Photosynthetic responses to Cu exposure are independent of light acclimation and uncoupled from growth inhibition in Fucus serratus (Phaeophyceae)

We have studied the effect of light acclimation on photosynthetic responses and growth during Cu²⁺ exposure (0–0.84 μM) in the brown seaweed Fucus serratus. Measurements of chlorophyll fluorescence parameters showed that Cu²⁺ exposure amplified ETR, reduced the chlorophyll content at the cellular level and that there was no effect of light adaptation on the Cu²⁺ resistance of the algae. In contrast to the inhibitory effects of Cu²⁺ on chlorophyll fluorescence, O₂ evolution and the total content of chlorophyll and carotenoid of the algae was unaffected by Cu²⁺. We conclude that photoinhibition and perhaps pigment degradation in the meristoderm was compensated for by cells deeper in the thallus with the result that the overall photosynthetic fitness of the algae was maintained. The pronounced inhibitory effects of Cu²⁺ on algae growth was not a consequence of photoinhibition and could be attributed to direct inhibitory effects on the growth process.     

Towards a more ecologically relevant assessment of the impact of heavy metals on the photosynthesis of the seagrass,Zostera capricorni

This in situ study used photosynthetic activity (measured as chlorophyll a fluorescence) and photosynthetic pigment concentrations to assess the effect of copper, cadmium, lead and zinc on the seagrass Zostera capricorni. Custom-made portable in situ exposure (PIE) chambers were developed so seagrasses could be dosed within the meadow. Z. capricorni was exposed to 0.1 and 1 mg l⁻¹ of metal solutions for 10 h. During this time and for the subsequent four-day recovery period, the effective quantum yield of photosystem II (PS II) (ΔF/Fm^′) was measured. While the results were variable, copper and zinc exposed samples had a depressed ΔF/Fm^′ during the exposure period. Samples exposed to zinc recovered to pre-exposure levels but those exposed to copper did not. Cadmium and lead did not impact on the chlorophyll a fluorescence and the chlorophyll pigment data supported these findings. This study presents an innovative new application of chlorophyll a fluorescence stress assessment.     

Adaptation to high light irradiances enhances the photosynthetic Cu resistance in Cu tolerant and non-tolerant populations of the brown macroalgae Fucus serratus

The relationship between light acclimation and Cu²⁺ tolerance was studied in two populations of Fucus serratus known to be naturally non-tolerant and tolerant to Cu²⁺. Acclimation to high irradiances increased the photosynthetic tolerance to Cu²⁺. The xanthophyll cycle was apparently not involved in protecting the photosynthetic apparatus against Cu²⁺ toxicity, as results showed that Cu²⁺ did not induce dynamic photoinhibition. The higher photosynthetic Cu²⁺ resistance of high light algae did not result in increased growth. The excess energy acquired by high light-adapted algae appeared to be utilized in Cu²⁺ defense mechanisms in the Cu²⁺ non-tolerant population. The polyphenol content of the algae was reciprocal to the Cu_T content, suggesting that polyphenol may be the primary Cu²⁺ defense of non-tolerant low light algae, acting through secretion and extracellular chelating of Cu²⁺, while the compounds do not seem to be involved in the primary Cu²⁺ tolerance mechanism in Cu²⁺ tolerant algae.     

The Impact of the Herbicide Diuron on Photosynthesis in Three Species of Tropical Seagrass

