Underwater Light Regime Research Articles

Phytoplankton and Its Role in the Formation of the Underwater Light Regime in Artificial Reservoirs

Phytoplankton and Its Role in the Formation of the Underwater Light Regime in Artificial Reservoirs

Subtidal macrophyte diversity and potentials in Nha Trang Bay - baseline data for monitoring a rising natural resource

Tropical coastal ecosystems provide a unique complex marine habitat with a high diversity of algal species, Viet Nam being a particular hotspot. These algae may host a variety of potential unknown or underestimated bioactive algal compounds. In parallel the worldwide rising interest in macroalgae-based products leads to increasing activities in seaweed natural harvest and mariculture within coastal waters. With this growing interest, the present work provides baseline data for a systematic and science-based macroalgal monitoring program in Nha Trang Bay, Viet Nam, to keep track of potentially interesting taxa and to identify driving environmental factors which may also naturally influence biodiversity and species abundance. The present study investigated macrophyte diversity and abundance by combining a qualitative and quantitative macroalgal survey approach with environmental sampling (e.g. physicochemical properties of water column, underwater light regime, and sediment characteristics). Surveys were performed in the dry season (May/June) 2019 in different water depths (3, 6 and 10 m) at seven sites within Nha Trang Bay. The study revealed a coastal patchwork of diverse habitats inhabited by complex macrophyte communities, including estuarine dense Sargassum forests and Turbinaria meadows, sheltered sandy seagrass (Halodule spp.) beds with upcoming Lyngbya blooms, low diverse Padina deserts and highly turbid aquaculture (lobster and fish farms) impacted sites with surprisingly high macroalgal diversity. During our study a total of 86 macrophyte species were encountered in the subtidal (>1 m water depth), whereas only 6 species (Padina australis, Sargassum mcclurei, Turbinaria ornata, Halimeda discoidea, Amphiroa fragilissima, Tricleocarpa cylindrical) were frequently found at every survey site. The observed high patchiness and presence of economically important (e.g. Sargassum spp., Gracilaria spp., Caulerpa spp., Gelidiella acerosa, Acanthophora spicifera) and potentially economically interesting (e.g. Padina australis, Turbinaria ornata, Stypopodium zonale, Chondria armata) taxa during the survey underlines the high potential of the present macrophytic bio-resource, which apparently is strongly structured and will be altered by the changing heterogenic environment.

Spectral differences in the underwater light regime caused by sediment types in New Zealand estuaries: implications for seagrass photosynthesis

The underwater light regime is fundamental to the ecological health of aquatic systems because it is a limiting factor for photosynthesis in marine plants such as seagrasses. Although seagrass meadows are a key component of coastal systems, their survival has been threatened by increased turbidity levels, both from resuspension of marine sediments and input of terrestrial material. The objective of this study was to investigate how marine (typically grey/white in colour) and terrestrial (typically more yellow-orange in colour with finer texture) sediments affect underwater light quality. Two experimental systems were used: (1) a large outdoor tank and (2) laboratory controlled small sampling container, using natural terrestrial and marine sediment samples (with different colours and grain sizes) from New Zealand. In the tank experiments, high concentrations of sediment reduced transmittance considerably, particularly below 450 nm. Since seagrasses absorb light optimally at wavelengths < 500 nm, as well as between 650 nm and 700 nm, the photosynthetic capacity will be less efficient with pigment absorption occurring mainly at the 650–700 nm wavebands. The difference in colour (white and grey) between marine sediments with the same grain sizes was tested in the laboratory. White sediment resulted in lower transmittance at the same concentration compared with grey sediments; concentration differences had more impact on the spectral distribution of light for white sediments. Within the ranges tested, sediment concentration contributed most to changes in overall light transmittance, with grain size being slightly less important. Sediment colour was important in changing the distribution of light, with orange and white sediments increasing attenuation of shorter wavelengths, which are most needed for seagrass photosynthesis. Our results emphasise the importance of quantifying the spectral changes to underwater light regimes in managing estuaries that are subjected to regular catchment runoff.

