis (A. Scott), is found in association with colonies of the nitrogen-fixing (diazotrophic), bloomforming cyanobacterium Trichodesmium spp. in tropical and subtropical waters. M. gracilis is one of the few direct grazers of these often toxic cyanobacteria. Experiments investigating NH ₊ ⁴ regeneration by M. gracilis were conducted in the Caribbean in September 1992 and the Coral Sea, Australia in November 1994. Rates of M. gracilis ingestion of Trichodesmium thiebautii labelled with ¹⁵N₂ measured in the eastern Caribbean indicated that M. gracilis could consume 33 to 45% of total T. thiebautii colony N d^-1 and >100% of new N fixed d^-1. We also measured the release of NH ₊ ⁴ by M. gracilis feeding on T. thiebautii, as well as by non-feeding copepods, using ¹⁵N isotope dilution methods. In non-feeding copepods, rates of NH ₊ ⁴ release increased as numbers of copepods were increased as both copepod numbers and food availability increased. In the presence of T. thiebautii colonies, M. gracilis had an average rate of NH ₊ ⁴ regeneration of 7.7±1.5 nmol N copepod^-1 h^-1 (±SE), which was significantly higher than when food was absent (1.9±0.7 nmol N copepod^-1 h^-1). Rates of M. gracilis excretion were relatively high based on excretion: ingestion ratios, which could be due to having a high-N food source readily available, to sloppy-feeding effects, or as a response to toxins in the cyanobacterium. Incubations of M. gracilis with and without T. erythraeum resulted in significant increases in [NH ₊ ⁴ ] as a function of copepod density only. Ammonium leakage from the cyanobacterium and/or microheterotroph associates was relatively low. M. gracilis, through excretion and possible mechanical breakage of cells while grazing, appears to provide a direct link between atmospherically derived new nitrogen and regenerated NH ₊ ⁴ in the oligotrophic systems where Trichodesmium spp. are abundant.     

Rates of primary productivity and growth in<Emphasis Type="Italic"> Ecklonia radiata</Emphasis> measured at different depths,over an annual cycle,at West Island,South Australia

The pattern of growth (biomass accumulation) in Ecklonia radiata throughout the year and across a depth profile was investigated using the traditional hole-punch method, and the information presented in context with concurrently measured in situ net productivity rates. The rate of net daily productivity showed a lack of consistent seasonal variability, remaining constant throughout the year at two of the four depths measured (3 m and 12 m), and becoming higher during winter at another (5 m). Throughout the year, rates of net daily productivity differed significantly across the depth profile. Net daily productivity rates averaged 0.017 g C g^–1 dwt day^–1 and 0.005 g C g^–1 dwt day^–1 at a depth of 3 m (1,394 mol O₂ g^–1 dwt day^–1) and 10 m (382 mol O₂ g^–1 dwt day^–1) respectively. In contrast, the biomass accumulation rate of E. radiata was highly seasonal, with low rates of growth occurring in autumn (0.002 g dwt g^–1 dwt day^–1 at both 3 and 10 m) and summer (0.007 and 0.004 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively) and higher rates in spring (0.016 and 0.007 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively) and winter (0.015 and 0.008 g dwt g^–1 dwt day^–1 at 3 and 10 m respectively). The proportion of assimilated carbon used for biomass accumulation varied throughout the year, between 5% and 41% at 3 m and between 28% and 128% at 10 m. The rates of biomass accumulation at all depths represented only a small proportion of the amount of carbon assimilated annually.Communicated by P.W. Sammarco, Chauvin     

Nitrate-reductase activity and in vivo nitrate-reduction rate in Ulva rigida illuminated by blue light