The impact and recovery from exposure to the herbicide diuron [DCMU; 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea] was assessed for three tropical seagrasses, maintained in outdoor aquaria over a 10-day period. Photosynthetic stress was detected using chlorophyll a fluorescence, measured with a Diving-PAM (pulse amplitude modulated fluorometer). Exposure to 10 and 100 μg l⁻¹ diuron resulted in a decline in effective quantum yield (ΔF/F_m^′) within 2 h of herbicide exposure in Cymodocea serrulata, Halophila ovalis and Zostera capricorni. Effective quantum yield also declined over the first 24 h of exposure in H. ovalis at even lower diuron concentrations (0.1 and 1.0 μg l⁻¹). Effective quantum yield in H. ovalis and Z. capricorni was significantly depressed at all diuron concentrations (0.1–100 μg l⁻¹) after 5 days exposure, whereas effective quantum yield in C. serrulata was only significantly lower in plants exposed to highest diuron concentrations (10 and 100 μg l⁻¹). Effective quantum yield depression was present 5 days after plants exposed to 10 and 100 μg l⁻¹ diuron were returned to fresh seawater. These results indicate that exposure to herbicide concentrations present in nearshore Queensland sediments present a potential risk to seagrass functioning.     

Zn2+浓度对固氮鱼腥藻(Anabaena azotica Ley)光能转化特性的影响

本实验对在不同Zn^2 浓度条件下培养的固氮鱼腥藻（Anabaena azotica Ley）的生长、光合放氧速率和叶绿素荧光参数Fv/Fm进行了测定。结果表明,当Zn^2 浓度为1.0μmol/L时,其比生长速率（Specifie growth rate）最大,光合放氧速率和Fv/Fm值最高,当Zn^2 浓度大于等于5.0μmol/L是会抑制A.azotica Ley的生长和光合作用,对在0μmol/L和5.0μmol/L Zn^2 发生反应的藻蓝蛋白溶液的可见光吸收光谱的分析,发现前者624nm处藻胆体的吸收峰和后者620nm处藻蓝蛋白的吸收峰都因Zn^2 的作用而下降,推测藻蓝蛋白为Zn^2 影响光合作用的位点之一,碳酸酐酶活性的测定表明Zn^2 的浓度水平会影响其活性大小,推测是Zn^2 影响光合作用的另一途径。     

A Simple In Vivo Fluorescence Method for the Selective Detection and Quantification of Freshwater Cyanobacteria and Eukaryotic Algae

A method for the rapid detection of cyanobacteria (blue‐green algae) and their differentiation from eukaryotic algae in natural phytoplankton assemblages is presented. Fluorescence emission of photosynthetic pigments at 670 nm was measured using a microplate fluorescence reader when excited at two different wavelengths – 485 nm and 590 nm. The ratio of fluorescence excited at these wavelengths (590 nm/485 nm) was proportional to the ratio of cyanobacteria and eukaryotic algae, which was determined by the in situ spectrofluorometer for the phytoplankton quantification. The fluorescence intensity was equal to the total chlorophyll‐a content. These two fluorescence values can provide the first warning on a development of potentially toxic cyanobacteria in water.     

Impacts of domestic sewage effluent on phytoplankton from Bedford Basin, eastern Canada

The impact of domestic sewage effluent (SE) on the dynamics of phytoplankton assemblages from Bedford Basin was evaluated in the laboratory. Phytoplankton production and chlorophyll a increased proportionally with SE enrichment. Phytoplankton species composition also changed. The potentially harmful diatoms, Pseudonitzschia spp., present initially in small numbers (600 cells 1⁻¹) in Bedford Basin seawater, and became dominant (3–5×10⁶ cells 1⁻¹) when the seawater was enriched with 0.5–5% untreated SE. With higher proportions of SE, other harmful species such as Fragilaria spp. and Euglena spp. became dominant (7−15×10⁶ and 2.2×10⁴ cells 1⁻¹, respectively). Treatment of SE with UV light or activated charcoal seems to favour growth of benign species, such as Chaetoceros socialis, Skeletonema costatum and Thalassiosira spp., but not harmful species such as Pseudonitzschia spp. Further research on UV treatment of domestic sewage is recommended.     

