Annual Variation of Environmental Variables, Phytoplankton Species Composition and Photosynthetic Parameters in a Coastal Lagoon

Physical–chemical variables, phytoplankton biomass, speciescomposition and photosynthesis–irradiance (P-I) parameterswere analysed during 1 year in the Santo André Lagoon,SouthwestPortugal – a land-locked coastal ecosystem withtemporary connections with the sea. When the lagoon stayed closedthe observed phytoplankton blooms were mainly caused by Prorocentrumminimum, a potentially toxic dinoflagellate. It was dominantduring most of the year but the seawater inflow to the lagoontriggered a decrease in phytoplankton biomass and an abruptshift in species composition. The maximum photosynthetic rate(P_max) ranged from 2.0 to 22.5 mg C (mg chlorophyll a)^–1h^–1 and the light saturation index (I_k), ranged from 5.2to 335.0 µE m^–2 s^–1, with winter minima andsummer maxima. P_max and I_k were both positively correlated totemperature. Abundance ofP. minimum was associated with highnitrate concentrations whereas diatoms appear when ammonium,salinity and wind velocity are high. A mathematical model todescribe photosynthetic rate as a function of irradiance andtemperature [P (I, t)] was applied to the samples in which P.minimum was the dominant species     

The Relationship between Phytoplankton Diversity and Community Function in a Coastal Lagoon

The decrease of biodiversity related to the phenomena of global climate change is stimulating the scientific community towards a better understanding of the relationships between biodiversity and ecosystem functioning. In ecosystems where marked biodiversity changes occur at seasonal time scales, it is easier to relate them with ecosystem functioning. The objective of this work is to analyse the relationship between phytoplankton diversity and primary production in St. André coastal lagoon – SW Portugal. This lagoon is artificially opened to the sea every year in early spring, exhibiting a shift from a marine dominated to a low salinity ecosystem in winter. Data on salinity, temperature, nutrients, phytoplankton species composition, chlorophyll a (Chl a) concentration and primary production were analysed over a year. Modelling studies based on production-irradiance curves were also conducted. A total of 19 taxa were identified among diatoms, dinoflagellates and euglenophyceans, the less abundant group. Lowest diversities (Shannon–Wiener index) were observed just before the opening to the sea. Results show a negative correlation (p<0.05) between diversity and chlorophyll a (Chl a) concentration (0.2–40.3 mg Chl a m⁻³). Higher Chl a values corresponded to periods when the community was dominated by the dinoflagellate Prorocentrum minimum (>90% of cell abundance) and production was maximal (up to 234.8 mg C m⁻³ h⁻¹). Maximal photosynthetic rates (P_max) (2.0–22.5 mg C mg Chl a⁻¹ h⁻¹) were higher under lower Chl a concentrations. The results of this work suggest that decreases in diversity are associated with increases in biomass and production, whereas increases correspond to opposite trends. It is suggested that these trends, contrary to those observed in terrestrial and in some benthic ecosystems, may be a result of low habitat diversity in the water column and resulting competitive pressure. The occurrence of the highest photosynthetic rates when Chl a is low, under some of the highest diversities, suggests a more efficient use of irradiance under low biomass–high diversity conditions. Results suggest that this increased efficiency is not explained by potential reductions in nutrient limitation and intraspecific competition under lower biomasses and may be a result of niche complementarity.     

Approaching a functional measure of vulnerability in marine ecosystems

Ecosystem vulnerability is a major concern for management purposes, especially when directed toward conservation and sustainable exploitation. We estimate the relative vulnerability of selected marine-ecosystems in the Gulf of Mexico through simulation experiments based on trophic models. The same perturbation pattern was applied to different functional groups at different trophic levels. Perturbation consisted of increasing biomass extraction for a single group up to 98% at a constant rate over 50 years. The ratio Ascendency to Capacity of Development, A/C, was estimated as a measure of ecosystem order. The maximum negative difference respect to the initial A/C represents the gain of entropy. The slope of the relationship between entropy gained and the trophic level provides an estimate of the relative vulnerability of the ecosystem. This was applied to five ecosystems in the Gulf of Mexico: Florida coral reef; Mexican coastal lagoon, Terminos Lagoon; and three continental shelves, the northern Gulf of Mexico, USA; Yucatan and the Campeche Sound, Mexico. The pattern of vulnerability among ecosystems is related to ecosystem complexity. The coral reef exhibited a lower slope, corresponding to higher vulnerability, which is related to higher connectivity, production efficiency, and net ecosystem production. Increasingly higher slopes, corresponding to lower vulnerability, followed a gradient from the coral reef to the continental shelves to the least vulnerable system, the coastal lagoon. Middle trophic levels contribute to higher vulnerability. This interpretation is supported by the concept of energy flows within trophic networks. The relevance of these findings for management is discussed.     

