Patterns of vertical cyst distribution and survival in 100-year-old sediment archives of three spring dinoflagellate species from the Northern Baltic Sea

The history of expansion of bloom-forming cold water dinoflagellates in the Northern Baltic Sea was studied using 100-year-old sediment archives of their resting cysts. Vertical cyst distributions of Biecheleria baltica and Apocalathium malmogiense, two dinoflagellates indistinguishable by light microscopy and not recognized as distinct species in monitoring, and chain-forming Peridiniella catenata were analysed in Pb²¹⁰ and Cs¹³⁷ dated layers of a sediment core from deep, hypoxic accumulation bottoms of the Gulf of Finland. Cyst profiles showed that B. baltica and A. malmogiense were already present in the Baltic spring phytoplankton community at the beginning of the 20th century. This confirms that B. baltica, which was only recognized in the late 1980s, is a native species in the area. A drastic increase in B. baltica cyst concentrations in the 1930s to 1960s coincided with the acceleration of anthropogenic eutrophication. Large cyst deposits accumulated over several decades in the sediment which, by the 1980s, amounted to the seed stock necessary to inoculate dominant blooms. In the cyst records A. malmogiense always contributed a minor fraction of the two species. P. catenata had a relatively short cyst record in Gulf of Finland sediments despite demonstrated long-term presence in the plankton, which emphasizes that cyst-based historic surveys are not suitable for all cyst-forming dinoflagellates. This was corroborated by correspondence analyses of long-term plankton and cyst records which validated the trends from the sediment archive for B. baltica and A. malmogiense, but failed to do so for P. catenata. Germination experiments with 100-year-old cysts revealed a remarkable long-term survival capacity of A. malmogiense, making this species a suitable model for resurrection studies testing adaptation in heavily impacted systems such as the Baltic Sea.     

Net N input and riverine N export from Illinois agricultural watersheds with and without extensive tile drainage

Some of the largest riverine N fluxes in the continental USA have been observed in agricultural regions with extensive artificial subsurface drainage, commonly called tile drainage. The degree to which high riverine N fluxes in these settings are due to high net N inputs (NNI), greater transport efficiency caused by the drainage systems, or other factors is not known. The objective of this study was to evaluate the role of tile drainage by comparing NNI and riverine N fluxes in regions of Illinois with similar climate and crop production practices but with different intensities of tile drainage. Annual values of NNI between 1940 and 1999 were estimated from county level agricultural production statistics and census estimates of human population. During 1945–1961, riverine nitrate flux in the extensively tile drained region averaged 6.6kgNha^–1year^–1 compared to 1.3 to 3.1kgNha^–1 for the non-tile drained region, even though NNI was greater in the non-tile drained region. During 1977–1997, NNI to the tile-drained region had increased to 27kgNha^–1year^–1 and riverine N flux was approximately 100% of this value. In the non-tile-drained region, NNI was approximately 23kgNha^–1year^–1 and riverine N flux was between 25% and 37% of this value (5 to 9kgNha^–1year^–1). Denitrification is not included in NNI and, therefore, any denitrification losses from tile-drained watersheds must be balanced by other N sources, such as depletion of soil organic N or underestimation of biological N fixation. If denitrification and depletion of soil organic N are significant in these basins, marginal reductions in NNI may have little influence on riverine N flux. If tile drained cropland in Illinois is representative of the estimated 11 million ha of tile drained cropland throughout the Mississippi River Basin, this 16% of the drainage area contributed approximately 30% of the increased nitrate N flux in the Lower Mississippi River that occurred between 1955 and the 1990s.     

