Decreased Light Penetration Research Articles

The effects of dynamic optical properties during interstitial laserphotocoagulation

A nonlinear mathematical model was developed and experimentally validated toinvestigate the effects of changes in optical properties during interstitiallaser photocoagulation (ILP). The effects of dynamic optical properties werecalculated using the Arrhenius damage model, resulting in a nonlinearoptothermal response. This response was experimentally validated by measuringthe temperature rise in albumen and polyacrylamide phantoms. A theoreticalstudy of ILP in liver was conducted constraining the peak temperatures belowthe vaporization threshold. The temperature predictions varied considerablybetween the static and dynamic scenarios, and were confirmed experimentally inphantoms. This suggests that the Arrhenius model can be used to predictdynamic changes in optical and thermal fields. An increase in temperature risedue to a decrease in light penetration within the coagulated region during ILPof the liver was also demonstrated. The kinetics of ILP are complex andnonlinear due to coagulation, which changes the tissue properties duringtreatment. These complex effects can be adequately modelled using an Arrheniusdamage formulation.

Phytoplankton dynamics and structure: a comparative analysis in natural and man-made water bodies of different trophic state

Previous investigations on Sicilian man made lakes suggested that physical factors, along with the specific morphology and hydrology of the water body, are important in selecting phytoplankton species. In particular, the variations of the zmix/zeu ratio due to the operational procedure to which reservoirs are generally subject were recognised as a trigger allowing the assemblage shift. To investigate if these variations may be considered analogous to those occurring in natural lakes as trophic state and phytoplankton biomass increase, causing a transparency decrease and a contraction of the euphotic depth, phytoplankton were collected in two natural water bodies, one mesotrophic (Lake Biviere di Cesaro) the other eutrophic (Lake Soprano), and compared with those collected in two reservoirs with analogous trophic characteristics (Lake Rosamarina, mesotrophic and Lake Arancio, eutrophic). Particular attention was paid to the dynamics of two key groups: Cyanophytes and chlorophytes. In all four water bodies, transparency mainly depended on chlorophyll level. Annual average value of phytoplankton biomass in the mesotrophic environments was below 2.0 mg l−1, whereas in the eutrophic systems it was well above 10 mg l−1. All water bodies showed the presence of cyanophytes (e.g. Anabaena spp., Anabaenopsis spp., Microcystis spp., Planktothrix spp.) and chlorophytes (e.g. Chlamydomonas spp., Botryococcus spp., Oocystis spp., Scenedesmus spp., Pediastrum spp.), but their relative proportions and body size dimensions were different. In particular, small colonial chlorophytes and large-colony forming cyanophytes were most common in the most eutrophic water bodies, whereas larger colonies of green algae in those with a lower trophic state. The results showed that, under the same climatic conditions, autogenic (increase of biomass, decrease in light penetration and euphotic depth) and allogenic (use of the stored waters, anticipated breaking of the thermocline, increase of the mixing depth) processes may shift the structure of phytoplankton assemblage in the same direction even though the quantity of biomass remains linked to nutrient availability.

Seasonal variations in P–I responses of Chara hispida L. and Potamogeton pectinatus L. from stream mediterranean ponds

The relationship between primary production and irradiance was analyzed over an annual cycle in the submerged macrophytes, Chara hispida and Potamogeton pectinatus, of two mediterranean stream ponds. The effect of temperature on photosynthesis was also studied. The population of P. pectinatus reached maximum production values of 19.67 mgO 2 g −1 AFDW h −1 in December at 20°C. In summer and owing to the development of floating filamentous algae, P. pectinatus growth decreased. This can be observed in a decrease in production and chlorophyll content. At the end of autumn a `self-pruning' mechanism in P. pectinatus caused increase of light penetration and new shoots were observed. C. hispida showed maximum production values of 10.86 mg O 2 g −1 AFDW h −1 in May. In summer, in agreement with temperature increase C. hispida fructifies and decays, increasing P. pectinatus growth and cover in the same pond. Photoinhibition was observed in C. hispida in October, in agreement with a low radiation adaptation owing to an unusual decrease in light penetration in the pond, caused by inputs of suspended material from a fire suffered in the watershed at the end of summer. Variations in temperature had no effect on Pm/ R ratio in P. pectinatus and decreased sharply from young to old tissue. The effect of temperature on the Pm/ R ratio in C. hispida was significant. P. pectinatus showed great adaptation capacity to temperature fluctuations and light and this adaptation is higher than in C. hispida, which is affected by temperature increase and light penetration reduction.