Variability of photosynthesis-irradiance curves and ecosystem respiration in a small river

• 1 We investigated photosynthesis‐irradiance relationships (P‐I curves; P = oxygen production rate due to photosynthesis, I = light irradiance rate at the water surface) and ecosystem respiration in a 9 km long reach of a river that is characterised by light conditions favouring primary production, high ambient nutrient concentrations, a high re‐aeration rate, and frequent spates. We addressed the question of how disturbances (spates) and season influence photosynthesis and ecosystem respiration.
• 2 We used an oxygen mass‐balance model of the river to identify ecosystem respiration rates and the two parameters of a hyperbolic P‐I function (P_max = maximum oxygen production rate due to photosynthesis, α = the initial slope of the P‐I function). The model was fitted to dissolved oxygen concentrations quasi‐continuously recorded at the end of the reach. We estimated parameters for 137 three‐day periods (during the years 1992–97) and subsequently explored the potential influence of season and disturbances (spates) on P_max, α and ecosystem respiration using stepwise regression analysis.
• 3 Photosynthesis‐irradiance relationships and ecosystem respiration were subject to distinct seasonal variation. Only a minor portion of the variability of P‐I curves could be attributed to disturbance (spates), while ecosystem respiration did not correlate with disturbance related parameters. Regular seasonal variation in photosynthesis and ecosystem respiration apparently prevailed due to the absence of severe disturbances (a lack of significant bedload transport during high flow).

     

Multiannual phytoplankton trends in relation to environmental changes across aquatic domains: A case study from Sardinia (Mediterranean Sea)

We investigated multiannual trends in phytoplankton in relation to several environmental drivers. We analyzed ecological data collected during the past two decades from three aquatic sites: an artificial lake, a coastal lagoon, and a marine coastal area. Hydrographic, nutrient, and phytoplankton data were statistically analyzed to detect trends and interactions. In all ecosystems, the chlorophyll a concentration decreased with increasing abundance of small-sized phytoplankton. Phytoplankton dynamics were related to decreased nutrient concentrations in the lake, to dynamics of phosphorus and decreased salinity in the lagoon, and probably to combined top-down and bottom-up processes in the marine gulf.     

Characterisation of a Tunisian coastal lagoon through hyperspectral underwater irradiance

North African coastal lagoons are unique ecosystems that often suffer degradation due to human activities. Therefore, monitoring methods are required to identify stressors and assist with the management of these valuable and often understudied ecosystems. A synthetic indicator of water ecological quality would be desirable for regular monitoring of these ecosystems under pressure. In 2008 an optical procedure was developed and applied in Ghar El Melh, a Tunisian lagoon which has been increasingly impacted by pollutant loading, especially from agriculture. In situ hyperspectral irradiance was measured at several stations, from which the apparent optical properties (AOPs), namely the irradiance attenuation coefficient K(λ) and the reflectance ratio R(λ), were obtained in order to relate them to water composition, in terms of light-attenuating substances (LASs). The significant relationships observed between R and LAS values enabled the application of a hyperspectral optical classification, which effectively highlighted threatened sectors of the lagoon. The pattern of differing water quality across the lagoon system that was derived from the hyperspectral classification agreed well with that obtained from a conventional optical classification that included AOPs and LASs. We suggest that hyperspectral analysis and classification is a useful monitoring tool for the assessment of change in coastal lagoons, and perhaps also in other shallow-water ecosystems.     

Ecosystem services transcend boundaries: estuaries provide resource subsidies and influence functional diversity in coastal benthic communities

Background

Estuaries are highly productive ecosystems that can export organic matter to coastal seas (the ‘outwelling hypothesis’). However the role of this food resource subsidy on coastal ecosystem functioning has not been examined.