The distribution and abundance of krill faecal material and oval pellets in the Scotia and Weddell Seas (Antarctica) and their role in particle flux

Summary The abundance and depth distribution of zooplankton faeces in spring to early summer were investigated along meridional transects (47°W and 49°W) that extended from the Scotia Sea (57°S) across the Weddell-Scotia Confluence and into the Weddell Gyre (62°S). The sea ice edge retreated from 59°30S to 61°S during the study. Faeces were sampled with nets, Niskin bottles and sediment traps and subsequently analysed by light and electron (SEM) microscopy. Krill faecal strings and oval faecal pellets of unknown origin were by far the most important zooplankton faeces and highest concentrations were always found in the Confluence often close to the ice border. Krill faeces were usually more abundant in the uppermost layer (0–50m) where they contributed an average of 130 g dry weight m^–3. There was an exponential decrease with depth, with a minimum of 0.6 g dry weight m^–3 in the 500–1000 m stratum. Oval pellets were more evenly distributed in the upper 1000 m of the water column, with an average of 9 g dry weight m ^–3, although there was a small peak (20 g dry weight m^–3) in the subsurface layer (50–150 m depth). Consecutive collections (day-night) of krill faeces using drifting sediment traps showed that only the larger strings sank from 50 to 150 m depth. Peritrophic membranes appeared to deteriorate during sinking. Diatoms (in particular Nitzschia and Thalassiosira spp.) contributed by far the bulk of material in krill and oval faeces. In samples collected near or under the pack ice, remains of crustaceans in both krill- and oval faeces were also found.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Feeding rate inhibition in crowded Daphnia pulex

Feeding rates of Daphnia pulex fed a range of levels of the alga Chlamydomonas reinhardi of 15 °C are strongly density-dependent. At lower densities, Daphnia (30 1^–1) fed at higher rates than crowded (270 1^–1) Daphnia which manifest a relatively depressed saturation feeding response. At 30 individuals/liter, Daphnia consumed 8.5 – 15.7 × 10⁴ cells d^–1h^–1 (on a volume basis, 12.1 – 22.2 × 10⁶ m³), at 270 L^–1 3.7 – 3.9 × 10⁴ (5.2 – 5.5 = 10⁶ m³ cells d^–1h^–1 when feeding on algae at 80 000 cells ml^–1 (11.3 × 10⁶ m³ ml^–1). The feeding rate data best fit an Ivlev feeding function. An autoallelopath might be causing the repression. Water preconditioned with crowded Daphnia completely repressed feeding in uncrowded Daphnia after six hours.     

Scales of horizontal patchiness in chlorophyll a, chemical and physical properties of landfast sea ice in the Gulf of Finland (Baltic Sea)

Horizontal variation of first-year landfast sea ice properties was studied in the Gulf of Finland, the Baltic Sea. Several scales of variation were considered; a number of arrays with core spacings of 0.2, 2 and 20 m were sampled at different stages of the ice season for small-scale patchiness. Spacing between these arrays was from hundreds of meters to kilometers to study mesoscale variability, and once an onshore–offshore 40-km transect was sampled to study regional scale variability. Measured variables included salinity, stable oxygen isotopes (¹⁸O), chlorophyll a (chl-a), nutrients and dissolved organic carbon. On a large scale, a combination of variations in the under-ice water salinity (ice porosity), nutrient supply and the stage of ice development control the build-up of ice algal biomass. At scales of hundreds of meters to kilometers, there was significant variability in several parameters (salinity, chl-a, snow depth and ice thickness). Analyses of the data from the arrays did not show evidence of significant patchiness at scales <20 m for algal biomass. The results imply that the sampling effort in Baltic Sea ice studies should be concentrated on scales of hundreds of meters to kilometers. Using the variations observed in the study area, the estimate for depth-integrated algal biomass in landfast sea ice in the Gulf of Finland (March 2003) is 5.5±4.4 mg chl-a m^–2.     

Seasonal changes in sediment phosphorus forms in relation to sedimentation and benthic bacterial biomass in Lake Erken