Acute effects of back-to-back hurricanes on the underwater light regime of a coral reef

Some of the best-known disturbances affecting coral reefs are storms, yet their impacts on light are poorly known. Here, we describe the underwater light on a reef off St. John, US Virgin Islands (18°18′37.04N, 63°43′23.17W), during two hurricanes and multiple tropical waves that occurred between 17 August 2017 and 30 November 2017. Photosynthetically active radiation (PAR) was recorded at 19-m depth and at the surface, and rainfall was measured as a cause of turbidity affecting underwater light. Hurricanes Irma and Maria reduced maximum daily underwater photosynthetic photon flux density (PPFD) to < 14 µmol photons m−2 s−1 (< 0.001% of surface PPFD) and, from 1 September to 30 November, were associated with rainfall that was higher (107 cm) than has been recorded over this period since 1972. These reductions in underwater PPFD are unprecedented since at least 2014, and the corresponding values of the diffuse attenuation coefficients (Kd-PAR 0.268–0.426) are among the lowest recorded on a coral reef. Our study reveals the capacity for hurricanes to render a photic habitat temporarily aphotic, and over 69 days (and relative to 2016), to contribute to a 20% reduction in summed, daily integrated underwater PPFD. Large reductions in underwater PPFD have negative implications for photosynthetic taxa, but through the turbidity that reduces underwater light, they create opportunities for population growth by suspension feeding invertebrates.

Estimating Underwater Light Regime under Spatially Heterogeneous Sea Ice in the Arctic

The vertical diffuse attenuation coefficient for downward plane irradiance ( K d ) is an apparent optical property commonly used in primary production models to propagate incident solar radiation in the water column. In open water, estimating K d is relatively straightforward when a vertical profile of measurements of downward irradiance, E d , is available. In the Arctic, the ice pack is characterized by a complex mosaic composed of sea ice with snow, ridges, melt ponds, and leads. Due to the resulting spatially heterogeneous light field in the top meters of the water column, it is difficult to measure at single-point locations meaningful K d values that allow predicting average irradiance at any depth. The main objective of this work is to propose a new method to estimate average irradiance over large spatially heterogeneous area as it would be seen by drifting phytoplankton. Using both in situ data and 3D Monte Carlo numerical simulations of radiative transfer, we show that (1) the large-area average vertical profile of downward irradiance, E d ¯ ( z ) , under heterogeneous sea ice cover can be represented by a single-term exponential function and (2) the vertical attenuation coefficient for upward radiance ( K L u ), which is up to two times less influenced by a heterogeneous incident light field than K d in the vicinity of a melt pond, can be used as a proxy to estimate E d ¯ ( z ) in the water column.

Light requirements of water lobelia (Lobelia dortmanna L.)

