An in situ study of photosynthetic oxygen exchange and electron transport rate in the marine macroalga Ulva lactuca (Chlorophyta)

Direct comparisons between photosynthetic O₂ evolution rate and electron transport rate (ETR) were made in situ over 24 h using the benthic macroalga Ulva lactuca (Chlorophyta), growing and measured at a depth of 1.8 m, where the midday irradiance rose to 400–600 μmol photons m⁻² s⁻¹. O₂ exchange was measured with a 5-chamber data-logging apparatus and ETR with a submersible pulse amplitude modulated (PAM) fluorometer (Diving-PAM). Steady-state quantum yield ((F_m′−F_t)/F_m′) decreased from 0.7 during the morning to 0.45 at midday, followed by some recovery in the late afternoon. At low to medium irradiances (0–300 μmol photons m⁻² s⁻¹), there was a significant correlation between O₂ evolution and ETR, but at higher irradiances, ETR continued to increase steadily, while O₂ evolution tended towards an asymptote. However at high irradiance levels (600–1200 μmol photons m⁻² s⁻¹) ETR was significantly lowered. Two methods of measuring ETR, based on either diel ambient light levels and fluorescence yields or rapid light curves, gave similar results at low to moderate irradiance levels. Nutrient enrichment (increases in [NO₃ ⁻], [NH₄ ⁺] and [HPO₄ ^2-] of 5- to 15-fold over ambient concentrations) resulted in an increase, within hours, in photosynthetic rates measured by both ETR and O₂ evolution techniques. At low irradiances, approximately 6.5 to 8.2 electrons passed through PS II during the evolution of one molecule of O₂, i.e., up to twice the theoretical minimum number of four. However, in nutrient-enriched treatments this ratio dropped to 5.1. The results indicate that PAM fluorescence can be used as a good indication of the photosynthetic rate only at low to medium irradiances. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of flow on meiofauna colonization in artificial streams and reference sites within the Illinois River, Arkansas

Despite frequent disturbances from flow, stream meiofauna form diverse and abundant assemblages suggesting that they are resistant and/or resilient to flow disturbances. Stream flow profoundly influences benthic invertebrate communities but these effects remain poorly understood. We examined the influence of flow on meiofauna colonization at small spatial scales (2–3 m) using artificial streams in conjunction with similar sites (flow, depth, substrates) in the reference stream (Illinois River, Arkansas). Colonization of meiofauna was found to be rapid and generally increased with flow rates examined (1–2, 6–7, and 11–12 cm s⁻¹). Six of the 10 most abundant taxa successfully completed colonization in artificial channels (equaled or exceeded reference benthic densities) within 5 days. Benthic meiofauna were more abundant in fast flows in artificial channels and in fast and slow flows in reference stream sites. A diverse assemblage of meiofauna was collected from the plankton which was dominated by rotifers, copepods (mostly nauplii), dipterans, and cladocerans. Densities of drifting meiofauna (potential colonists of the benthos) were low (5 no. l⁻¹) and similar among artificial channels and reference sites regardless of flow rates (F _1,18 = 2.19, p = 0.1407). Although densities were low, the numbers of drifting meiofauna were more than sufficient to colonize the benthos. Less than 0.65% of the drifting meiofauna were needed to colonize the substrates of artificial streams. The benthic assemblage paralleled that of the plankton, consisting mainly of rotifers, copepods (mainly nauplii), and dipterans. Evidence for active control over dispersal was observed as meiofauna densities varied between the plankton and benthos over the diel cycle (F _1,18 = 6.02, p = 0.0001 and F _1,18 = 9.88, p = 0.006, respectively). Rotifers, copepods, and nematodes were more abundant in the plankton during the day and in the substrates at night. These results suggest that meiofauna assemblages can change rapidly in response to alterations of habitat patches by disturbance.     