Surficial sediment and sedimenting material were sampled during spring and summer 1991 in Lake Erken. Sediment was analyzed for redox potential, P concentrations and bacterial biomass. Sedimentation and chlorophyll a concentrations of sedimenting matter were determined. Additionally, different phosphorus forms in surficial sediment were quantified using sequential fractionation. The resulting dataset was used to study the effects of sedimentation events following phytoplankton blooms and benthic bacterial biomass on the size of the various phosphorus pools in the sediment.Sedimentation of spring diatoms caused a rapid increase in the NH₄Cl- and NaOH-extractable P (NH₄Cl–P and NaOH–rP) in the sediment. During sedimentation, NaOH–rP and NH₄Cl–P increased within 3 days from 422 ± 17 g g^–1 DW to 537 ± 8.0 g g^–1 DW and from 113 ± 13 g g^–1 DW to 186 ± 26 g g^–1 DW, respectively. The NaOH–nrP (non-reactive P) fraction made up about 17% of Tot-P in sediment samples, whereas NaOH–rP and HCl–P made up 25% each. All P forms showed considerable seasonal variation. Significant relationships were found between bacterial biomass and the NaOH–nrP and NH₄Cl–P fractions in the sediment, respectively. Also regressions of NaOH–nrP and NH₄Cl–P versus the chlorophyll a concentration of sedimenting matter were highly significant. These regressions lend support to the conjecture that NaOH–nrP is a conservative measure of bacterial poly-P.     

Copepod eggs survive a decade in the sediments of the Baltic Sea

Many planktonic calanoid copepod species have been proved to spend a part of their life cycle as benthic resting eggs. In addition to avoiding seasonally occurring unfavourable conditions resting stages may also be used as a long-term survival strategy. The aim of this study was to find out for how long eggs of calanoid copepods retain their viability in the sediments of the Baltic Sea. The occurrence of viable copepod eggs in sea bottom sediment was studied in Pojovik Bay, SW coast of Finland. Eggs were found throughout a 25 cm deep core but deeper than 20 cm they were very scarce. Eggs were incubated at 12 °C in order to check the viability, and their age was estimated by determining the sediment accumulation rate with ¹³⁷Cs-method. Viable eggs were estimated to be 10–13 yrs old, some possibly even 18–19 yrs. Most eggs in the top 8 cm were viable, their age being up to 7–8 yrs. Nauplii that hatched from the eggs belonged to Acartia bifilosa and Eurytemora affinis, A. bifilosa dominating the hatchers of the top sedment layers and E. affinis the deeper layers. Preliminary evidence is presented that E. affinis produces true diapause eggs in the Baltic Sea.     

Phonotaxis in femaleOrmia ochracea (Diptera: Tachinidae), a parasitoid of field crickets

Gravid females of Ormia ochracealocate their hosts by homing on their hosts' calling songs. At Gainesville, Florida, O. ochraceafemales were attracted in greatest numbers to broadcast sounds that simulated the calling song of Gryllus rubens.Other candidate hosts and the attractiveness of their songs relative to the simultaneous song of G. rubenswere G. fultoni(9%), G. integer(4%), G. firmus(3%), Orocharis luteolira(1%), Scapteriscus borellii(1%), and S. vicinus(0%). The response of female O. ochraceato simulated G. rubenssongs that have different pulse rates changes with temperature in parallel with temperature-induced changes in the pulse rate of natural songs. Speaker stations 16 m apart in an apparently uniform environment produced strikingly different fly counts (e.g., 852 and 2163). The song of G. rubensat 21 °C approximates a continuous sequence of 4.6-kHz pulses at a rate of 45 s ^–1 and with a duty cycle of 50%. When two of these parameters were held constant and the third systematically varied in steps of 0.4kHz, 10s ^–1,and 10–20%, maximum attraction occurred at 4.4 kHz, 45 s ^–1,and 20–80%. Omitting as many as half the pulses in a rubenssimulation (e.g., 1, 2, 4, or 16 pulses followed by an equivalent silence, and repeat) did not significantly reduce the counts of O. ochracea,proving that chirping (producing pulses in brief groups) is no safeguard from call-seeking O. ochracea.Phase shifting of pulses in successive chirps sometimes decreased fly counts. When songs were first broadcast, flies came within seconds. Flies that landed at sound often stayed for minutes, even when the sound was turned off.     