The Effect of Copper on the Settlement Success of Larvae from the Scleractinian Coral Acropora tenuis

This study examined the effect of copper on the settlement success of planula larvae of the reef-building coral Acropora tenuis during 1994 and 1996 at Magnetic Island, Great Barrier Reef. Copper concentrations of 2, 10, 20 μg l⁻¹ did not inhibit larval settlement after 48-h exposure. However, copper concentrations of 42 μg l⁻¹ and 81 μg l⁻¹ significantly reduced settlement success of A. tenuis larvae after 48-h exposure compared with controls using normal seawater. At 200 μg l⁻¹ copper, all larvae died. EC₅₀ values for the effect of copper on A. tenuis larval settlement were calculated from the 1996 results using measured copper concentrations. The 48-h EC₅₀ was 35 μg l⁻¹ with an upper and lower 95% confidence limit of 37 μg l⁻¹ and 32 μg l⁻¹, respectively. The 48-h NOEC value for both experiments was 20 μg l⁻¹ copper. These experiments provide some of the first data on sub-lethal effects of trace metals on tropical marine organisms, and demonstrate that relatively low copper concentrations impair or inhibit settlement of coral larvae.     

Hydrocarbon and coprostanol levels in seawater, sea-ice algae and sediments near Davis station in eastern Antarctica: A regional survey and preliminary results for a field fuel spill experiment

A survey of hydrocarbons and the sterol coprostanol, together with a hydrocarbon degradation experiment, was conducted in a coastal marine environment in East Antarctica. Aliphatic hydrocarbon levels in sea-ice algae were 1.9–12.5 mg m⁻² and in seawater particulate matter 0.07–0.17 μg l⁻¹. Sea-ice algae contained the diatom biomarker, the highly branched isoprenoid (ip) diene ipC_25:2, and Southern Ocean seawater particulate matter samples were distinguished from near shore samples by the presence of nC_21:6. Sea-ice algae and seawater particulate matter samples showed a predominance of even chain n-alkanes. Hydrocarbon levels in sediment samples from anoxic fjord basins were high (45–48 μg g⁻¹) compared to a sub-tidal marine sample (0.7 μg g⁻¹), and were predominantly of bacterial origin. Contaminants detected were linear alkyl benzenes in sewage effluent from Davis station, and polycyclic aromatic hydrocarbons (PAH) which were present in very low levels (parts per trillion) throughout the environment. High levels of 2,6-dimethylnaphthalene were found in anoxic sediment from Ellis Fjord and may arise from a novel bacterial source. Coprostanol concentrations in sediments ranged from 67 to 1280 ng g⁻¹. A dual origin is proposed from marine mammalian faeces and, at several sites, from conversion of algal-derived sterols by anaerobic bacteria. Future studies examining the impact of human sewage from scientific bases or other ventures should use care in interpreting results when such high baseline values, from marine mammalian input, may occur naturally around the Antarctic coast. The potential exists, however, for the technique to distinguish between human and mammalian inputs through measurement of the coprostanol to epicoprostanol ratio, particularly if undertaken with appropriate comparative sampling. Results for a hydrocarbon degradation experiment where a light fuel was applied to an Antarctic beach, showed loss of up to 99% of the fuel within 2 months, mainly by volatilization.     

Glacier hydrochemistry, solute provenance, and chemical denudation at a surge-type glacier in Kuannersuit Kuussuat, Disko Island, West Greenland

In 1995–1998, Han 11 km terrestrial surge of Kuannersuit Glacier, an outlet glacier of the largest ice cap on Disko Island, West Greenland, affected the catchment dramatically. In order to estimate solute fluxes and provenances, bulk meltwaters were sampled at the main subglacial outlet during the initial part of the quiescent phase. The hydrochemistry is significantly influenced by a subglacial basaltic weathering regime with absence of carbonate minerals. The results show that marine and aerosol derived solutes have minimal contribution to the total ion content, whereas sequestration of atmospheric CO₂ associated with carbonation of Ca-rich feldspar and reactive volcanic glass is more dominant than previously reported from glacierized catchments. Application of a sampling strategy dividing water samples into four groups to determine the content of dissolved HCO₃⁻ and CO₃²⁻ shows that the cationic equivalent weathering rate range is 683–860 Σmeq⁺ m⁻² a⁻¹ and solute flux ranges between 76 and 98 t km⁻² a⁻¹. The crustal denudation rate is estimated to 26 t km⁻² a⁻¹, and the transient CO₂ drawdown amounts to 8500–13700 kg C km⁻² a⁻¹.     