In the marine green alga Ulva rigida C. Agardh, nitrate reductase (NR) is synergetically induced by blue light and nitrate. The present study examines the effect of blue light and a large NO ₃ ^– pulse (0.3 mM) on relevant variables of NO ₃ ^– -assimilation such as NO ₃ ^– -uptake, intracellular NO ₃ ^– -storage, NR activity, in vivo NO ₃ ^– -reduction rate and NO ₂ ^– and NH ₄ ⁺ -accumulation. Nitrate uptake started immediately upon addition of NO ₃ ^– , suggesting the presence of a constitutive carrier, however in the first 1.5 to 2 h, periods of net NO ₃ ^– efflux were frequent. After this time, NO ₃ ^– -uptake and intracellular NO ₃ ^– -accumulation proceeded linearly with time, suggesting the existence of a different NO ₃ ^– -uptake mechanism, which seems to be inducible. Our results indicate that in vivo NO ₃ ^– -reduction is not exclusively dependent on the potential NR activity. In U. rigida, during the first 2 h after a NO ₃ ^– pulse (300 M) there were clear indications that the induction state of the NO ₃ ^– -carrier limits the reduction rate of NO ₃ ^– . Once the induction of the NO ₃ ^– -transporter had been completed (1.5 to 2 h), the NO ₃ ^– -assimilation pathway reached a steady state, NO ₃ ^– -uptake rate, NO ₃ ^– -reduction rate and NO ₂ ^– and NH ₄ ⁺ -accumulation being linear with time. Since the reduction of NO ₃ ^– leads mainly to the accumulation of NH ₄ ⁺ , we conclude that, after the NO ₃ ^– -reduction itself, NH ₄ ⁺ -fixation into carbon skeletons is the limiting step in the assimilation of NO ₃ ^– by U. rigida under blue light.     

Regional studies of daily,seasonal and size fraction variability in ammonium remineralization

Rates of ammonium remineralization were determined using a ¹⁵N isotope dilution technique for two oceanic regions, one coastal region, and one estuarine region, covering a wide range of ambient nutrient, light, and temperature conditions. Results showed that NH ₄ ⁺ assimilative and regenerative fluxes were primarily in balance, even when the ambient nitrogenous pool was completely dominated by NO ₃ ^- . Variations in uptake and remineralization rates relative to time of day and season were also determined. Size fraction studies at several of the sites showed that the smallest size fraction (<10 m) was usually the most important in remineralizing NH ₄ ⁺ , and the importance of the apparent bacterial fraction (<1 m) may increase following blooms. The results support the concept that, over a wide variety of conditions, the fluxes of NH ₄ ⁺ remineralization and uptake are tightly coupled; phytoplankton are able to utilize NH ₄ ⁺ at the rate that it is produced by heterotrophic processes.     

Ingestion-independent rates of ammonium excretion by the copepod Calanus pacificus

The relationship between food ingested and NH ₊ ⁴ excretion rate was investigated for female Calanus pacificus collected in August, 1982, from the San Juan Archipelago, Washington State, USA. The copepods were preconditioned to 6 densities of the diatom Thalassiosira weissflogii (0 to 10⁴ cells ml^–1) for 30 h before the experiment. The experiment was conducted with nutrients added in excess to maintain equal rates of NH ₊ ⁴ uptake by the diatoms at all densities. Although ingestion rates of C. pacificus varied from 0 to over 20% of body N d^–1 at the different food levels, excretion was a constant 6.6 nM NH ₊ ⁴ copepod^–1 h^–1 or about 10% of body N d^–1. This ingestion-excretion relationship, which is consistent with previous respiration and fecundity studies, suggests that the ecological dominance of C. pacificus only under conditions of high food abundance may be due to a dramatic increase in its growth efficiency as ingestion increases above the level supporting a constant metabolic rate. The maintenance of a constant level of metabolism during relatively short periods of low food abundance may be advantageous if it allows the copepod to exploit more effectively short-term variability in its food resulting from environmental heterogeneity or vertical migration.Contribution No. 1360 from the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Nitrogen supply,tissue composition and frond growth rates for Macrocystis pyrifera off the coast of southern California