Observations on the distribution patterns,behaviour, diets and reproductive cycles of sand-dwelling clinids (Perciformes: Clinidae) from South Africa

Synopsis Twenty-two samples of sand-dwelling fishes were collected from sublittoral sandy substrata off the Cape Peninsula, South Africa using a novel quantitative technique in which the ichthyocide, rotenone, was introduced beneath 6.25 m² plastic sheets weighted around the perimeter with chain. A total of 94 fish of eight species and four families were recorded. Of these, four species of the family Clinidae made up 92% of the material. Fish densities varied from 0.10–2.96 fish m^–2 (0.17–1.14 g m^–2). Distribution patterns amongst the Clinidae were explained by sediment particle size, with all of the newly discovered species, Cancelloxus longior, inhabiting fine sand (median grain size 0.25–0.50 mm), and all Xenopoclinus leprosus, C. elongatus and Pavoclinus smalei occurring in gravel (> 1 mm). Xenopoclinus kochi was found in all sediment types, but was most abundant in coarse substrata. Diets of all four species were similar, consisting chiefly of amphipods and isopods. However, small differences in prey preference were evident. All four species reached sexual maturity at 25–30 mm standard length, were viviparous, exhibited superembryonation and gave birth to live young of approximately 14 mm. Breeding seasonality occurred in all species except P. smalei.     

Improved stability and expression of a recombinant shuttle plasmid in Escherichia coli during fedbatch cultivation

The effect of higher cell densities on the expression and segregational stability of a recombinant E. coli- B. subtilisshuttle plasmid coding for carboxymethylcellulase (CMCase) activity, was studied in E. coli DH5. Of the various feeding policies adopted for maximal expression and stability, exponential feeding resulted in the highest biomass of 15g dry cell weight (DCW) l^–1 and plasmid stability of 45%. A CMCase activity of 11400 Uml^–1 was achieved as compared to 230 Uml^–1 during batch cultivation. In the case of other feeding strategies viz., constant feeding, linear feeding or intermittent feeding, the plasmid stability varied between 20% to 60%. Biomass achieved ranged from 5.0 g DCW l^–1 to 9.0 g DCW l^–1 and enzyme activities were between 2550 Uml^–1 and 6000 Uml^–1.     

Bioturbation experiments in the Venice Lagoon

Short experiments (14–21 days) were carried out during autumn 1998 and spring 1999 at one selected site of the Venice Lagoon to measure bioturbation activities and mixing rates, as well as to obtain quantitative information on benthos functionality. Fluorescent sediment particles (luminophores, 63–350 m) were introduced as pulse inputs at the sediment surface. The concentration–depth profiles of the tracer were simulated with a new advection–diffusion–non local model applied under non-steady state conditions. This allowed the quantification of the mixing parameters associated with different mechanisms: biodiffusion (D _b), bioadvection (W) and non-local mixing (Ke, z₁, z₂). A parameter RS (removed sediment) was also calculated to account for the flux of sediment due to non-local transport. Results show that bioturbation was dominated by biodiffusion in autumn and by bioadvection in spring. Mean mixing parameters Db, W, and RS changed from 3.09 to 0.87 cm² y^–1, from 0.93 to 15.50 y^–1 and from 5.85 to 7.79 g cm^–2 y^–1, respectively.     

Short-term variations in particulate matter sedimentation off Kapp Norvegia,Weddell Sea,Antarctica: relation to water mass advection,ice cover,plankton biomass and feeding activity

Summary A multi-cup sediment trap was deployed at 250m in the shelf area off Kapp Norvegia, Weddell Sea (630 m water depth) to determine the relative importance of water mass advection, sea ice movement, phytoplankton biomass and plankton feeding. Short-term fluctuations in sedimentation were determined using a sampling frequency of 2.7 days over 54 days during January and February 1988. Three periods of enhanced sedimentation were associated with water mass exchange, settling of diatoms following break-up of ice cover and release of fecal matter by krill feeding on particulate matter derived from phytoplankton and ice algae. An initial sedimentation pulse (28 Jan) was mainly due to sinking pelagic diatoms and krill fecal strings containing algae released from sea ice passing over the trap position. The ¹³C-composition of the sedimented organic carbon was about-24. The isotope ratio decreased sharply by about 5.5 at the end of the first pulse indicating the source of sinking matter becoming pelagic diatoms of the retreating ice-edge. At this time the diatom Corethron criophilum contributed a very high proportion of the organic flux causing an increase of the opal/C_org ratios. The second pulse (6 Feb) was due to empty diatom frustules, minipellets and small planktonic aggregates. Much of the organic carbon was transported by round fecal pellets. During the third pulse (14 Feb), round fecal pellets transported even more; the percentage of C. criophilum to the diatom organic carbon flux was more than 80% (>2mg C m^–2 day^–1).Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Ontogeny of vision in the Antarctic fish Pagothenia borchgrevinki (Nototheniidae)