Effects of the herbicide diuron on the early life history stages of coral

The effects of the herbicide diuron on the early life history stages of broadcast spawning and brooding corals were examined in laboratory experiments. Fertilisation of Acropora millepora and Montipora aequituberculata oocytes were not inhibited at diuron concentrations of up to 1000 microg l(-1). Metamorphosis of symbiont-free A. millepora larvae was only significantly inhibited at 300 microg l(-1) diuron. Pocillopora damicornis larvae, which contain symbiotic dinoflagellates, were able to undergo metamorphosis after 24 h exposure to diuron at 1000 microg l(-1). Two-week old P. damicornis recruits on the other hand were as susceptible to diuron as adult colonies, with expulsion of symbiotic dinoflagellates (bleaching) evident at 10 microg l(-1) diuron after 96 h exposure. Reversible metamorphosis was observed at high diuron concentrations, with fully bleached polyps escaping from their skeletons. Pulse amplitude modulation (PAM) chlorophyll fluorescence techniques demonstrated a reduction in photosynthetic efficiency (Delta F/F(m)') in illuminated P. damicornis recruits after a 2 h exposure to 1 microg l(-1) diuron. The dark-adapted quantum yields (F(v)/F(m)) also declined, indicating chronic photoinhibition and damage to photosystem II.     

Mesocosm experiments for evaluating the biological efficacy of ozone treatment of marine ballast water

Ballast water is a major pathway for the transfer of non-indigenous species in aquatic environments. The objectives of this study were to determine the ability of ozone to reduce the numbers of a spectrum of marine organisms collected from Puget Sound, Washington in replicated mesocosm (280 l) experiments, and estimate the minimum ozone concentrations as measured by total residual oxidant (TRO) required to reduce organism densities. Ozone treatment was effective in removing bacteria, phytoplankton, and mesozooplankton with initial TRO concentrations of 2–5 mg l⁻¹ as Br₂. Persistence of TRO resulted in an extended period of toxicity and cumulative mortality. TRO decay allowed bacteria populations to multiply when TRO levels fell below 0.5–1.0 mg l⁻¹ as Br₂. Phytoplankton chlorophyll a concentrations were rapidly reduced by ozone treatment and did not increase in any treatments or controls because of lack of light. Overall mesozooplankton viability was rapidly reduced by 90–99% in treatment TRO levels above 1.85 mg l⁻¹ as Br₂. Our study outlines novel protocols that can be used for testing different potential ballast water treatment systems in replicated and controlled mesocosm experiments.     

Relationships between water availability and Eucalyptus camaldulensis growth in a riparian forest