Seasonal changes in ambient NO ₃ ^– and NH ₄ ⁺ , tissue composition (N, C, and C/N ratio), and frond growth rates for Macrocystis pyrifera (L.) Agardh were examined. Ambient NO ₃ ^– showed distinct seasonal variations. Frond growth rates were variable, but showed no clear correlation to ambient NO ₃ ^– . The average N content of plant tissue did, however, show the same seasonal variations as ambient NO ₃ ^– . The longitudinal distribution of total tissue N and various components of tissue N along fronds were also analyzed. Several distinct patterns were found: high levels of protein N at growing tips and elevated levels of soluble N in lower parts of the frond. Free amino acids accounted for a major portion of the soluble N, but neither NO ₃ ^– nor NH ₄ ⁺ accumulated in the plant tissue. The longitudinal distribution of N along the fronds is compared to reported variations in C metabolism, and it is concluded that C and N sourcesink relations do not always coincide and bidirectional translocation may occur.     

A study on the landfill leachate and its impact on the groundwater quality of the greater area

Characterisation of the leachate originating from the Ano Liosia landfill (situated in Attica region, Greece) as well as assessment on the quality of the local aquifer were carried out. The experimental results showed that most of the parameters examined in the leachate samples such as colour, conductivity, TS, COD, NH₃–N, PO₄–P, SO₄ ^2–, Cl^–, K⁺, Fe and Pb were found in high levels. The organic load was quite high since the COD concentrations were in the range of 3250–6125mgL^–1. In addition, the low BOD/COD ratio (0.096–0.195), confirmed that the majority of this organic matter is not easily biodegradable. The groundwater near the landfill site was characterised as not potable and not suitable for irrigation water, since most of the physical and chemical parameters examined – such as colour, conductivity, DS, hardness, Cl^–, NH₃–N, COD, K⁺, Na⁺, Ca²⁺, Fe, Ni and Pb exceeded the permissible limits given by EE, EPA and the Greek Ministry of Agriculture. Furthermore, this study presents the application of the hydrologic evaluation of landfill performance (HELP) model for the determination of the yearly leakage from the base of the landfill after the final capping.     

Effect of algal bloom deposition on sediment respiration and fluxes

Using sediment cores collected in November 1989 from Aarhus Bight, Denmark, the fluxes of O₂, CO₂ (total CO₂), NH ₄ ⁺ , NO ₃ ^– +NO ₂ ^– and DON (dissolved organic nitrogen) across the sediment-water interface were followed for 20 d in an experimental continous flow system. On day 7, phytoplankton was added to the sediment surface, to see the result of simulated algal bloom sedimentation. Benthic O₂ consumption and CO₂ efflux, 38 to 41 mmol O₂ m^-2 d^-1 and 25 to 30 mmol CO₂ m^-2 d^-1, respectively, immediately increased by 39% and 100% after the algal addition, but gradually declined to the orginal level. Fluxes of NH ₄ ⁺ (1.0 to 1.2 mmol m^-2 d^-1) and DON (0.3 to 0.9 mmol m^-2 d^-1) increased due to the organic substrate addition. NH ₄ ⁺ and NO ₃ ^– flux changed direction, becoming an efflux and influx, respectively, for a few days and a large amount of DON (max. rate 4.0 mmol m^-2 d^-1) was immediately produced either by bacterial hydrolytic activity or from autolysis of the algae. DON was the most significant nitrogen component in pore water and in terms of N-flux from sediment. A temporary stimulation of anaerobic respiration processes (sulfate reduction and denitrification) and a decrease in nitrification were indicated. After the effect of the organic addition had declined, the fluxes gradually reverted to the original rates. The halflife of the added algal material, of which 20 to 30% was very labile, was estimated to be 2 to 3 wk.     