The influence of vines on an oligohaline marsh community: results of a removal and fertilization study.

The effects of competitive suppression by vines on the non-vine plant community have received little attention in temperate habitats. This study investigated the impact vines have on their herbaceous hosts in a wetland community at two soil fertility levels. Plots in an oligohaline marsh were treated in a 2 × 2 factorial design with vine removal and fertilization over two growing seasons. There was no significant interaction between removal and fertilization treatments on any of the measured variables. Vine removal initially caused an increase in light penetration through the canopy, but by the end of the study, plots with vines removed had less light due to a 25% increase in biomass by the plants released from competition with vines. For plots with vines removed, species richness was higher during a brief period in the spring of the second year, but by the end of the study, richness in removal plots decreased relative to controls. Fertilization caused a 40% increase in biomass overall, although only two species, Sagittaria lancifolia L. and Polygonum punctatum Ell., showed dramatic increases. Despite fertilization causing a 40% decrease in light penetration to the ground, no change in species richness was observed. Overall, these results show that vine cover in this wetland suppresses non-vine species and reduces community biomass. Removal of vines increased biomass of non-vine dominants but resulted in only an ephemeral change in species richness. Fertilization did not increase the effects of vines on the non-vine community.

Deep‐water aquatic plant communities in an oligotrophic lake: physiological responses to variable light

SUMMARY1. Oligotrophic Lake Waikaremoana, New Zealand, is used for hydroelectric power generation and the lake levels are manipulated within an operating range of 3 m. There was concern that rapidly changing water levels adversely affected the littoral zone by decreasing light availability in two ways: local turbidity caused by shoreline erosion at low water levels; and decreased light penetration to the deep littoral zone caused by high water levels in summer.2. The littoral zone was dominated by native aquatic plants with vascular species to 6 m and a characean meadow below this to 16 m. The biomass and heights of the communities in the depth zone 0–6 m were reduced at a site exposed to wave action relative to those at a sheltered site. However, the community structure below 6 m was similar at exposed and sheltered sites. The lower boundary of the littoral zone was sharply delimited at 16 m and this bottom boundary remained constant throughout the year despite large seasonal changes in solar radiation and the 3 m variation in lake level.3. There was evidence that the deep‐water community consisting of Chara corallina had adapted physiologically to low‐light conditions. Net light saturated photosynthesis (CO2 exchange) per unit chlorophyll a (Chl a) was reduced to 1.7 μg C (μg Chl a)−1 h−1 at the lower boundary, half of that recorded at 5 m. The concentration of Chi a per gram of biomass (dry weight), was considerably greater at the lower boundary than higher in the profile [c. 7 mg Chl a (g dry wt)−1 at 16 m vs. 4 mg Chl a (g dry wt)−1 at 5 m]. Chl b also increased with depth and there was no change in the ratio of Chl a and Chl b with increasing depth. The saturation light intensity (Ik) of the community at the lower boundary was only 78 μmol photons m−2 s−1. Photosynthetic parameters (Ik and α) as well as the Chl a content remained relatively constant throughout the seasonal and short‐term changes in radiation.4. The photosynthetic characteristics of the littoral community were therefore not greatly affected by the lake level change caused by the present hydroelectric operations. However, the sharpness of the lower boundary and its extreme shade characteristics imply that the deep‐water community would be sensitive to any further changes in underwater light availability.