The effects of short-term flooding on soil water content and subsequent tree response were examined in a riparian Eucalyptus camaldulensis forest which was dissected by a series of shallow ephemeral channels, locally known as runners. Twelve isolated plots, each approximately 0.8 ha, were established in three blocks of four treatments. One of the blocks was underlain by a moist, sandy aquifer 2–4 m below the surface. The four treatments were (1) flooding each spring; (2) flooding each summer; (3) flooding each spring plus each summer; (4) control (zero flooding). Depth of water percolation after a summer flooding varied from 1.3 to over 6 m below the surface. Horizontal movement away from the edge of the floodwater ranged from almost zero on some plots to at least 38 m. The extensive horizontal movement was confined within narrow aquifers which occurred under some plots. Trees in plots underlain by a shallow aquifer always had higher xylem pressure potential (XPP, MPa) than other trees, and flooding these plots increased XPP by a non-significant quantity (−0.14 MPa to −0.12 MPa). However, on the other plots, flooding resulted in a statistically significant increase in XPP from −0.45 to −0.10 MPa. The effect of flooding on XPP was evident for between 22.5 and 37.5 m from the floodwater. This was ascribed to root interception and some horizontal movement of water. Increased flood frequency from zero to one to two per year resulted in mean leaf areas of 11.0 cm², 12.2 cm² and 13.2 cm², respectively. Trees in the runner, at 8 or at 38 m from the channels, had mean leaf areas of 12.9 cm², 13.6 cm² and 9.9 cm², respectively. The presence of shallow aquifers increased mean leaf area from 11.5 to 13.3 cm². Increased flood frequency significantly increased relative growth rate of trees up to 22.5 m from the edge of the floodwater. We conclude that short-term flooding of channels that occupied 15–20% of the forest floor temporarily improved tree moisture status and this increased tree growth rate in up to 70% of the forest.     

Accretion, core formation, H and C evolution of the Earth and Mars

Recent understandings of planetary accretion have suggested that accumulation of a small number of large planetesimals dominates intermediate to final growth stages of the terrestrial planets, with impact velocity high enough to induce extensive melting of the planetesimal and target materials, resulting in formation of a large molten region in which gravitational segregation of silicate and metal, that is, core formation proceeds. In case of homogeneous accretion, volatiles contained in each planetesimal are likely subjected to partitioning among gas, silicate melt, and molten metallic iron at significantly high temperatures and pressures in such a massive molten region. Each phase would subsequently form the proto-atmosphere, -mantle, or-core, respectively. Such chemical reprocessing of H and C associated with core formation, which is followed by both degassing from mantle and atmospheric escape, may result in a diverse range of H₂O/CO₂ in planetary surface environments, which mainly depends on the H and C content relative to metallic iron in planetary building stones. This may explain inferred difference in volatile distribution between the Earth's (relatively H₂O-rich, CO₂-poor) and the martian (H₂O-poor, CO₂-rich) surface environments. Such volatile redistribution may be systematically described by using the retentivity of H₂O, ξ, defined as follows: ξ = 1 − ([CO]⁰ + 2[CH₄]⁰ + 2[C(gr)]⁰)/[H₂O]⁰, where [i]⁰ represents mol number of species i partitioned into non-metallic phases, that is, gas and silicate melt in impact-induced molten region. When ξ > 0.5, relatively H₂O-rich and CO₂-poor surface environment may eventually evolve, although a small portion of H₂O partitioned into the NON=metallic phases are possibly consumed by subsequent chemical reactions with reduced C-species with producing CO₂ and H₂. When ξ< 0.5, on the contrary, H₂O consumption by the above reactions and selective loss of H₂ to space may result in relative H₂O-depleted and CO₂-rich surface environment. Given the building stone composition by the two-component model by Ringwood (1977) and Wänke (1981), ξ is found to decrease with increasing the mixing fraction of the volatile-rich component: ξ > 0.5 for the mixing fraction smaller than about 15–20% and ξ < 0 for the mixing fraction larger than about 20–30%. This is not significantly dependent on temperature and pressure in molten region and H/C ratio in the building stone. The estimated mixing fraction of the volatile-rich component, about 10% for the Earth and 35% for Mars, is consistent with the observed difference in volatile distribution between the surfaces of both planets.     

Intensifying groundwater acidification at Birkenes, southern Norway

Groundwater chemical data from Birkenes, southern Norway, collected during the period October 1980 to November 1993, reveal intensifying acidification in the 1990s, as evidenced by decreases in pH, acid-neutralising capacity and alkalinity, and increases in hardness/alkalinity ratio, ‘acidification’, nitrate, non-marine sulphate (SO₄^*), non-marine hardness (Ca^* + Mg^*) and dissolved aluminium. The whole monitoring period is characterised by slopes of four or more on a plot of (Ca^* + Mg^*) vs. alkalinity.