Wet deposition chemistry studies at Suva, Fiji, a remote tropical island site in the south Pacific

A field project encompassing wet-only rainwater sampling was initiated as a bilateral Fiji/Australia activity. Normally, biweekly samples were collected, using a wet-only rainwater sampler, and analysed for H⁺, Na⁺, K⁺, Mg²⁺, NH₄ ⁺, Cl^–, NO₃ ^–, SO₄ ^2–, PO₄ ^3-, methane sulphonic acid, oxalic acid, formic acid and acetic acid. The pH of the rainwater ranged between 5.730 and 4.480 with an average value of 5.176, slightly lower than the pH of unpolluted rainwater saturated with atmospheric CO₂(pH = 5.650). Na⁺and Cl^–were the major ions with average concentrations of 98.15 M and 109.57 M respectively. There is an excellent correlation between the cation sum (average 147.71 eq L^-1) and the anion sum (average 142.12 eq L^-1) attesting to the quality of the data generated. This paper presents the detailed results of the study for a relatively clean remote island site in Suva, Fiji, latitude 18° 09 S, longitude 178° 27 E, height 6 m, and outlines prospects for further work.     

Application of a 15N tracer method to the study of dissolved organic nitrogen uptake during spring and summer in Chesapeake Bay

The dissolved organic nitrogen (DON) pool in marine waters contains a diverse mixture of compounds. It is therefore difficult to accurately estimate planktonic uptake of DON using the limited number of radiolabeled compounds commercially available. We describe a method to estimate DON uptake rates using ¹⁵N-labeled DON recently released from phytoplankton. To make ¹⁵N-labeled DON, we incubated surface water with ¹⁵NH ₄ ⁺ and then isolated the DON, including any recently released DO¹⁵N, with ion retardation resin. This DON was then added to a freshly collected water sample from the same environment to quantify the rate of DON uptake. The technique was applied to investigate rates of DON uptake relative to inorganic nitrogen in the mesohaline Chesapeake Bay during May 1990 and August 1991. The May experiment took place after the spring bloom, and rates of DON uptake [ranging from 0.31 to 0.53 g-atom (g-at) Nl^-1 h^-1] often exceeded rates of NH ₄ ⁺ and NO ₃ ^- uptake combined. The rates of DON uptake at this time were higher than estimated bacterial productivity and were not correlated with bacterial abundance or bacterial productivity. They were, however, correlated with rates of NO ₃ ^- uptake. In May, we estimate that only 7 to 32% of DON uptake was a result of urea utilization. In contrast, in August, when regenerated nutrients predominate in Chesapeake Bay, rates of DON uptake (ranging from 0.14 to 0.51 g-atom Nl^-1 h^-1) were an average of 50% of the observed rates of NH ₄ ⁺ uptake. Consistent with the May experiment, rates of DON uptake were not correlated with bacterial production. A sizable fraction of DON uptake, however, appeared to be due to urea utilization; rates of urea uptake, measured independently, were equivalent to an average of 74% of the measured rates of DON uptake. These findings suggest that, during both periods of study, at least a fraction of the measured DON uptake may have been due to utilization by phytoplankton.     

Native legume establishment on acidic coal mining overburden at Collie,Western Australia

Nitrogen is often provided to impoverished overburden dumps through the establishment of legumes. Low indigenous soil nutrient levels, summer drought conditions and an acidic mining overburden represent major obstacles to successful rehabilitation of open-cut coal mining at Collie in southwest Western Australia. In this study,Acacia pulchella, a native Western Australian species often used in rehabilitation of mined lands, was shown to nodulate and grow in coal mining overburden with pH values less than 4.0 under glasshouse conditions. Plant growth (both top and root dry weight), nodule fresh weight, and nodulation success was best at a pH near 5.0, a value only slightly lower than the typical soil pH of the native jarrah (Eucalyptus marginata) forest. Acetylene reduction rates were reduced by acidity and ranged from 8.2 m C₂H₄g^–1hr^–1 at pH 6.77 to 3.0 m C₂H₄g^–1hr^–1 at a pH of 3.98. Four additional plant species were found to occur and to nodulate on acid overburden material at Collie.     