Methodology/Principal Findings

We investigated the influence of estuarine primary production as a resource subsidy and the influence of estuaries on biodiversity and ecosystem functioning in coastal mollusk-dominated sediment communities. Stable isotope values (δ¹³C, δ¹⁵N) demonstrated that estuarine primary production was exported to the adjacent coast and contributed to secondary production up to 4 km from the estuary mouth. Further, isotope signatures of suspension feeding bivalves on the adjacent coast (Dosinia subrosea) closely mirrored the isotope values of the dominant bivalves inside the estuaries (Austrovenus stutchburyi), indicating utilization of similar organic matter sources. However, the food subsidies varied between estuaries; with estuarine suspended particulate organic matter (SPOM) dominant at Tairua estuary, while seagrass and fringing vegetation detritus was proportionately more important at Whangapoua estuary, with lesser contributions of estuarine SPOM. Distance from the estuary mouth and the size and density of large bivalves (Dosinia spp.) had a significant influence on the composition of biological traits in the coastal macrobenthic communities, signaling the potential influence of these spatial subsidies on ecosystem functioning.

Conclusions/Significance

Our study demonstrated that the locations where ecosystem services like productivity are generated are not necessarily where the services are utilized. Further, we identified indirect positive effects of the nutrient subsidies on biodiversity (the estuarine subsidies influenced the bivalves, which in turn affected the diversity and functional trait composition of the coastal sediment macrofaunal communities). These findings highlight the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystems.     

Seasonal variation in light-shade adaptation of natural populations of the symbiotic sea anemone Aiptasiapulchella (Carlgren, 1943) in Hawaii

The productivity and biomass parameters of the symbiotic anemone Aiptasia pulchella (Carlgren, 1943) from a shaded mangrove lagoon (maximum summer irradiance of 100 μE m⁻² · s⁻¹) and a sunlit reef flat (maximum summer irradiance of 1400 μE · m⁻² · s⁻¹) were examined in Hawaii. Light-shade adaptation was evident in the summer populations (1981) but not observed during the fall (1982). In the summer, zooxanthellae from the lagoon A. pulchella (shade anemones) contained 2.97 pg Chl a cell ⁻¹ and those from the reef flat (sun anemones) contained 1.70 pg Chl a · cell⁻¹; but Chl a : c₂ ratios were 2.5 in zooxanthellae from both shade and sun anemones. During the fall, there were no significant differences in Chl a and c₂ of zooxanthellae (2.25 pg Chl a · cell⁻¹) in shade and sun anemones, but Chl a : c₂ ratios averaged 3.9. During both seasons, shade anemones were larger and contained higher densities of zooxanthellae than sun anemones. In addition to differences between shade and sun habitats, there was localized photoadaptation of zooxanthellae within individual anemones due to microhabitat variations in ambient irradiance. Growth rates of zooxanthellae in A. pulchella differed in shade and sun anemones. Specific growth rates for zooxanthellae in situ were the same for shade populations in both summer and fall (0.016 day⁻¹). However, zooxanthellae in sun anemones grew four times faster in the fall (0.033 day⁻¹) than during the summer (0.008 day⁻¹). These results suggest that growth of zooxanthellae in these anemones was independent of ambient irradiance. Photosynthesis-irradiance (P-I) responses of shade and sun anemones during the summer showed that shade anemones had greater photosynthetic efficiencies (α) but lower photosynthetic capacities (P_max) than sun anemones. Dark-respiration rates of sun anemones were twice those obtained with shade anemones. In the fall, these populations of anemones did not exhibit P-I responses characteristic of light-shade adaptation. Both α and P_max of shade and sun anemones were higher in the fall, indicating that zooxanthellae in A. pulchella adapted to seasonal reduction in irradiance.     