Owing to its proximity to the sea, the Birkenes catchment receives seasalt-influenced precipitation, which results in episodic, natural acidification of the groundwater via cation exchange of marine Na⁺ with soil-bound H⁺ and/or Al³⁺. However, it is uncertain whether all of the recent groundwater acidification can be attributed to intensifying seasalt deposition alone: the steep slopes on the (Ca^* + Mg^*) vs. alkalinity plot and the increase in groundwater SO₄^* suggest that strong acids, of possible anthropogenic origin, may be involved. Additionally, seasalt deposition appears not to have increased during the 1990s: Cl⁻ content in precipitation has not increased significantly and river water pH has not decreased significantly over the period 1990–1993. The suggestion is made that the observed intensification in groundwater acidification at Birkenes partly results from the exhaustion or weakening of an acid buffering system caused by soil acidification, under persisting, even if abating, anthropogenic acid loading.     

Correlation of magnetic properties with Δ(Sr) of igneous rocks from the northeast Pacific and Gulf of California

The magnetic properties of basalts and diabases recovered under Legs 63 and 64 of the Deep Sea Drilling Project are summarized. They are first correlated with the measured grain sizes of the opaque minerals and the overall alteration states of the rocks, the latter measured by conventional chemical parameters such as water content, total volatiles content, and Fe₂O₃/FeO ratio. It is shown, however, that the decrease in ⁸⁷Sr/⁸⁶Sr upon leaching (Δ(^87/86Sr)) is perhaps the best quantifier of the overall alteration state of the whole rocks as well as of the degree of low-temperature oxidation suffered by the titanomagnetites. It is well correlated with Curie-point variations caused by changes in the Fe³⁺/Fe²⁺ ratio.     

Sublethal effects of cupric ion activity on the grazing behaviour of three calanoid copepods

This research (1) characterized the effects of sublethal cupric ion activities on the grazing behavior of two estuarine copepods (Acartia tonsa, Acartia hudsonica) and one nearshore, neritic copepod (Temora longicornis) and (2) compared the sensitivity of short-term sublethal behavioral assays with that of longer-term acute toxicity tests. A nitrilotriacetate-trace-metal-ion buffer system at 27‰. S was used to quantify and control the free cupric ion activity. Acute toxicity tests were used to determine the mortality of A. tonsa and T. longicornis over 72 h within the approximate cupric ion activity range of 10⁻¹³ to 10^−9.5 M. 24 h survival was not affected within the approximate cupric ion activity range of 10⁻¹³ to 10^−9.7 M, the range used for subsequent grazing activity experiments after 24 h exposure to Cu. Grazing activity was significantly diminished at cupric ion activities of ≈ 10⁻¹⁰ M for A. tonsa and T. longicornis, and at ≈ 10⁻¹¹ M for A. hudsonica. A hormetic pattern of response in feeding activity was observed with A. tonsa and T. longicornis. Grazing activity was found to be a sensitive measure of sublethal Cu stress compared with the acute toxicity tests. Grazing activity was affected at environmentally relevant cupric ion activities.     

Effects of some environmental parameters on the reproduction and development of a tropical marine harpacticoid copepod Nitocra affinis f. californica Lang