Annual cycles of benthic community oxygen uptake in a deep oligotrophic lake (Attersee,Austria)

Benthic community oxygen uptake of Lake Attersee sediments was measured between 1976 and 1979, along two profiles at 25, 50 and 100 m depth. Profile I was situated in the bay of Unterach into which the main tributary, Mondsee-Ache, discharges a high load of organic matter. Profile II was chosen at Weyregg to avoid the eutrophying effect of Mondsee-Ache. Oxygen uptake rates of Unterach sediments at 25 and 50 m depth were found to be higher when compared to the other sites (mean rates: Unterach 25 m = 15.56, 50 m = 11.05 mg O₂ · m⁻² · h⁻¹; Weyregg 25 m = 6.43, 50 m = 5.14 mg O₂ · m⁻² · h⁻¹). Organic content of the uppermost sediment layer was also higher in the bay of Unterach than at Weyregg. Oxygen uptake rates of undisturbed sediment cores vary considerably throughout the year, but no simple correlation existed with variations in organic content of the sediments. Peaks of organic matter were found to concur with following peaks of oxygen uptake rates, which implies that a certain time span is necessary for transforming freshly sedimented organic matter into a state digestable for the benthic community. The retardation between increasing organic matter of the sediment and the corresponding increase of benthic oxygen uptake was different at Unterach and Weyregg respectively, which is explained by the different quality of sedimenting material.     

Nitrogen and Phosphorous Excretion Rates by Tubificids from the Prahova River (Romania)

Nitrogen and phosphorous exchange at the water–sediment interface is controlled both by complex physico-chemical factors and biological processes. Zoobenthos excretion is one of the most important processes in the mineralization of sedimented organic mater. In polluted freshwaters, tubificid worms are among the dominant components of the benthic community. Rates of ammonium and inorganic phosphate excretion by tubificids were experimentally assessed. They were related to the tubificid abundance in a stream ecosystem polluted with municipal and industrial wastewater. The relationship between these rates and temperature were investigated within the range of 4–23 °C. Relatively constant excretion rates were obtained for both nutrients in the first 8 h of excretion, ranging between 0.076 and 0.226 μg N mg d.w.⁻¹ h⁻¹ and 0.0065–0.01 μg P mg d.w.⁻¹ h⁻¹_, respectively. Q₁₀ values of 2.52 for ammonium and 1.31 for phosphate were calculated. If we presume that all excreta eventually enters the water column, then we can calculate that these invertebrates potentially add 39.17 mg N m⁻² day⁻¹ and 0.49 mg P m⁻² day⁻¹. These values accounts for 17.16 and 7.56% of the nutrient load in the river water, respectively.     

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO<Subscript>2</Subscript> in a tropical coastal lagoon (Chilka Lake,India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO₂ production in the lake. Surface partial pressure of CO₂ (pCO₂), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO₂ in the lake seems to be governed by pCO₂ levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO₂ efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m⁻² d⁻¹ equivalent to 2.64 GgC d⁻¹ at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m⁻² d⁻¹ ≈ 0.06 GgC d⁻¹). 15% of CO₂ efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m⁻² d⁻¹ ≈ −3.77 GgC d⁻¹) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m⁻² d⁻¹ ≈ 2.80 GgC d⁻¹) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO₂ during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.     