Summary Pagothenia borchgrevinki ranging in size from 63 to 245 mm were captured from beneath sea ice in McMurdo Sound by fishing and diver collection. Changes in ocular morphology with increasing body size were measured, and assessed in relation to ingested prey. Relative eye size was highest among smaller fish (<100mm total length), and declined with increasing fish size. This was accompanied by a decrease in cone density in the retina from a maximum of 14,200 mm^–2 in the smallest fish examined (63 mm), to 1000 mm^–2 in a 220 mm long fish. Theoretical acuity was lowest among fish at either end of the size range examined (minimum separable angle 40–50) but approximately constant over the remainder of the size range (25–40). Rod density also decreased with increasing body size but rod numbers per unit visual arc were relatively constant, except in the smallest fish, where angular rod density was low. The same prey taxa occurred in fish of all sizes; however, prey items smaller than about 1.5 mm were not taken by fish of any body size.     

Abundance,biomass and spatial distribution pattern of brittle stars (Echinodermata: Ophiuroidea) on the Kolbeinsey Ridge north of Iceland

Species composition, abundance, biomass and spatial distribution pattern of brittle star assemblages of the Kolbeinsey Ridge, north of Iceland, were investigated by analysing seafloor photographs and epibenthic sled catches. Sampling was conducted in July 1992 along a 34 km long cross-ridge transect at 67°55N in depths ranging from 830 to 1100 m. Five brittle star species were found. Only one species, Ophiocten gracilis, occurred with densities of more than 1 ind m^–2. Both ridge slopes were characterized by soft bottom habitats, the ophiuroid distribution, however, revealed differences between slopes. Generally, brittle star densities were significantly higher on the eastern slope. There, the highest mean abundance per station of O. gracilis was 497 ind m^–2. Except for one station, a distinct patchiness on the 100 m-scale was obvious in the spatial distribution of O. gracilis. Small settling stages with disc diameters <1 mm accounted for up to 98% of the population of O. gracilis near the ridge top, but their abundance share decreased with depth to only 6% on the eastern slope foot. Biomasses of O. gracilis were extrapolated by combining abundances estimated from photographic counts with a size-weight relationship and size frequencies established from sled catches. The highest value (120 mg AFDW m^–2) was found on the deep eastern slope where large O. gracilis with disc diameters >4 mm were most abundant. Our findings reflect a cross-ridge gradient in terms of hydrographic regime and, hence, probably pattern of food supply for the benthos.     

Secretory capacity of the lachrymal salt gland of hatchling sea turtles,Chelonia mydas

Summary The lachrymal salt glands ofChelonia mydas were functional when hatchlings emerged from the nest. Osmotic concentrations up to 720 mosmol kg^–1 were recorded in spontaneously produced tears (salt gland secretions). When injected with a Na⁺ load (1500–2700 mol (100 g)^–1) newly emerged hatchlings produced tears ranging in osmotic concentration from 1000–1900 mosmol kg^–1 with Na⁺ secretion rates from single glands of 200–475 mol (100 g·h)^–1. In these circumstances the rate of sodium excretion, via the salt glands, was equivalent to the sodium content of 0.2 to 0.5 ml of sea water per hour. Since the apparent drinking rate of hatchlings within the first two days of entering sea water was approximately 0.5 to 1.7 ml per day, the excretion of Na⁺ imbibed by drinking is well within the secretory capacity of the lachrymal salt glands.In feeding hatchlings extraordinarily high Na⁺ secretion rates were induced by Na⁺ loading. Hatchlings which were loaded with Na⁺ by injection (1500–5400 mol (100 g)^–1) produced tears having osmotic concentrations between 1500 and >2000 mosmol kg^–1. The Na⁺ secretion rates from single glands were 750–4185 mol (100 g·h)^–1 with extremely high short term rates of 10700 mol (100 g·h)^–1 (50 mol min^–1 for 28 g hatchlings).In terms of gland mass the highest long term secretion rate translates into 21 mmol of Na⁺ per gram of salt gland per hour and is the highest secretion rate yet recorded for a reptilian salt gland. This rate is almost three times the highest rate recorded for sea snakes (8 mmol g·h^–1) and is similar to rates commonly observed in avian salt glands (25 mmol g·h^–1).Secretion by the lachrymal salt glands was initiated by increased blood concentrations of Na⁺ or K⁺, K⁺ being as effective as Na⁺ but with the composition of the teras being virtually unchanged compared to tears from Na⁺ stimulated hatchlings. Preliminary experiments indicated that secretion was not initiated by increased Cl^– concentration in the blood or by increased volume or osmotic concentration of the blood.Abbreviation O.P. osmotic pressure     