Relative growth of different species of marine algae in wasterwater-seawater mixtures

In recent studies, we developed a combined nutrient removal-marine aquaculture process for the tertiary treatment of wastewater and the production of commercially important shellfish. Part of this process consists of an outdoor mass cultivation system for marine algae. During our previous experiments we observed that marine diatoms almost exclusively are the dominant algal species in our outdoor cultures. To better understand this phenomenon of diatom dominance we grew 16 species of marine algae in continuous monoculture under laboratory conditions simulating to some degree the conditions prevailing in our outdoor experiments. Species such as Skeletonema costatum, Monochrysis lutheri and Tetraselmis sp., which were never dominant in our outdoor cultures, grew as well in monoculture, as Phaeodactylum tricornutum, frequently, the prevalent species outdoors. However, when monocultures of Dunaliella tertiolecta and Thalassiosira pseudonana (3H) were purposely contaminated with P. tricornutum, the latter species quickly became dominant. It is suggested that a complex interaction of environmental factors is usually responsible for the dominance of a particular species; one such factor may be the nitrogen source in the growth media. Under conditions of virtually, complete nitrogen assimilation, the carbon: nitrogen ratio in the algae was high (7 to 8) when NO ₃ ^- –N was the source of nitrogen, and low (4 to 6) when NH ₄ ⁺ –N was the prime form of nitrogen. When algal growth was low, resulting in a large inorganic nitrogen residue, the carbon:nitrogen ratio was low regardless of whether NO ₃ ^- –N or NH ₄ ⁺ –N was the main nitrogen source.Contribution No. 3297 from the Woods Hole Oceanographic Institution.     

Relationships between microbial distributions and the anaerobic decomposition of organic matter in surface sediments of long Island Sound,USA

Relative rates of the anaerobic decomposition of organic matter in the upper 10 cm of sediment from two stations in central Long Island Sound, USA, were compared. Sediment samples from discrete depth intervas were incubated anoxically and changes in SO ₄ ⁼ , NH ₄ ⁺ , bacterial numbers, extractable adenosine triphosphate (ATP), organic matter, and organic carbon were measured as a function of time and temperature. At both stations (15 and 34 m water-depths, respectively), the calculated rates of SO ₄ ⁼ reduction and NH ₄ ⁺ production decreased exponentially (approximately) with depth below the sediment-water interface. Over the same depth interval, ATP concentrations dropped by a factor of 6 to 7 and bacterial numbers were lower by a factor of 2 to 3. These decreases in SO ₄ ⁼ reduction, NH ₄ ⁺ production, bacterial numbers, and ATP, reflect a change in the physiological state of microbial populations with depth in the sediment and are consistent with the conclusions that the quantity of easily utilizable organic matter changes rapidly below the sediment surface and that food limitation controls the basic depth distribution of microbial activity. The average rates of SO ₄ ⁼ reduction, 29 to 39 mM year^-1 (22°C), in the top 10 cm are similar at both stations studied here, as well as at an additional station from a previous study. In contrast, average NH ₄ ⁺ production differs by a factor of 2 at the two stations, reflecting differences in the C:N ratio of the organic matter supplied to the sediment surface and differences in particle reworking by macrofauna at each site. The apparent activation energy of SO ₄ ⁼ reduction was 19±1 kcal mole^-1 and that of NH ₄ ⁺ production, 18±3 kcal mole^-1. The overall quantity of carbon required to support the calculated average SO ₄ ⁼ reduction rate in the top 10 cm is 23 g C m^-2 year^-1 and represents 36% of all the carbon available to the benthos annually and 11% of the net primary production in the water column. Directly measured fluxes of NH ₄ ⁺ from sediments to overlying water at both stations agree well with those predicted from production rates obtained by the incubation techniques.     