Microphytobenthos production in the Gulf of Fos, French Mediterranean coast

Microphytobenthic oxygen production was studied in the Gulf of Fos (French Mediterranean coast) during 1991/1992 using transparent and dark benthic chambers. Nine stations were chosen in depths ranging from 0.5 to 13 m, which represents more than 60% of bottoms in the Gulf. Positive net microphytobenthic oxygen production was seasonally detected down to 13 m; the maximum value attained was 60 mg O₂ m⁻² h⁻¹ (0.7–0.8 g O₂ m⁻² d⁻¹) in sediments at 0.5 m depth during spring and winter. Respiration rates were maximum in the sediments located at the mussel farm (5 m), in the center of the Gulf, with 135 mg O₂ m⁻² h⁻¹ in spring (3.2 g O₂ m⁻² d⁻¹); in the other locations, it ranged from 3.3 to 58.2 mg O₂ m⁻² h⁻¹ (0.08–1.4 g O₂ m⁻² d⁻¹). Compared to phytoplankton, microphytobenthos production was higher only in the bottoms < 1 m depth. In deeper bottom waters, phytoplankton production could be absent due to light limitation, while microphytobenthos was still productive. Phytoplankton production m⁻² was generally higher than microphytobenthic production. Microphytobenthic biomass, higher than phytoplanktonic, varied from 27 to 379 mg Chl a m⁻², the maximum in the mussel farm sediments, with the minimum in sandy shallow bottoms. Pigment analysis showed that microphytobenthos consisted mainly of diatoms (Chl c and fucoxanthin) but other algal groups containing Chl b could become seasonally important. A Principal Component Analysis suggested that the main statistical factors explaining the distribution of our observations may be interpreted in terms of enrichment in phaeopigments and light; the role of Chl a appearing paradoxically as secondary in benthic production rates. Phaeopigments are mainly constituted by phaeophorbides, which indicate grazing processes. The influence of the mussel farm on the oxygen balance is noticeable in the whole Gulf.     

Feeding dynamics and respiration of the bottom-dwelling caridean shrimp Nauticaris marionis Bate, 1888 (Crustacea: Decapoda) in the vicinity of Marion Island (Southern Ocean)

The feeding dynamics and oxygen uptake of the bottom-dwelling caridean shrimp Nauticaris marionis were studied during the April/May 1984, 1996 and 1997 cruises to Marion Island (Prince Edward Islands, Southern Ocean). N. marionis is thought to have an opportunistic feeding mode. Prey composition varied considerably between the years and sites investigated. Overall, benthic (mainly hydrozoans and bottom-dwelling polychaetes) and, at times, pelagic (largely euphausiids and copepods) prey items dominated in the stomachs of N. marionis both by occurrence and by volume. Generally, pelagic prey contributed more to the diets of smaller shrimps, while benthic prey was a more important component in the guts of larger specimens. Wet, dry and ash-free dry weight were determined for specimens used in respiration experiments. The respiration rates of N. marionis females with carapace length 6.6–11.1 mm ranged from 80 to 250 μl O₂ individual⁻¹ · h⁻¹, or from 0.588 to 2.756 μl O₂ · mg⁻¹ dry weight h⁻¹. Regression analyses showed highly significant correlations between oxygen consumption and carapace length for N. marionis. Daily ingestion rates estimated using an in situ gut content analysis technique (4.4% of body dry weight) and an energy budget approach (average 4.7% of body dry weight, range 2.0–7.5%) showed good agreement with each other. Accepted: 29 July 1998     

Diverse communities of active Bacteria and Archaea along oxygen gradients in coral reef sediments

Microbial communities inhabiting highly permeable sediments of Checker Reef in Kaneohe Bay, Hawaii, were characterized in relation to porewater geochemistry (O₂, NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, phosphate). The physiologically active part of the population, assessed by sequencing cDNA libraries of 16S rRNA amplicons, was very diverse, with an estimated ribotype richness ≥1,380 in anoxic sediment. Quantitative analysis of community structure by rRNA-targeted fluorescence in situ hybridization (FISH) indicated that the archaeal population (9–18%) was dominated by marine Crenarchaeota (5–9%). Planctomycetales were the most abundant group in the oxic and interfacial habitat (17–19%) but were a minority (<5%) in anoxic reef sediment, where γ-Proteobacteria were numerically dominant (18%). Another 9–14% of the microbial benthos belonged to β-Proteobacteria, predominantly within the order Nitrosomonadales, many cultured representatives of which are NH₄ ⁺ oxidizers. The results of this study contribute to the phylogenetic characterization of benthic microbial communities that are important in organic matter degradation and nutrient recycling in coral reef ecosystems.     