Species Dynamics and Nutrient Accumulation During Early Primary Succession in Coastal Sand Dunes

1 The present study reports on a primary succession series which started on bare soil on the Dutch island of Schiermonnikoog after the building of a sand dike. Vegetational changes were studied for 18 years by means of permanent transects along a topographic gradient from a moist plain to dry dunes. Soil development and vegetation structure were reconstructed using a chronosequence. A fertilizer experiment was set up in an intermediate successional stage in the plain and on the dune, in order to determine which soil resources limited productivity. 2 Differences in salinity, flooding and moisture content were important determinants of the differences in species composition along the topographic gradient. In addition, year-to-year fluctuations of these factors seem to be responsible for the year-to-year fluctuations in frequency of occurrence of many short-lived species. These factors did not, however, show a consistent long-term trend over time. 3 From soil analyses and the nutrient addition experiment, it is concluded that nitrogen limited above-ground biomass production. Over a period of about 16 years the total amount of nitrogen in the organic layer of the soil increased from 7 to 50 g N m-2 in the plains and from 1 to 15 g N m-2 on the dunes. 4 The accumulation of nitrogen during the successional series is accompanied by an increased biomass, a decreased light penetration to the soil surface, a decreased root/shoot ratio, increasing dominance of tall species, and a decreasing abundance of small, short-lived species. These data suggest that the importance of light competition is increasing during succession. 5 The importance of plant height versus light reduction at the soil surface in determining the outcome of light competition is discussed.

Allocation and the Transient Dynamics of Succession on Poor Soils

Biomass and nitrogen allocation to leaf, root, stem, and reproduction was determined in a 35—field chronosequence that spans the first 60 yr of secondary succession on a Minnesota sand plain. Biomass (grams per square metre) in leaf and root increased during succession, but reproductive biomass declined, and that in stem remained constant. Because root biomass increased twice as rapidly as leaf biomass, the proportion of total biomass in root increased during succession, whereas that in leaf, reproduction, and stem declined. In an additional study, biomass allocation was determined on a species—by—species basis for 46 species common at different times during succession. This study showed a similar pattern of increasing proportional root allocation and declining proportional reproductive and stem allocation during succession. These changes were accompanied by an increase in total soil nitrogen and a decrease in light penetration to the soil surface during succession. Increasing root allocation and decreasing reproductive allocation suggest that succession on these nutrient—poor soils is the transient dynamics of colonization and competitive displacement, with later successional species being superior nitrogen competitors because of higher root allocation. Allocation trade—offs root, stem, leaf, and seed can lead to initial dominance by species with high seed and leaf allocation, presumably because of greater colonization and/or maximal growth rates. Thus, this succession differs markedly from successions on rich soils, for which stem allocation is increasingly important. These results contradict the resource ratio hypothesis as an explanation for the pattern of early succession on impoverished soils.

The effect of the Mt. Mazama ashfall on the planktonic diatom community of Lake Washington1

Preserved diatom assemblages are found in deep water sediments in Lake Washington above and below an ash layer originating from an eruption of Mt. Mazama 6800 b.p. Total abundance of frustules did not change after the ash event, but a significant shift in community structure occurred. Melosira italica var. subarctica increased after the ashfall, and Fragilaria crotonensis decreased. The changes most likely indicate an increase in the Si: P ratio and decreased light penetration in the euphotic zone during winter and spring.

Trend in the transparency of the northern Adriatic Sea 1911–1982

In the northern Adriatic Sea, the Secchi disk has been used in oceanographic studies since 1911. An analysis of the data collected during summers between 1911 and 1982 demonstrates that, on the average, the Secchi disk depth has decreased in time. It is likely that a decrease in light penetration has reduced the benthic primary production and thus has increased the probability of an occurrence of anoxic events near the bottom. The long-term nutrient enrichment of the freshwaters discharging into the northern Adriatic Sea appears to be the main factor which has caused the above changes.

Quantitative light dosimetry in vitro and in vivo

Knowledge of the radiant energy fluence rate in tissues during laser irradiation is important for the understanding and improvement of the results of preclinical as well as clinical treatments. Quantitative data are extremely rare, however. In this paper, quantitative measurements of energy fluence rates are reported, in vitro as well as in vivo, with emphasis on light dosimetry for photodynamic therapy. Examples are given of fluence rate distributions that may occur during surface irradiation, intracavity irradiation of hollow organs (bladder) and interstitial irradiation. For the same incident irradiance, the energy fluence rate in tissue may vary considerably, depending on type of tissue, wavelength and geometry. During experimental and clinical interstitial PDT-treatments a considerable decrease in light penetration into tumours was observed, apparently indicating changes in optical properties as a result of treatment. This demonstrates the importance of in vivo light dosimetry.