Abstract Maximum depth of colonization (zC) and total area covered by a population of Lobelia dortmanna, as well as underwater light regime were studied in 25 soft water lobelia lakes in north-western Poland. Variations in underwater light conditions among the lakes were described by Secchi disc depths (zSD), and by attenuation coefficients of irradiance within photosynthetically active radiation range (Kd,PAR), and euphotic zone depths (zEU) derived from photometric measurements conducted twice a year (in midspring and midsummer) during the period 2014–2015. Maximum depth of colonization of water lobelia ranged from 0.1 to 2.2 m (median zC = 0.8 m; mean zC = 1.0 m). Nine lakes showed the relative coverage of the littoral zone (RCLZ) by L. dortmanna to be greater than the mean value, which was 4.8%. Studies showed that light requirements of water lobelia increase when the maximum depth of colonization also increases. This pattern could be partially related to the greater energy needs of deeper growing individuals due to enlarged seed production and their incubation, and for the creation of much heavier inflorescences. Assessment of the light requirements of L. dortmanna along the depth gradient indicates that relative irradiance (percentage of subsurface irradiance of PAR) should be at the level of: (i) 47–50% (annual total of quantum irradiance 3083–3280 mol m−2 yr−2) for plants growing within a depth range of 2.0–2.5 m; (ii) 44–47% (2886–3083 mol m−2yr−1) for plants growing within a depth range of 1.5–2.0 m; (iii) 41–44% (2690–2886 mol m−2yr−2) for plants growing within a depth range of 1.0–1.5 m; and (iv) 34–41% (2230–2690 mol m−1 yr−1) for those growing in the littoral zone at a depth of between 0.5 and 1.0 m. In average conditions in the Pomeranian lakes, the maximum depth of colonization by L. dortmanna accounts for approximately a third of the Secchi disc depth and a fifth of the depth of the euphotic zone with irradiance of PAR at zC equal to about 43% of subsurface irradiance. It has also been demonstrated that the light factor is a crucial one that limits the absolute maximum depth of lobelia population occurrence in Pomeranian lakes. The cleanest and most transparent lakes of this region have light attenuation coefficients (Kd,PAR) within the range of 0.35–0.42 m−1, which corresponds to the maximum colonization depths of 1.8–2.2 m.

Schulman acquires Citadel

The underwater light regime of a Mediterranean coastal lagoon (Albufera des Grau, Balearic Islands) was studied during four years in order to characterise the spatial and temporal variations in the light attenuation coefficient (K) and to assess the relative contribution of the different water components to total light attenuation.During the studied period K averaged 1.42 m−1 and ranged from 0.63 m−1 to 3.80 m−1. High temporal variability was observed in light attenuation coefficients, but the lagoon was spatially uniform.Percentage bottom irradiance in relation to specific requirements for the dominant macrophyte species (Ruppia cirrhosa) was used as an indicator of benthic light limitation. Macrophyte light limitation was expected to occur in the deepest areas of the lagoon during winter, the most turbid period of the annual cycle. During the macrophyte growing season, higher bottom irradiances were observed but a significant percentage of the lagoon benthos (17% in spring and 7% in summer) was expected to be light limited.In the deepest areas of the lagoon (>2 m) changes in bottom irradiance were related more to variations in the light attenuation coefficient than to variations in water level. However, water level appeared to play an important role in determining benthic light limitation at intermediate depths (1.5 m) for the range of K from 1.8 m−1 to 3.3 m−1.The partitioning of the light attenuation coefficient showed that phytoplankton was the main driver of the temporal dynamics of K, but only accounted for 44% of total light attenuation on average. The mean contributions of the other water components to K were: DOC (47%), tripton (6%), and water (3%). At low values of K, attenuation by DOC was responsible for up to 75% of total attenuation. An equation to predict K from the concentration of water components explained 93% of the variance.

Contrasting optical properties of surface waters across the Fram Strait and its potential biological implications

Underwater light regime is controlled by distribution and optical properties of colored dissolved organic matter (CDOM) and particulate matter. The Fram Strait is a region where two contrasting water masses are found. Polar water in the East Greenland Current (EGC) and Atlantic water in the West Spitsbergen Current (WSC) differ with regards to temperature, salinity and optical properties. We present data on absorption properties of CDOM and particles across the Fram Strait (along 79° N), comparing Polar and Atlantic surface waters in September 2009 and 2010. CDOM absorption of Polar water in the EGC was significantly higher (more than 3-fold) compared to Atlantic water in the WSC, with values of absorption coefficient, aCDOM(350), m−1 of 0.565±0.100 (in 2009) and 0.458±0.117 (in 2010), and 0.138±0.036 (in 2009) and 0.153±0.039 (in 2010), respectively. An opposite pattern was observed for particle absorption with higher absorption found in the eastern part of the Fram Strait. Average values of particle absorption (aP(440), m−1) were 0.016±0.013 (in 2009) and 0.014±0.011 (in 2010), and 0.047±0.012 (in 2009) and 0.016±0.014 (in 2010), respectively for Polar and Atlantic water. Thus absorption of light in eastern part of the Fram Strait is dominated by particles — predominantly phytoplankton, and the absorption of light in the western part of the strait is dominated by CDOM, with predominantly terrigenous origin. As a result the balance between the importance of CDOM and particulates to the total absorption budget in the upper 0–10m shifts across Fram Strait. Under water spectral irradiance profiles were generated using ECOLIGHT 5.4.1 and the results indicate that the shift in composition between dissolved and particulate material does not influence substantially the penetration of photosynthetic active radiation (PAR, 400–700nm), but does result in notable differences in ultraviolet (UV) light penetration, with higher attenuation in the EGC. Future changes in the Arctic Ocean system will likely affect EGC through diminishing sea-ice cover and potentially increasing CDOM export due to increase in river runoff into the Arctic Ocean. Role of attenuation of light by CDOM in determining underwater light regime will become more important, with a potential for future increase in marine productivity in the area of EGC due to elevated PAR and lowered UV light exposures.