Hyporheic Production is Substantially Greater than Benthic Production for a Common New Zealand Caddisfly

Life-history and production of Olinga feredayi in both benthic and hyporheic stream habitats were investigated in a pristine Waikato, New Zealand, forest stream over two years to investigate the contribution of hyporheic habitat to total secondary production. O. feredayi had a univoltine life-history with adult emergence occurring from November to March. Larvae with case lengths < 2 mm were present on most dates suggesting delayed egg hatching. Benthic densities were inversely related to maximum peak daily flow in the month prior to sampling, and positively related to the dry mass of particulate organic matter present in samples. Reach-average benthic production calculated by the size-frequency method was 0.024 g DM m⁻² year⁻¹. Hyporheic production was 4.276 g DM m⁻³ year⁻¹ and 6.462 g DM m⁻³ year⁻¹ in colonisation baskets set at 15–30 cm and 30–45 cm within the substratum, respectively, 2.3–3.4 times greater than production in surface baskets (0–15 cm). Averaged out over the reach scale, it was estimated that 96% of annual secondary production of O. feredayi occurred in hyporheic habitats >10 cm below the streambed surface. Our study clearly demonstrates that only sampling benthic habitats can lead to gross under-estimation of population-level annual production, and provides evidence for the role of the hyporheos as a source of secondary production that may partly account for the Allen Paradox.     

Denitrification activity and oxygen dynamics in Arctic sea ice

Denitrification and oxygen dynamics were investigated in the sea ice of Franklin Bay (70°N), Canada. These investigations were complemented with measurements of denitrification rates in sea ice from different parts of the Arctic (69°N–85°N). Potential for bacterial denitrification activity (5–194 μmol N m⁻² day⁻¹) and anammox activity (3–5 μmol N m⁻² day⁻¹) in melt water from both first-year and multi-year sea ice was found. These values correspond to 27 and 7%, respectively, of the benthic denitrification and anammox activities in Arctic sediments. Although we report only potential denitrification and anammox rates, we present several indications that active denitrification in sea ice may occur in Franklin Bay (and elsewhere): (1) despite sea ice-algal primary production in the lower sea ice layers, heterotrophic activity resulted in net oxygen consumption in the sea ice of 1–3 μmol l⁻¹ sea ice per day at in situ light conditions, suggesting that O₂ depletion may occur prior to the spring bloom. (2) The ample organic carbon (DOC) and NO₃ ⁻ present in sea ice may support an active denitrification population. (3) Measurements of O₂ conditions in melted sea ice cores showed very low bulk concentrations, and in some cases anoxic conditions prevailed. (4) Laboratory studies using planar optodes for measuring the high-resolution two-dimensional O₂ distributions in sea ice confirmed the very dynamic and heterogeneous O₂ distribution in sea ice, displaying a mosaic of microsites of high and low O₂ concentrations. Brine enclosures and channels were strongly O₂ depleted in actively melting sea ice, and anoxic conditions in parts of the brine system would favour anaerobic processes.     

Optimization of polyhydroxybutyrate production by <Emphasis Type="Italic">Bacillus</Emphasis> sp. CFR 256 with corn steep liquor as a nitrogen source

Polyhydroxyalkanotes (PHAs), the eco-friendly biopolymers produced by many bacteria, are gaining importance in curtailing the environmental pollution by replacing the non-biodegradable plastics derived from petroleum. The present study was carried out to economize the polyhydroxybutyrate (PHB) production by optimizing the fermentation medium using corn steep liquor (CSL), a by-product of starch processing industry, as a cheap nitrogen source, by Bacillus sp. CFR 256. Response surface methodology (RSM) was used to optimize the fermentation medium using the variables such as corn steep liquor (5–25 g l⁻¹), Na₂HPO₄ 2H₂O (2.2–6.2 g l⁻¹), KH₂PO₄ (0.5–2.5 g l⁻¹), sucrose (5–55 g l⁻¹) and inoculum concentration (1–25 ml l⁻¹). Central composite rotatable design (CCRD) experiments were carried out to study the complex interactions of the variables. The optimum conditions for maximum PHB production were (g l⁻¹): CSL-25, Na₂HPO₄ 2H₂O-2.2, KH₂PO₄ − 0.5, sucrose − 55 and inoculum − 10 (ml l⁻¹). After 72 h of fermentation, the amount of PHA produced was 8.20 g l⁻¹ (51.20% of dry cell biomass). It is the first report on optimization of fermentation medium using CSL as a nitrogen source, for PHB production by Bacillus sp.     