Recurrent and Persistent Brown Tide Blooms Perturb Coastal Marine Ecosystem

Throughout the summers of 1985 and 1986 a small (2–3 μm diameter), previously underscribed chrysophyte bloomed monospecifically (>109 cells 1−1) in Long Island embayments. The bloom colored the water dark brown, decimated eelgrass beds through decreased light penetration and caused starvation (tissue weight loss) and recruitment failure of commercially important bay scallop populations. These perturbations portend longterm changes in subtidal communities Similar and concurrent blooms in bays of Rhode Island and New Jersey suggest a meteorological component of the environmental conditions promoting bloom formation. Culture experiments with isolates of the microalga suggest the presence of stimulatory growth factors in the bloom seawater. *** DIRECT SUPPORT *** A01BY040 00002

A survey of the fish fauna of Transkei estuaries. Part 1. The Kei River estuary

During the period of survey the 8 km long Kei estuary was subjected to periodic floods which reduced salinities to 0%o and decreased light penetration to a few centimetres. Species composition, and temporal and spatial abundance were determined for fish samples collected by means of gill nets. Twenty-six species of fish were caught with Mugil cephalus, Liza tricuspidens and Argyrosomus hololepidotus predominating, in that order. The family Mugilidae comprised 67,8% of the numbers and 66,6% of the fish biomass caught. Definite seasonal patterns could not be determined for many of the fish although M. cephalus, L. richardsoni and A. hololepidotus were more abundant in summer. The effect of river flooding had unpredictable results on Mugilidae catches which either decreased or increased after floods while catches of P. commersonii and A. hololepidotus usually increased. Largest catches were made in the middle reaches although species diversity was greatest in the lower reaches of the estuary.

Environmental Impact Prediction and Assessment: the Southern Indian Lake Experience

The impoundment of Southern Indian Lake (SIL) and diversion from the lake of the Churchill River in northern Manitoba, Canada, were the subjects of two independent environmental impact statements. Subsequently, a case study measured change in the limnological and biological characteristics of the lake after development. Comparison of pre- and post-impoundment observations allows an assessment of the predictive capability that was applied to the lake by the preimpact statements. Predictions related to the physical environment, e.g. increased shoreline erosion, littoral sedimentation, higher turbidity, and decreased light penetration and visibility, were qualitatively correct; however, an unpredicted decrease in water temperature also occurred. Increased phosphorus availability and light limitation of primary production were also correctly forecasted in a qualitative manner. These aspects will be quantitatively predictable in future reservoirs because of studies at SIL and elsewhere. Biological responses above the primary trophic level were mostly not predicted or predicted incorrectly. Unpredicted changes that were especially significant to the fishery were rapid declines in the quantity and quality of whitefish (Coregonus clupeaformis) catch, increases in mercury concentrations in fish, and the need for extensive compensation programs to keep the fishery economically viable. Testable hypotheses to explain all unpredicted events have been formulated but require experimental verification. The paradigm of reservoir ecosystem development that is present in the literature requires reformulation if future environmental impact analyses of reservoirs are to be improved.

Phytoplankton as a factor in the decline of the submersed macrophyte Myriophyllum spicatum L. in Lake Wingra, Wisconsin, U.S.A.

A dramatic decline in biomass and areal coverage of the submersed macrophyte Myriophyllum spicatum in Lake Wingra, Wisconsin, USA during the mid-1970's is documented using aerial photography, vegetation surveys, and quadrat biomass sampling. Over the same period, light penetration as measured by Secchi disc transparency and extinction coefficient decreased substantially. During this period, extinction coefficient was closely correlated with chlorophyll a levels implicating phytoplankton as the major source of decreased light penetration. A growth model for M. spicatum predicts a substantial decline in macrophyte biomass when extinction coefficient is increased to the levels reported since 1977. Available data do not show whether the phytoplankton increase preceeded or followed the macrophyte decline. Nonetheless it is clear that phytoplankton growth can account for a substantial portion of the decline and that macrophyte recovery will be difficult given the phytoplankton-induced decrease in water clarity.