The effects of salinity, temperature, and light conditions on the reproduction and development of harpacticoid copepod, Nitocra affinis f. californica under controlled laboratory conditions were determined. Seven different salinity levels (5, 10, 15, 20, 25, 30, 35 ppt), four temperatures (20, 25, 30, 35 °C), three different light intensities (25, 56, 130 μmol m⁻² s⁻¹) and photoperiods (24 h:0 h, 1 h:23 h, 12 h:12 h LD cycle) were employed in this study. The highest (p < 0.05) overall reproduction and fastest development time were achieved by copepods reared under 30–35 ppt salinity. The optimum temperature required for the maximum reproduction was 30 °C while under 30 °C and 35 °C the copepod development time was shortest (p < 0.05) compared to other temperature levels. The overall reproduction was highest (p < 0.05) and development rate of N. affinis was shortest (p < 0.05) under lowest light intensity (25 μmol m⁻² s⁻¹). Continuous light (24 h:0 h LD) inhibited the egg production while, continuous darkness (1 h:23 h LD) and 12 h:12 h LD significantly favoured the overall reproductive activity of the female. Photoperiods 1 h:23 h and 12 h:12 h LD yielded highest total (p < 0.05) offspring female⁻¹ coupled with highest (p < 0.05) survival percentage. This study illustrated that although N. affinis can tolerate wide range of environmental conditions, prolonged exposure to subnormal environments affect its reproduction and development. This study showed that this species can be mass cultured for commercial purposes and has a potential to be used for toxicity studies due to its high reproductive performance fast development and a wide range of tolerance to environmental conditions.     

Use of statistical analysis to formulate conceptual models of geochemical behavior: water chemical data from the Botucatu aquifer in São Paulo state, Brazil

Water chemical data from the Botucatu Sandstone aquifer in the São Paulo State part of the Paraná Basin, Brazil, was evaluated using geochemical methods and two statistical analyses: cluster analysis and factor analysis. The results were used to develop a conceptual geochemical model, in which three geochemical regions were identified, and their chemical behavior was modeled. The characteristic chemicals, changing from the recharge area to the center of the basin, are: SiO₂—(HCO₃⁻ and Ca²⁺)—(Na⁺, CO₃²⁻, and SO₄²⁻). The distribution of the chemicals is interpreted as controlled by different water–rock interaction processes in the different regions. In the recharge area, dissolution of alkali–feldspar minerals in the sandstone is the main reaction observed; in the mid-section of the basin, calcite dissolution results in high calcium and bicarbonate concentration; in the center of the basin, leakage from underlying layers becomes the governing factor.     

Hydrogen peroxide production rates in clean and polluted coastal marine waters of the Mediterranean, Red and Baltic Seas

The main aim of this study was to assess the hydrogen peroxide (HP) production rates (HPPR) related to anthropogenic pollution in coastal waters by laboratory and field experiments. HPPR's were assessed by simultaneous measurements of HP concentrations, cumulative solar UV irradiation and dissolved organic matter (DOM) fluorescence in the seawater samples at clean and polluted sites in the Mediterranean, Red and the Baltic Seas. The natural HP concentrations at all sites (8–100 nM) fall within the normal range recorded elsewhere, and follows a diurnal pattern. The polluted stations in the Mediterranean and Baltic Seas showed higher HPPR (3.2–16.6 nM m² W⁻¹ h⁻¹) than the clean stations, while in the Red Sea no significant differences were found because the station that was considered a priori polluted was actually rather clean (2–3 nM m² W⁻¹ h⁻¹). Laboratory experiments demonstrated that HPPR was positively linearly correlated to DOM fluorescence, however, this relationship was not found in the natural seawater samples examined in the field. The lack of relationship between HPPR and DOM in the field was attributed to enzymatic breakdown of HP as observed in dark decay experiments. HP dark decay rates were highest in polluted stations, probably due to larger bacterioplankton populations in these samples. Moreover, the HP dark decay rates were much lower in filtered than in non-filtered samples. Sun incubation of filtered (0.2 μm) seawater samples were performed to assess whether elimination of particles would yield higher HPPRs than in non-filtered samples. In the Baltic Sea (low UV irradiation) HPPR was higher in the filtered samples, while at sites with relatively high UV irradiation (Mediterranean and Red Seas), filtration of seawater did not stimulate HPPRs, probably due to UV-damage to planktonic microorganisms.