Biogeochemistry of inter-reef sediments on the northern and central Great Barrier Reef

The deposition and cycling of carbon and nitrogen in carbonate sediments located between coral reefs on the northern and central sections of the Great Barrier Reef were examined. Rates of mass sediment accumulation ranged from 1.9 kg m⁻² year⁻¹ (inshore reefs) to 2.1–4.9 kg m⁻² year⁻¹ (between mid-shelf reefs); sedimentation was minimal off outer-shelf reefs. Rates of total organic carbon decomposition ranged from 1.7 to 11.4 mol C m⁻² year⁻¹ and total nitrogen mineralization ranged from 77 to 438 mmol N m⁻² year⁻¹, declining significantly with distance from land. Sediment organic matter was highly reactive, with mineralization efficiencies ranging from 81 to 99% for organic carbon and 64–100% for nitrogen, with little C and N burial. There was no evidence of carbonate dissolution/precipitation in short-term incubation experiments. Rates of sulfate reduction (range 0–3.4 mmol S m⁻² day⁻¹) and methane release (range 0–12.8 μmol CH₄ m⁻² day⁻¹) were minor or modest pathways of carbon decomposition. Aerobic respiration, estimated by difference between total O₂ consumption and the sum of the other pathways, accounted for 55–98% of total carbon mineralization. Rates of ammonification ranged from 150 to 1,725 μmol NH₄ m⁻² day⁻¹, sufficient to support high rates of denitrification (range 30–2,235 μmol N₂ m⁻² day⁻¹). N₂O release was not detected and rates of NH₄ ⁺ and NO₂ ⁻ + NO₃ ⁻ efflux were low, indicating that most mineralized N was denitrified. The percentage of total N input removed via denitrification averaged ≈75% (range 28–100%) with little regenerated N available for primary producers. Inter-reef environments are therefore significant sites of energy and nutrient flow, especially in spatially complex reef matrices such as the Great Barrier Reef.     

Oxygen and ammonium dynamics during a farming cycle of the bivalve <Emphasis Type="Italic">Tapes philippinarum</Emphasis>

Fluxes of dissolved oxygen and ammonium across the water sediment interface were measured in a control and in an experimental area farmed with the clam Tapes philippinarum. Young clams were seeded in March 2003 at mean (~500 ind m⁻²) and high (~1500 ind m⁻²) densities in a sandy area (2100 m²) of the Sacca di Goro Lagoon, Italy. Approximately every two months, until October 2003, intact sediment cores were collected and incubated in the light and in the dark and surface sediments (0–2 cm) were analysed for organic matter and nitrogen content. Clams farming induced pronounced changes in sediment characteristics and metabolism. Oxygen consumption and ammonium production at the high density area were, on average, 3 to 4 and 1.9 to 4.9 folds higher than those measured in the control field respectively; rates were positively correlated with clams biomass. Experimental fields resulted “Net and Total Heterotrophyc” in 3 out of 4 sampling dates and clams were the major factor shifting the benthic system towards this status. In only one occasion the appearance of the macroalgae Ulva spp. pushed the system rapidly towards hyperautotrophic conditions. Our results indicated that clams have the potential to drive benthic metabolism in farmed areas and to sustain macroalgal growth through regeneration of a limiting nutrient for seawater as inorganic N.     

Allochthonous litter inputs, organic matter standing stocks, and organic seston dynamics in upland Panamanian streams: potential effects of larval amphibians on organic matter dynamics