Benthic Primary Production Budget of a Caribbean Reef Lagoon (Puerto Morelos,Mexico)

High photosynthetic benthic primary production (P) represents a key ecosystem service provided by tropical coral reef systems. However, benthic P budgets of specific ecosystem compartments such as macrophyte-dominated reef lagoons are still scarce. To address this, we quantified individual and lagoon-wide net (P_n) and gross (P_g) primary production by all dominant functional groups of benthic primary producers in a typical macrophyte-dominated Caribbean reef lagoon near Puerto Morelos (Mexico) via measurement of O₂ fluxes in incubation experiments. The photosynthetically active 3D lagoon surface area was quantified using conversion factors to allow extrapolation to lagoon-wide P budgets. Findings revealed that lagoon 2D benthic cover was primarily composed of sand-associated microphytobenthos (40%), seagrasses (29%) and macroalgae (27%), while seagrasses dominated the lagoon 3D surface area (84%). Individual P_g was highest for macroalgae and scleractinian corals (87 and 86 mmol O₂ m⁻² specimen area d⁻¹, respectively), however seagrasses contributed highest (59%) to the lagoon-wide P_g. Macroalgae exhibited highest individual P_n rates, but seagrasses generated the largest fraction (51%) of lagoon-wide P_n. Individual R was highest for scleractinian corals and macroalgae, whereas seagrasses again provided the major lagoon-wide share (68%). These findings characterise the investigated lagoon as a net autotrophic coral reef ecosystem compartment revealing similar P compared to other macrophyte-dominated coastal environments such as seagrass meadows and macroalgae beds. Further, high lagoon-wide P (P_g: 488 and P_n: 181 mmol O₂ m⁻² lagoon area d⁻¹) and overall P_g:R (1.6) indicate substantial benthic excess production within the Puerto Morelos reef lagoon and suggest the export of newly synthesised organic matter to surrounding ecosystems.     

Dynamics of Phytoplankton Communities Under Photoinhibition

We analyzed a model of phytoplankton competition for light in a well-mixed water column. The model, proposed by Gerla et al. (Oikos 120:519–527, 2011), assumed inhibition of photosynthesis at high irradiance (photoinhibition). We described the global behavior through mathematical analyses, providing a general solution to the multi-species competition for light with photoinhibition. We classified outcomes of 2- and 3-species competitions as examples, and evaluated feasibility of the theoretical predictions using empirical relationships between photosynthetic production and irradiance. Numerical simulations with published p–I curves indicate that photoinhibition may often lead to strong Allee effects and competitive facilitation among species. Hence, our results suggest that photoinhibition may play a major role in organizing phytoplankton communities.     

Iron limitation of phytoplankton in an urbanized vs. forested southeastern U.S. salt marsh estuary

Over the last two decades, escalating rates of coastal development have altered macro- and micronutrient loading patterns to many southeastern U.S. estuaries. This study addresses a potential consequence of urbanization-associated coastal deforestation that may have important implications for estuarine ecosystem productivity: a reduction in bioavailable Fe. We compared the potential for Fe limitation at representative sites in two neighboring South Carolina salt marsh estuaries, one (“Parsonage Creek” site in Murrells Inlet estuary) impacted by urbanization-associated clear-cutting, and the other (“Oyster Landing” site in North Inlet estuary) undeveloped and surrounded by forests. The urbanized estuarine site was marked by lower dissolved Fe concentrations compared to the forested estuarine site. In bioassay experiments conducted from 1996 to 1999, the addition of chelated Fe to natural phytoplankton populations stimulated chlorophyll a concentrations in water from Parsonage Creek on 6 of 11 sampling dates. Chlorophyll responses in Oyster Landing water also were observed on two of those sampling times, dates that followed periods of relatively low rainfall. In one experiment, the addition of Fe+NO₃⁻ to Parsonage Creek water led to significantly higher chlorophyll concentrations than when NO₃⁻ was added alone, but Fe+NH₄⁺ addition did not result in higher chlorophyll than NH₄⁺ addition alone. It was hypothesized that the difference in NO₃⁻ and NH₄⁺ responses was related to a greater tendency towards Si depletion in Fe-stressed NO₃⁻-grown samples, or alternatively, to the higher Fe requirement for NO₃⁻ assimilation relative to that needed for NH₄⁺ assimilation. In bioassay experiments using Fe-deplete semi-continuous cultures of Synechococcus WH8101, Fe addition did not affect the net growth rate of cultures transferred to water from either estuary, but increased the chlorophyll a content of cells transferred to Parsonage Creek water. Finally, photosynthesis vs. irradiance parameters (α, P_max) obtained from Oyster Landing samples (<2 μm size fraction) were similar to those measured in Fe-replete Synechococcus continuous cultures, while the same parameters derived from Parsonage Creek samples were much lower and comparable to those from Fe-deplete cultures. The results indicate that phytoplankton population growth from high salinity salt marsh estuaries can be Fe-limited. Also, the potential for Fe limitation was more prevalent in a urbanized deforested estuary than in an undeveloped forested estuary, consistent with the hypothesis that organically bound Fe from coastal forests plays an important role in supplying Fe for the growth of estuarine phytoplankton.     