A preliminary evaluation of lake morphometric traits influence on the maximum growing depth of macrophytes

Underwater light regime is widely considered the principal determinant of aquatic plant depth distribution. The majority of previous studies dealing with macrophytes in lakes have singled out Secchi disk transparency (SD) values as the key empirical proxy to explain the maximum depth of macrophyte colonization (Zc). Few studies have investigated the role played by lake morphometry in structuring macrophyte beds. Using a balanced dataset including 20 Italian lakes (10 shallow and 10 deep lakes), we analysed transparency and lake morphometric traits to investigate their possible effects on Zc. Our results demonstrate that lake area plays a significant role, and confirm a direct influence of SD values on Zc. Considering lakes with an equal degree of transparency, smaller lakes may yield a lower Zc than larger ones. Morphology has a great influence on lake ecological characteristics especially on water thermal conditions and mixing depth. Based on our data, we argue that the thermal stratification plays a non negligible role in explaining macrophytes zonation, due to its influence on macrophytes life cycles and phytoplankton vertical distribution. Therefore, the present data suggest the need to enhance and refine our knowledge about the relationship between aquatic plants distribution and lake thermal conditions to better model the response of macrophytes to climate change and eutrophication.

Effects of retrogressive permafrost thaw slumping on sediment chemistry and submerged macrophytes in Arctic tundra lakes

Summary1. Global warming is predicted to cause changes in permafrost cover and stability in the Arctic. Zones of high ion concentration in regions of ice‐rich permafrost are a reservoir of chemicals that can potentially be transferred to fresh waters during thawing. Consequently, input of enriched runoff from the thaw and sediment and vegetation from the landscape could alter lakes by affecting their geochemistry and biological production.2. Three undisturbed lakes and five lakes disturbed by retrogressive permafrost thaw slumps were sampled during late summer of 2006 to assess the potential effects of thermokarst shoreline slumping on water and sediment chemistry, the underwater light regime, and benthic macrophyte biomass and community structure.3. Undisturbed lakes had sediments rich in organic material and selected micronutrients, while disturbed lakes had sediments richer in calcium, magnesium and strontium, greater transparency of the water column, and a well‐developed submerged macrophyte community.4. It is postulated that enriched runoff chemistry may alter nutrient availability at the sediment–water interface and also the degradation of organic material, thus affecting lake transparency and submerged macrophytes. The results suggest that retrogressive permafrost slumping can significantly affect food webs in arctic tundra lakes through an increase in macrophyte biomass and development of a more complex benthic habitat.