Allochthonous inputs of detritus represent an important energy source for streams in forested regions, but dynamics of these materials are not well studied in neotropical headwater streams. As part of the tropical amphibian declines in streams (TADS) project, we quantified benthic organic matter standing stocks and organic seston dynamics in four Panamanian headwater streams, two with (pre-amphibian decline) and two without (post-decline) healthy amphibian assemblages. We also measured direct litterfall and lateral litter inputs in two of these streams. Continuous litterfall and monthly benthic samples were collected for 1 year, and seston was collected 1–3 times/month for 1 year at or near baseflow. Direct litterfall was similar between the two streams examined, ranging from 934–1,137 g DM m⁻² y⁻¹. Lateral inputs were lower, ranging from 140–187 g DM m⁻¹ y⁻¹. Dead leaves (57–60%), wood (24–29%), and green leaves (8–9%) contributed most to inputs, and total inputs were generally higher during the rainy season. Annual habitat-weighted benthic organic matter standing stocks ranged from 101–171 g AFDM m⁻² across the four study reaches, with ∼4 × higher values in pools compared to erosional habitats. Total benthic organic matter (BOM) values did not change appreciably with season, but coarse particulate organic matter (CPOM, >1 mm) generally decreased and very fine particulate organic matter (VFPOM, 1.6–250 μm) generally increased during the dry season. Average annual seston concentrations ranged from 0.2–0.6 mg AFDM l⁻¹ (fine seston, <754 μm >250 μm) and 2.0–4.7 mg AFDM l⁻¹ (very fine, <250 μm >1.6 μm), with very fine particles composing 85–92% of total seston. Quality of fine seston particles in the two reaches where tadpoles were present was significantly higher (lower C/N) than the two where tadpoles had been severely reduced (P = 0.0028), suggesting that ongoing amphibian declines in this region are negatively influencing the quality of particles exported from headwaters. Compared to forested streams in other regions, these systems receive relatively high amounts of allochthonous litter inputs but have low in-stream storage. Handling editor: J. Padisak     

Distribution of benthic invertebrates in acid,brown water streams in the South Island of New Zealand

Acid, brown water streams are common on the west coast of the South Island, New Zealand. Acid precipitation is not a significant problem in this region where stream water acidity is brought about by high concentrations of humic substances. The interrelationships between pH, alkalinity, conductivity, DOC and total reactive aluminium were investigated at 45 running water sites. pH (range 3.5–8.1) was strongly correlated with alkalinity (range 0–49 g·m⁻³ CaCO₃) and less strongly with conductivity (range 2.0–9.7 mS·m⁻¹). A strong positive correlation was found between DOC and total reactive aluminium concentration both of which were correlated negatively with pH. In all brown water streams, most aluminium was probably in the non-toxic, organically complexed form. Benthic invertebrate assemblages were examined at 34 sites. Taxonomic richness was not correlated with pH and similar numbers of ephemeropteran, plecopteran and trichopteran taxa were taken from acidobiontic (pH ⩽ 5.5), acidophilic (pH 5.6–6.9) and moderately alkaline (pH ⩾ 7.0) groups of streams. Many species occurred over a wide pH range and had a lower limit of about pH 4.5. The mayfly, Deleatidium occurred at 33 sites and was amongst the five most abundant taxa at 32 of them. The stoneflies, Zelandobius confusus, Austroperla cyrene and Stenoperla maclellani, an elmid, Hydora sp. and a caddisfly, Psilochorema sp. also occurred in over half the streams and frequently were abundant. Few habitat specialists were found. Benthic assemblages were not associated strongly with measured physicochemical factors but streams in close proximity tended to have similar faunas. This suggests that the availability of suitable colonizers sets the limits to species richness and is important in determining the composition of benthic assemblages at a particular locality.     

Evaluation of ammonium and phosphate release from intertidal and subtidal sediments of a shallow coastal lagoon (Ria Formosa – Portugal): a modelling approach