Effects of light and nutrients on seasonal phytoplankton succession in a temperate eutrophic coastal lagoon

Rodeo Lagoon, a low-salinity coastal lagoon in the Golden Gate National Recreation Area, California, United States, has been identified as an important ecosystem due to the presence of the endangered goby (Eucyclogobius newberri). Despite low anthropogenic impacts, the lagoon exhibits eutrophic conditions and supports annual episodes of very high phytoplankton biomass. Weekly assessments (February–December 2007) of phytoplankton indicated diatoms, Nodularia spumigena, Chaetoceros muelleri var. muelleri, flagellated protozoa, a mixed assemblage, and Microcystis aeruginosa dominated the algal community in successive waves. Phytoplankton succession was significantly correlated (r ² = 0.37, p < 0.001) with averaged daily irradiance (max = 29.7 kW m⁻² d⁻¹), water column light attenuation (max = 14 m⁻¹), and orthophosphate and dissolved inorganic carbon concentrations (max = 1.5 and 2920 μM, respectively). Negative effects of phytoplankton growth and decay included excessive ammonia concentrations (exceeded EPA guidelines on 77% of sampling days), hypoxia (<3 mg l⁻¹ dissolved oxygen), and introduction of several microcystins, all in the latter half of the year. Our one-year study suggests that this coastal lagoon is a highly seasonal system with strong feedbacks between phytoplankton and geochemical processes.     

Annual patterns of phytoplankton density and primary production in a large, shallow lake: the central role of light

1. We studied the seasonal dynamics of suspended particulate matter in a turbid, large shallow lake during an annual period (2005–06). We relate the patterns of seston concentration (total suspended solids), phytoplankton biomass and water transparency to the seasonal pattern of incident solar radiation (I₀). We also report the seasonal trends of phytoplankton primary production (PP) and photosynthesis photoinhibition due to photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) (I_β and UV₅₀). 2. We first collected empirical evidence that indicated the conditions of light limitation persisted during the study period. We found that the depth‐averaged irradiance estimated for the time of the day of maximum irradiance (I_mean–noon) was always lower than the measured onset of light saturation of photosynthesis (I_k). 3. We then contrasted the observations with theoretical expectations based on a light limitation scenario. The observed temporal patterns of seston concentration, both on a volume and area basis, were significantly explained by I₀ (R² = 0.39 and R² = 0.37 respectively). The vertical diffuse attenuation coefficient (kd_PAR) (R² = 0.55) and the depth‐averaged irradiance (I_mean) (R² = 0.66), significantly increased with the I₀; while the irradiance reaching the lake bottom (I_out) significantly decreased with the incident irradiance (R² = 0.49). However, phytoplankton biovolume maxima were not coincident with the time of the year of maximum irradiance. 4. A significant positive relationship was observed between PP estimated on an area basis and I₀ (R² = 0.51, P < 0.001). In addition, the parameters describing the photosynthetic responses to high irradiances displayed marked seasonal trends. The photosynthesis photoinhibition due to PAR as well as to UV were significantly related to incident solar radiation (PAR: R² = 0.73; UV: R² = 0.74). These results suggest adaptation of the phytoplankton community in response to changes in incident solar radiation.     

Herbicide-induced macrophyte-to-phytoplankton shifts in Japanese lagoons during the last 50?years: consequences for ecosystem services and fisheries

As expressed in the Ramsar Convention, the wise use of wetlands, including coastal lagoons, is one of the greatest environmental concerns across the globe. Seagrass beds are the key element for sustaining coastal lagoon ecosystems. Eutrophication, especially by nitrogen and phosphorus, has degraded many coastal waters and has been invoked as a major cause of seagrass disappearance worldwide. Seagrass had been used as fertilizer in Japan, and the removal of seagrass contributed to the decline of nutrients in the water. The shifts of primary producers from rooted macrophytes to phytoplankton occurred simultaneously between the mid-1950s and 1965 in Japan??presumably because of the wide use of herbicides for rice paddies. Increased phytoplankton induced the increase of fishery yield of clam on one hand and non-edible biofouling species on the other. Because a phytoplankton-dominated ecosystem is easily oxygen depleted at the bottom with the accumulation of organic matter of phytoplankton origin, preventing hypoxia is the key factor to maintain high secondary production for bivalve fisheries and sustainable use of the ecosystem of coastal lagoons.     