Limnological effects of Egeria najas Planchon (Hydrocharitaceae) in the arms of Itaipu Reservoir (Brazil, Paraguay)

The limnological effects resulting from the metabolism of Egeria najas Planchon (Hydrocharitaceae) were evaluated in the arms of the Itaipu Reservoir (Brazil). Due to local population decline in biomass and coverage, decomposition was the main process that accounted for the observed limnological changes. Thus, total Kjeldahl nitrogen and total phosphorus in littoral regions were significantly higher during periods of E. najas occurrence. After plant stands disappeared, concentrations decreased in all arms. The nutrient pump effect explains these results, and E. najas stands probably act as sources of nutrients, especially nitrogen. However, low concentrations measured after aboveground biomass disappearance indicate that the internal loading effect is ephemeral and restricted to the littoral regions subjected to the action of naturally senescing populations. The underwater light regime was negatively affected by E. najas occurrence. Overall, these results and the small areas colonized by E. najas in the arms of the Itaipu Reservoir indicate that ecological problems (e.g., reduction of biodiversity) derived from E. najas metabolism are unlikely.

A vertically integrated eutrophication model and its application to a river-style reservoir — Fuchunjiang, China

Based on a 2-D hydrodynamic model, a vertically integrated eutrophication model was developed. The physical sub-model can be used for calculation of water density at different depths, and the water quality sub-model was used for calculation of algal growth. The cohesive and non-cohesive sediments were simulated separately with different methods. The light extinction coefficient used in the underwater light regime sub-model was linearly related to the sum of sediment and phytoplankton biomass. Some components less important to the model were simplified to improve practicability and calculation efficiency. Using field data from Fuchunjiang Reservoir, we calculated the sensitivity of ecological parameters included in this model and validated the model. The results of sensitivity analysis showed that the parameters strongly influenced the phytoplankton biomass, including phytoplankton maximum growth rate, respiration rate, non-predatory mortality rate, settling rate, zooplankton maximum filtration rate, specific extinction coefficient for suspended solids and sediment oxygen demand rate. The model was calibrated by adjusting these parameters. Total chlorophyll a (chl-a) concentrations at different layers in the water column were reproduced very well by the model simulations. The simulated chl-a values were positively correlated to the measured values with Pearson correlation coefficient of 0.92. The mean difference between measured and simulated chl-a concentrations was 12% of the measured chl-a concentration. Measured and simulated DO concentrations were also positively correlated (r = 0.74) and the mean difference was 4% of measured DO concentrations. The successful validation of model indicated that it would be very useful in water quality management and algal bloom prediction in Fuchunjiang Reservoir and a good tool for water quality regulation of other river-style reservoirs.

Light regime and components of turbidity in a Mediterranean coastal lagoon

The underwater light regime of a Mediterranean coastal lagoon (Albufera des Grau, Balearic Islands) was studied during four years in order to characterise the spatial and temporal variations in the light attenuation coefficient ( K) and to assess the relative contribution of the different water components to total light attenuation. During the studied period K averaged 1.42 m −1 and ranged from 0.63 m −1 to 3.80 m −1. High temporal variability was observed in light attenuation coefficients, but the lagoon was spatially uniform. Percentage bottom irradiance in relation to specific requirements for the dominant macrophyte species ( Ruppia cirrhosa) was used as an indicator of benthic light limitation. Macrophyte light limitation was expected to occur in the deepest areas of the lagoon during winter, the most turbid period of the annual cycle. During the macrophyte growing season, higher bottom irradiances were observed but a significant percentage of the lagoon benthos (17% in spring and 7% in summer) was expected to be light limited. In the deepest areas of the lagoon (>2 m) changes in bottom irradiance were related more to variations in the light attenuation coefficient than to variations in water level. However, water level appeared to play an important role in determining benthic light limitation at intermediate depths (1.5 m) for the range of K from 1.8 m −1 to 3.3 m −1. The partitioning of the light attenuation coefficient showed that phytoplankton was the main driver of the temporal dynamics of K, but only accounted for 44% of total light attenuation on average. The mean contributions of the other water components to K were: DOC (47%), tripton (6%), and water (3%). At low values of K, attenuation by DOC was responsible for up to 75% of total attenuation. An equation to predict K from the concentration of water components explained 93% of the variance.