Kentucky Bluegrass Seed Production Characteristics as Affected by Residue Management1

AbstractSeed productivity of Kentucky bluegrass (Poa pratensis L.) is usually improved by open field burning of post‐harvest residue. This field study compared the effects of field burning and mechanical removal of post‐harvest residue on Kentucky bluegrass plant growth and seed yields. ‘Merion’, ‘Nugget’, ‘Baron’, ‘Glade’, and ‘South Dakota’ Kentucky bluegrass cultivars were seeded on a Latahco soil, (fine‐silty, mixed, frigid argiaquic Agrialboll) in 1976. Residue removal treatments were stripped across the cultivars in 1977, 1978, 1979, and 1980. Seed yields where residue was removed by burning were 1.5 times greater in 1979,1.6 times greater in 1980, and 1.7 times greater in 1981 than yields where residue was mechanically removed to 2.5 cm. Seed yields where residue was removed by burning were 2.0 times greater in 1979, 1.9 times greater in 1980, and 2.6 times greater in 1981 than yields from plants where residue was mechanically removed to 7.6 cm. Panicle and tiller numbers were reduced where residue was mechanically removed to 2.5 or 7.6 cm compared to panicle and tiller numbers where residue was burned. Tiller leaf sheath lengths were not different where residue was burned or removed to 2.5 cm and were shorter than where residue was removed to 7.6 cm. Rhizome weights were reduced where residue was burned compared to weights where residue was clipped at 2.5 or 7.6 cm height in both 1980 and 1981. Root weights, when averaged across all five cultivars, were not affected by residue removal method. A significant cultivar × treatment interaction was noted for Glade, which produced more roots when clipped to 2.5 cm than when burned or clipped to 7.6 cm in both years. Significant cultivar × treatment interactions occurred in 1980 in tiller number which were reflected in panicle numbers and seed yields for Glade and South Dakota. Increased tiller production was associated with decreased rhizome weights. Since tiller apices have been found to control the upturning of rhizomes to produce new tillers, it was concluded that burning functions to reduce tiller apical control of rhizomes. The increased length of tiller leaf sheaths under decreasing heights of residue removal was concluded to be in response to decreased light penetration into the canopy.

Mesozoic and Tertiary Carbonate Buildups and Factors Which Control Their Distribution: ABSTRACT

With the development of global paleogeographic reconstructions, it is possible to examine the original distribution of carbonate buildups at various times in the earth's history. Through an extensive search using published geologic literature, carbonate buildups have been classified based on their association with shelf-edge, shallow-shelf, and basinal sediments. By plotting the data on well-constrained Mesozoic and Tertiary paleogeographic continental reconstructions, it is observed that carbonate buildups are generally restricted to low latitudes. However, the maximum excursion of buildups from the paleoequator varies significantly through geologic time. The distribution and abundance of carbonate buildups in the Mesozoic and Tertiary appear to be partly controlled by ancient ocean current systems. In a manner similar to the present, carbonate buildups extend further toward the poles along east-facing coastlines. Here, warm westward-flowing End_Page 916------------------------------ equatorial currents are diverted poleward. The reverse situation is true along west-facing coastlines where cooler waters are brought into low latitudes by polar currents. Here, buildups are usually restricted to near the ancient equator. Another control on the distribution of carbonate buildups might be related to a decrease in light penetration poleward due to an increase in the angle of incidence of light striking the oceans. Work done by others suggests that significant seasonal reduction in light penetration occurs between 30° and 40° from the equator. Maps showing the global distribution of carbonate buildups have been constructed for several intervals in the Mesozoic and Tertiary. These reconstructions illustrate the effects of ocean circulation, and continental placement and orientation on the distribution of carbonate buildups. End_of_Article - Last_Page 917------------