Application of a new multi-metric phytoplankton index to the assessment of ecological status in marine and transitional waters

Patterns of phytoplankton size spectra variation with gradients of environmental stress have been observed in freshwater, transitional waters and marine ecosystems, driving the development of size spectra based assessment tools.In this study, we have tested on transitional and coastal waters a new Index of Size spectra Sensitivity of Phytoplankton (ISS-Phyto), which integrates simple size spectra metrics, size class sensitivity to anthropogenic disturbance, phytoplankton biomass (chlorophyll a) and taxonomic richness thresholds. ISS-Phyto has been tested both among and within ecosystems along pressure gradients based on expert view assessment; the adequacy of symmetric and both left and right asymmetric models of phytoplankton size class sensitivity have been compared.The results showed that ISS-Phyto consistently discriminated between anthropogenic and natural disturbance conditions. Left asymmetric models of size spectra sensitivity, assuming greater disturbance tolerance with respect to eutrophication and organic enrichment of increasingly large size classes, showed the best fit comparing all ecosystems; in three of the four considered ecosystems (Varna, Helsinki, Mompás-Pasaia), they seemed to discriminate best between different levels of disturbance also within ecosystems. Moreover, they demonstrated significant and inverse patterns of variation along the overall pressure gradient as well as along the inorganic phosphorus (DIP), chlorophyll a and trophic index (TRIX) gradients.Therefore, ISS-Phyto, originally developed for transitional waters, seems to be an adequate assessment tool of ecological status also in coastal marine waters; moreover, it seems adequate to describe within ecosystem disturbance gradients. Hence, ISS-Phyto helps to understand the relationships between anthropogenic impact and ecosystem response from the individual point of view, with reference to the simple parameter of body size.     

Comparative composition and dynamics of harmful dinoflagellates in Mediterranean salt marshes and nearby external marine waters

The taxonomic structure of phytoplankton populations in two Mediterranean coastal lagoons were compared with those of nearby marine waters (external waters). Mediterranean confined lagoons remain isolated for most the year and concentrate phytoplankton to a very high biomass. Coastal lagoons on the Mediterranean may, therefore, act as accumulators of neritic phytoplankton (including species related to harmful algal blooms). We examined whether coastal lagoons act as concentrators of marine toxic dinoflagellates during confinement periods, and the common environmental factors that favour growth of specific harmful species in the two ecosystems considered: coastal lagoons and external waters. An alternation between the dominance of diatoms and dinoflagellates was observed, coinciding with that described in Margalef's mandala, occurring in external waters as well as in coastal lagoons. Moreover, the temporal patter was different in the two ecosystems. Dinoflagellate species composition and their bloom period were highly variable in time and space, thus, species had to be analysed individually. Most of the dinoflagellate species found in this study were potentially harmful and high biomass producers. Harmful dinoflagellate species performed well in both, external waters and lagoons, but the specific species-dependent affinity to each of these environments determined which organisms bloom there. Thus, expansion of harmful algal blooms (HAB) to inland waters is not likely and some environmental factors such as the oxidised state of available nitrogen, became determinant to the success and bloom of a species in the coastal lagoon ecosystem.     