Effect of phosphate and ammonium levels on photosynthetic and respiratory responses of the red alga Gracilaria verrucosa

Gracilaria verrucosa (Hudson) Papenfuss exposed to nutrient enriched media (0.1 mM PO₄; 1.0 mM NH ₄ ⁺ ) by pulse feeding 2 h every third day for a period of 5 wk at 20°C and 25–30 salinity showed significantly higher rates of photosynthesis regardless of photon flux density correlated with increased pigment levels. Algae in nonenriched media showed significantly higher levels of soluble carbohydrates and decreased levels of phycoerythrin and chlorophyll a. Photosynthetic and respiratory responses to temperature 15°, 25°, 30°C and salinity (15, 25, 30 S) combinations indicate broad tolerances by both nutrient enriched and non-nutrient enriched algae. Photosynthetic and respiratory rates were highest at the high temperatures. Pulse-fed algae had significantly higher photosynthetic rates than non-nutrient enriched plants at all temperature and salinity combinations. Non-nutrient enriched algae had significantly higher respiratory rates than nutrient enriched algae at only 30°C and 15. The respiratory rates of both nutrient enriched and non-nutrient algae decreased under combinations of higher temperatures and salinities. G. verrucosa, grown without nutrients, has lower tolerances to environmental stresses.     

Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Effect of ammonia on survival and osmoregulation in different life stages of the lobsterHomarus americanus

Acute toxicity of ammonia was determined for cultured larval, postlarval, and wild adult lobsters (Homarus americanus) in 1988. Ammonia tolerance was found to increase with ontogenetic development. Based on 96-h LC₅₀ values of 58 mg l^–1 NH₄ ⁺ + NH₃ l^–1 seawater (0.72 mg NH₃ l^–1) for Stage I larvae, 87 mg NH₄ ⁺ + NH₃ l^–1 (1.7 mg NH₃ l^–1) for Stage II larvae, 125 mg NH₄ ⁺ + NH₃ l^–1 (2.13 mg NH₃) for Stage III larvae, 144 mg NH₄ ⁺ + NH₃ l^–1 (2.36 mg NH₃ l^–1) for Stage IV postlarvae, 377 mg NH₄ ⁺ + NH₃ l^–1 (5.12 mg NH₃ l^–1) for adult lobsters at 5°C and 219 mg NH₄ ⁺ + NH₃ l^–1 (3.25 mg NH₃ l^–1) for adult lobsters at 20°C, recommendations for safe levels of total ammonia and un-ionized ammonia were calculated using an application factor of 0.1. Effects of ammonia on osmoregulatory capacity were studied on postlarvae and adults. Ability of postlarvae and adults to hyper-regulate in low-salinity media decreased after exposure to ammonia. In postlarval lobsters, osmoregulatory capacity was significantly affected in ammonia concentrations exceeding 32 mg l^–1. Osmoregulatory capacity in adult lobsters (5 and 20°C) was affected at 150 mg l^–1. In postlarval lobsters, a minimum exposure time of 12 h was required to impair osmoregulatory capacity. The decrease in hemolymph osmotic pressure was caused by lower hemolymph sodium concentrations. The presence of ammonia in the external medium could markedly affect the Na⁺/NH₄ ⁺ transport mechanism by permanently, temporarily, or partially impairing the transport sites for sodium.     

Nutrient cycling studies in Carmarthen Bay: phytoplankton production,nitrogen assimilation and regeneration

The results from a series of five cruises (August 1979–January 1981) to examine rates of primary production, nitrogen assimilation and regeneration in Carmarthen Bay, S. Wales, are presented. Chlorophyll, primary production and regeneration were distributed irregularly throughout the bay, with the highest concentrations and rates being found around the shallow periphery. Nitrate was the dominant form of nitrogen assimilated by the phytoplankton, apart from periods of high biomass during the summer when ammonium assumed a more important role. The rates of ammonium regeneration from microheterotrophs were greater than the ammonium demand over much of the year, indicating that the bay was a net exporter of ammonium and was a physically dominated open system. During the summer, the rates of ammonium regeneration and assimilation were similar, indicating efficient recycling of nutrients.