The aboral haemal system of the sea star,Asterias rubens (Echinodermata,Asteroidea): An ultrastructural and histochemical study

Summary The aboral parts of the haemal system of the sea star Asterias rubens are described, based on light and electron microscopy. These parts are (1) the mesenteric strands along the pyloric caeca and the pyloric stomach, (2) the gastric haemal tufts, and (3) the aboral haemal ring. The mesenteric haemal strands are very limited in size and distribution and, therefore, do not seem to have a major function in nutrient translocation. The myoepithelial cells of the gastric haemal tufts have the typical features of choanocytes; their ultrastructural characteristics corroborate the possible absorptive role of the gastric haemal tufts. The myoepithelial cells of the aboral haemal ring often show distinct apical bulbs of cytoplasm suggesting apocrine secretion of PAS-positive materials which are found in the surrounding aboral coelomic ring. These cells contain large stores of particulate glycogen and typical 1–2 m electrondense globules.The ground substance of the haemal tissues, which contains collagen fibers, reticular fibrils, and numerous amoeboid phagocytes, has been analyzed histochemically. Sulfated glycosaminoglycans are almost completely absent; the predominant components are polysaccharides, proteins, and/or glycoproteins. Lipids have not been demonstrated.The possible functions of the haemal tissues and associated coelomic channels are discussed.     

Feeding biology of the Antarctic brittle star Ophionotus victoriae (Echinodermata: Ophiuroidea)

Summary The diet and feeding behavior of the brittle star Ophionotus victoriae were studied in 1981–1983. Frequency-of-occurrence and points methods were used to examine stomach contents of 2475 specimens from 19 stations representing seven general localities along the Antarctic Peninsula and South Sandwich Islands. Results of the two methods were in agreement and indicated this species is an opportunistic generalist with high diet plasticity. The most abundant component of the stomach contents was sediment but the diet derived from at least 13 phyla. Crustaceans, notably euphausiids and amphipods, were most important at some locations. Diet composition varied significantly between locations and years, but did not differ among brittle star size groups. Stomach content data and laboratory studies indicate that O. victoriae utilizes a variety of feeding methods but not suspension feeding.     

In situ primary production in young Antarctic sea ice

An in situ incubation technique used successfully to measure the photosynthetic carbon assimilation of internal algal assemblages within thick multiyear Arctic ice was developed and improved to measure the photosynthetic carbon assimilation within young sea ice only 50 cm thick (Eastern Weddell Sea, Antarctica). The light transmission was improved by the construction of a cylindrical frame instead of using a transparent acrylic-glass barrel. The new device enabled some of the first precise measurements of in situ photosynthetic carbon assimilation in newly formed Antarctic sea ice, which is an important component in the sea ice ecosystem of the Antarctic Ocean. The rates of carbon assimilation of the interior algal assemblage (top to 5 cm from bottom) was 0.25 mg C m^–2 d^–1 whereas the bottom algal community (lowest 5 cm) attained only 0.02 mg C m^–2 d^–1. Chl a specific production rates (P^Chl) for bottom algae (0.020 – 0.056 g C g chl a ^–1 h^–1) revealed strong light limitation, whereas the interior algae (P^Chl = 0.7 – 1.2 g C g chl a ^–1 h^–1) were probably more limited by low temperatures (< –5 °C) and high brine salinities.     