Light-Related Photosynthetic Characteristics of Lotic Macroalgae

Photosynthetic characteristics in response to irradiance were analysed in 42 populations of 33 macroalgal species by two distinct techniques (chlorophyll fluorescence and oxygen evolution). Photosynthesis–irradiance (PI) curves based on the two techniques indicated adaptations to low irradiance reflected by low saturation values, high to moderate values of photosynthetic efficiency (α) and photoinhibition (β), for Bacillariophyta and Rhodophyta, which suggests they are typically shade-adapted algae. In contrast, most species of Chlorophyta were reported as sun adapted algae, characterized by high values of I _k and low of α, and lack of or low photoinhibition. Cyanophyta and Xanthophyta were intermediate groups in terms of light adaptations. Photoinhibition was observed in variable degrees in all algal groups, under field and laboratory conditions, which confirms that it is not artificially induced by experimental conditions, but is rather a common and natural phenomenon of the lotic macroalgae. Low values of compensation irradiance (I _c) were found, which indicate that these algae can keep an autotrophic metabolism even under very low irradiances. High ratios (>2) of photosynthesis/respiration were found in most algae, which indicates a considerable net gain. These two physiological characteristics suggest that macroalgae may be important primary producers in lotic ecosystems. Saturation parameters (I _k and I _s) occurred in a relatively narrow range of irradiances (100–400 μmol photons m^?2s^?1), with some exceptions (higher in some filamentous green algae or lower in red algae). These parameters were way below the irradiances measured at collecting sites for most algae, which means that most of the available light energy was not photochemically converted via photosynthesis. Acclimation to ambient PAR was observed, as revealed by lower values of I _k and I _cand higher values of α and quantum yield in algae from shaded streams, and vice versa. Forms living within the boundary layer (crusts) showed responses of shade-adapted species and had the highest values of P _max, α and quantum yield, whereas the opposite trend was observed in gelatinous forms (colonies and filaments). These results suggests adaptation to the light regime rather than functional attributes related to the growth form.     

Integrative ecosystem analyses of phytoplankton dynamics in the York River estuary (USA)

The relative importance of biotic (top-down) vs. abiotic (bottom-up) controls on phytoplankton dynamics was investigated in the York River estuary, Virginia (USA) by a combination of extensive analyses of long-term data sets collected by the U.S. Environmental Protection Agency (EPA) monitoring program over 17 years (1984–2001), field studies (1996–1997) and ecosystem modeling analyses. Results from the analysis of long-term data records collected at three stations along the salinity gradient suggested that phytoplankton are more likely controlled by abiotic mechanisms such as resource limitation than biotic mechanisms such as grazing since annual cycles of primary production and phytoplankton biomass were similar and no grazing effects were evident based on the observed relationship between phytoplankton and zooplankton biomass (R ² < 0.2, p > 0.1). This scenario was supported by short-term field observations made over an annual cycle at three stations in the mid-channel of the estuary where both chlorophyll a and primary production demonstrated similar patterns of seasonal variation. Ratios of fluorescence before and after acidification at all study sites were relatively high suggesting low grazing pressure in the estuary. A tidally-averaged, size-structured plankton ecosystem model was previously developed and verified for the lower York River estuary. The validated ecosystem model was also used to examine this issue and simulation results supported the importance of bottom-up control in the York River estuary.     

Bathymetric adaptations of reef-building corals at davies reef,great barrier reef,Australia. II. Light saturation curves for photosynthesis and respiration

Light saturation (P-I) curves for oxygen production and consumption were constructed in the laboratory for corals collected from depths of 1 to 45 m on the southeastern (seaward) side of Davies Reef, Great Barrier Reef, Australia. This depth range represents a gradient of from 82.6 to 2.9% of surface light, which was measured as photosynthetic photon flux density (PPFD) between 400 and 700 nm. Adaptative changes in photokinetic parameters were observed over the entire range of light intensities. The compensation intensity (I_c), I_k, the intensity at which photosynthesis was 95% light saturated (I_0.95), and the respiration rate (?R), all decreased with decreasing light intensity. The maximal rate of photosynthesis when normalized on the basis of coral chlorophyll-a content (P_ma^g) tended to decrease with decreasing irradiance but the change was not statistically significant. The $\text{P}\text{R}$ ratio ($\text{P}{}_{\mathrm{m}}{}^{\mathrm{g}}\text{?R}$), the initial slope of a light saturation curve (α), and the maximum rate of photosynthesis when normalized by coral protein content (P_mp^g), all increased with decreasing light intensity. The logarithms of the values for each variable parameter were proportional to the logarithms of the fraction of the surface PPFD (T) transmitted to the depth at which the corals grew. This enabled changes in these parameters to be accurately estimated for corals growing at any depth on the reef. Comparison of experimental data with published values for “saturating” irradiance and P_ma^g, suggests that the endosymbiotic algae within reef-building corals are photosynthetically intermediate between classical “sun” and “shade” plants.