During an annual cycle, overlying water and sediment cores were collected simultaneously at three sites (Tavira, Culatra and Ramalhete) of Ria Formosa’s intertidal muddy and subtidal sandy sediments to determine ammonium, nitrates plus nitrites and phosphate. Organic carbon, nitrogen and phosphorus were also determined in superficial sediments. Ammonium and phosphate dissolved in porewater were positively correlated with temperature (P < 0.01) in muddy and sandy sediments, while the nitrogen-oxidized forms had a negative correlation (P < 0.02) in muddy sediments probably because mineralization and nitrification/denitrification processes vary seasonally. Porewater ammonium profiles evidenced a peak in the top-most muddy sediment (380 μM) suggesting higher mineralization rate when oxygen is more available, while maximum phosphate concentration (113 μM) occurred in the sub-oxic layer probably due to phosphorus desorption under reduced conditions. In organically poor subtidal sandy sediments, nutrient porewater concentrations were always lower than in intertidal muddy sediments, ranging annually from 20 μM to 100 μM for ammonium and from 0.05 μM to 16 μM for phosphate. Nutrient diffusive fluxes predicted by a mathematical model were higher during summer, in both muddy (104 nmol cm⁻² d⁻¹––NH₄⁺; 8 nmol cm⁻² d⁻¹––HPO₄⁻²) and sandy sediments (26 nmol cm⁻² d⁻¹––NH₄⁺; 1 nmol cm⁻² d⁻¹––HPO₄⁻²), while during lower temperature periods these fluxes were 3–4 times lower. Based on simulated nutrient effluxes, the estimated annual amount of ammonium and phosphate exported from intertidal areas was three times higher than that released from subtidal areas (22 ton year⁻¹––NH₄⁺; 2 ton year⁻¹––HPO₄⁻²), emphasizing the importance of tidal flats to maintain the high productivity of the lagoon. Global warming scenarios simulated with the model, revealed that an increase in lagoon water temperature only produces significant variations (P < 0.05) for NH₄⁺ in porewater and consequent diffusive fluxes, what will probably affect the system productivity due to a N/P ratio unbalance.     

A Stable Isotope Tracer Study of the Influences of Adjacent Land Use and Riparian Condition on Fates of Nitrate in Streams

The influence of land use on potential fates of nitrate (NO₃ ⁻) in stream ecosystems, ranging from denitrification to storage in organic matter, has not been documented extensively. Here, we describe the Pacific Northwest component of Lotic Intersite Nitrogen eXperiment, phase II (LINX II) to examine how land-use setting influences fates of NO₃ ⁻ in streams. We used 24 h releases of a stable isotope tracer (¹⁵NO₃-N) in nine streams flowing through forest, agricultural, and urban land uses to quantify NO₃ ⁻ uptake processes. NO₃ ⁻ uptake lengths varied two orders of magnitude (24–4247 m), with uptake rates (6.5–158.1 mg NO₃-N m⁻² day⁻¹) and uptake velocities (0.1–2.3 mm min⁻¹) falling within the ranges measured in other LINX II regions. Denitrification removed 0–7% of added tracer from our streams. In forest streams, 60.4 to 77.0% of the isotope tracer was exported downstream as NO₃ ⁻, with 8.0 to 14.8% stored in wood biofilms, epilithon, fine benthic organic matter, and bryophytes. Agricultural and urban streams with streamside forest buffers displayed hydrologic export and organic matter storage of tracer similar to those measured in forest streams. In agricultural and urban streams with a partial or no riparian buffer, less than 1 to 75% of the tracer was exported downstream; much of the remainder was taken up and stored in autotrophic organic matter components with short N turnover times. Our findings suggest restoration and maintenance of riparian forests can help re-establish the natural range of NO₃ ⁻ uptake processes in human-altered streams.     

Oxygen and nutrient dynamics of the upside down jellyfish (Cassiopea sp.) and its influence on benthic nutrient exchanges and primary production