Defense by symbiotic crustacea of host corals elicited by chemical cues from predator

Summary Observations and experiments carried out on a coral reef off the Pacific coast of Panamá demonstrated that shrimp (Alpheus lottini) and crab (Trapezia spp.) symbionts that protect their host coral (Pocillopora elegans) can detect an approaching sea star predator (Acanthaster planci) by chemical cues. Simulated feeding attacks by Acanthaster in sealed transparent bags elicited only 0.5 defensive responses (snipping at spines and tube feet, jerking the sea star, and snapping) per 3 min; defensive behavior increased significantly to 4 and 5 responses, respectively, for Acanthaster in perforated bags and for Acanthaster in direct contact with coral. Neutralized (boiled) Acanthaster elicited only 3 defensive interactions per 3 min compared with 12 interactions for live Acanthaster. Simulated feeding attacks by Oreaster, a non-corallivorous sea star, elicited only 0.5 defensive responses per 3 min, whereas Oreaster introduced with Acanthaster water increased the level of defensive responses to 7. These results suggest that chemical, and to a lesser extent visual (physical presence and movements of sea star), cues stimulate the defensive behavior of the symbiotic crustaceans. The ability to detect a predator at a distance is probably advantageous because in responding only to an actual threat it minimizes the time the defending symbionts spend in an exposed position on the terminal branches of the host coral and because it alerts the crustaceans to sea stars feeding at night.     

Effects of aluminum,iron, oxygen and nitrate additions on phosphorus release from the sediment of a Danish softwater lake

The effects of oxygen, aluminum, iron and nitrate additions on phosphate release from the sediment were evaluated in the softwater Lake Vedsted, Denmark, by a 34-day experiment with undisturbed sediment cores. Six treatments were applied: (1) Control - O₂ (0–20% saturation), (2) O₂ (100% saturation) (3) Al³⁺ – O₂, (4) Fe³⁺ + O₂, (5) Fe³⁺ – O₂, and (6) NO₃ ^– – O₂. Al₂(SO₄)₃*18 H₂O and FeCl₃*4H₂O were added in amounts that theoretically should immobilize the exchangeable P-pool in the top 5 cm of the sediment, while sodium nitrate concentrations were increased to 5 mg N l^–1. The four treatments with metals or NO₃ ^– reduced the P efflux from the sediment significantly as compared to the suboxic control treatment. Mean accumulated P-release rates for suboxic treatments with Al³⁺, Fe³⁺, and NO₃ ^– were: –0.27 mmol m^–2 (st. dev = 0.02 mmol m^–2, N = 5), 0.58 mmol m^–2 (st. dev = 0.30 mmol m^–2, N = 5) and 1.40 mmol m^–2 (st. dev = 0.14 mmol m^–2, N = 5), respectively. The oxic treatment with Fe³⁺ had a P efflux of 0.36 mmol m^–2 (st. dev = 0.08 mmol m^–2, N = 5). The two highest P-release rates were observed in the control treatment and the treatment with O₂ (14.50 mmol m^–2 (st. dev = 3.90 mmol m^–2, N = 5) and 2.31 mmol m^–2 (st. dev = 0.80 mmol m^–2, N = 5), respectively). In order to identify changes in the P and Fe binding sites in the sediment as caused by the treatments, a sequential P extraction procedure was applied on the sediment before and after the efflux experiment. Addition of O₂, Fe³⁺ and NO₃ ^– to the sediment increased the amounts of oxidized Fe³⁺ and P_BD. Al³⁺ addition resulted in a lower fraction of P_BD but a correspondingly higher fraction of Al-bound P. Addition of Al³⁺ decreased the Fe-efflux from the suboxic sediment as well as the amount of oxidized Fe³⁺ in the sediment. This questions the use of Al compounds that contain sulfate because of the possible formation of FeS, which will restrict upward migration of Fe²⁺ and the formation of new Fe-oxides in the surface sediment. Instead, we suggest the use of AlCl₃ for lake restoration purposes.