The oxygen and nutrient dynamics of the zooxanthellate, upside down jellyfish (Cassiopea sp.), were determined both in situ and during laboratory incubations under controlled light conditions. In the laboratory, Cassiopea exhibited a typical Photosynthesis–Irradiance (P–I) curve with photosynthesis increasing linearly with irradiance, until saturation was reached at an irradiance of ~400 μE m⁻² s⁻¹, with photosynthetic compensation (photosynthesis = respiration) being achieved at an irradiance of ~50 μE m⁻² s⁻¹. Under saturating irradiation, gross photosynthesis attained a rate of almost 3.5 mmol O₂ kg WW⁻¹ h⁻¹, whereas the dark respiration rate averaged 0.6 mmol O₂ kg WW⁻¹ h⁻¹. Based upon a period of saturating irradiance of 9 h, the ratio of daily gross photosynthesis to daily respiration was 2.04. Thus, photosynthetic carbon fixation was not only sufficient to meet the carbon demand of respiration, but also to potentially support a growth rate of ~3% per day. During dark incubations Cassiopea was a relatively minor source of inorganic N and P, with the high proportion of NO _X (nitrate + nitrite) produced indicating that the jellyfish were colonised by nitrifying bacteria. Whereas, under saturating irradiance the jellyfish assimilated ammonium, NO _X and phosphate from the bathing water. However, the quantities of inorganic nitrogen assimilated were small by comparison to carbon fixation rates and the jellyfish would need to exploit other sources of nitrogen, such as ingested zooplankton, in order to maintain balanced growth. During in situ incubations the presence of Cassiopea had major effects on benthic oxygen and nutrient dynamics, with jellyfish occupied patches of sediment having 3.6-fold higher oxygen consumption and 4.5-fold higher ammonium regeneration rates than adjacent patches of bare sediment under dark conditions. In contrast at saturating irradiance, jellyfish enhanced benthic photosynthetic oxygen production almost 100-fold compared to the sediment alone and created a small sink for inorganic nutrients, whereas unoccupied sediment patches were sources of inorganic nutrients to the water column. Overall, Cassiopea greatly enhanced the spatial and temporal heterogeneity of benthic fluxes and processes by creating “hotspots” of high activities which switched between being sources or sinks for oxygen and nutrients over diurnal irradiance cycles, as the metabolism of the jellyfish swapped between heterotrophy and net autotrophy.     

Branchial morphology and ion–water transport proteins in Antarctic teleosts with different modes of life

In the Antarctic Ocean salt concentration differs from the bottom to the surface owing to the seasonal forming and melting of sea ice. Antarctic teleosts present different lifestyle from benthic to pelagic. While benthic animals face a constant seawater salinity, benthic–pelagic animals have to face different salt concentration. Branchial morphology and ion–water transport proteins were compared in animals with different lifestyle. The ultrastructure of the gills was investigated by scanning electron microscopy (SEM). Na⁺/K⁺/ATPase, Na⁺/K⁺/Cl⁻ cotransport protein NKCC₁ and Aquaporin 3 (AQP3), were investigated by immunohistochemistry. The immunoreactivity for the ion transporter proteins were more intense in the active benthic–pelagic animals and in the icefishes than in the sluggish benthic ones. Conversely, AQP immunoreactivity was stronger in the animals with sedentary lifestyles. The SEM showed the secondary lamellae in the benthic–pelagic animals more densely packed with the exception of the haemoglobin free teleosts.     

Invertebrate drift and benthic community dynamics in a lowland neotropical stream, Costa Rica

In this study we quantified invertebrate drift and related it to the structure of the benthic community, over a 6–8 month period, in a 4th-order tropical stream in Costa Rica. Relative to reports from similar-sized temperate and tropical streams, drift densities were high (2-fold greater: mean 11.2 m⁻³; range 2.5–25 m⁻³), and benthic insect densities were relatively low (>3-fold lower: mean 890 m⁻²; range 228–1504 m⁻²). Drift was dominated by larval shrimps that represented more than 70% of total drift on any given date; the remaining 30% was composed of 54 insect taxa. Among insects, Simuliidae and Chironomidae (Diptera) and Baetidae, Leptohyphes and Tricorythodes (Ephemeroptera) comprised 24% of total drift. Drift periodicity was strongly nocturnal, with peaks at 18:00 h (sunset) and 03:00 h. Our results, and those of previous experiments in the study stream, suggest that nighttime drift is driven by the presence of predatory diurnal drift-feeding fishes and nocturnal adult shrimps. There were no clear seasonal patterns over both ‘dry’ and wet seasons, suggesting that benthic communities are subject to similar stresses throughout the year, and that populations grow and reproduce continuously. This revised version was published online in July 2006 with corrections to the Cover Date.