Modeling absorption by CDOM in the Baltic Sea from season, salinity and chlorophyll

In the Baltic Sea the underwater light regime is to a large extent governed by absorption by colored dissolved organic matter (CDOM), where the largest source is riverine input. We have applied a conservative mixing model of CDOMs optical properties to a dataset collected in the Baltic Sea during 11 years of continuous observations in 1993–2004. The majority of observations agreed well with the model indicating that conservative mixing is important in the area, as terrestrial organic matter is diluted with open sea water. Deviations from the conservative mixing pattern mainly occurred at salinities over 6.8, and chlorophyll a concentrations over 1.5 mg m − 3 , and were located in open sea waters, coastal zone and Pomeranian Bay. The seasonal dependence between the light absorption coefficient by CDOM a CDOM(375) and salinity and chlorophyll a concentrations was explored. In March, April and November, months of intensive mixing and high riverine discharge, most of the variability in a CDOM(375) values could be explained by dilution of terrestrially derived CDOM alone. In February, May, and September, months of thermal stratification, reduced riverine discharge and enhanced biological activity, inclusion of chlorophyll a concentration resulted in significantly better models. Autochthonous production of CDOM was found to be a significant source of CDOM in the Southern Baltic Sea in these months. A series of algorithms for the prediction of CDOMs optical properties is presented.

Sediment resuspension and light attenuation in Peoria Lake: can macrophytes improve water quality in this shallow system?

We examined sediment resuspension and light attenuation in relation to the potential for macrophytes to improve water quality conditions in Peoria Lake, Illinois (U.S.A.). The lake exhibited high total suspended solids (TSS) loading and retention of predominantly fine-grained particles in 2000. Large fetches along prevailing wind rose, coupled with shallow morphometry and sediment particles composed of >90% silt and clay resulted in frequent periods of sediment resuspension. As calculated (wave theory) shear stress increased above the critical shear stress (measured experimentally), turbidity increased substantially at a resuspension monitoring station. Resuspension model explorations suggested that establishment of submersed aquatic macrophytes could substantially reduce sediment resuspension in Peoria Lake. However, Kd is currently very high, while Secchi transparency low, at in-lake stations. Thus, in order to establish a persistent macrophyte population in the lake to control resuspension, the underwater light regime will have to improve quite dramatically.

The influence of solar radiation on bleaching of shallow water reef corals in the Andaman Sea, 1993–1998

Historically, elevated sea-temperature excursions have been used almost exclusively to explain incidences of natural coral bleaching. Little attention, however, has been paid to instances where environmental conditions appear conducive to bleaching but none has been observed. In this paper we examine contemporaneous records of sea temperature and solar radiation (photosynthetically active radiation; PAR, 400–700 nm) over a 6-year period at a site in the Andaman Sea where regular monitoring of shallow water reef coral communities has been conducted since 1979. Four years (1991, 1995, 1997, and 1998) when anomalous sea temperatures were recorded in May are compared. We suggest a complex interaction of PAR and sea temperature, whereby elevated solar radiation prior to sea-temperature maxima may actually protect corals against subsequent bleaching. In addition, we demonstrate the important role that sea-level anomalies play in modifying the underwater light regime to bring about such conditions.

Effect of climate change relative to ozone depletion on UV exposure in subarctic lakes.

The effect of stratospheric ozone depletion on increases in ambient levels of solar ultraviolet (UV) radiation in high-latitude regions' has raised concerns about the response of northern ecosystems to environmental change. The concentration of coloured dissolved organic material, which is derived from terrestrial vegetation and acts as a screen for ultraviolet radiation, is low in high-latitude lakes. The underwater light environment in these lakes is therefore likely to be sensitive to small variations in the supply of this material, in addition to the effects of ozone depletion. Here we use fossil diatom assemblages in combination with bio-optical models to estimate the magnitude of past variations in the underwater light regime of a lake at the boreal tree line. We find large shifts in underwater UV-B, UV-A and photosynthetically available radiation associated with changes in the input of coloured dissolved organic material into subarctic lakes during the Holocene. The inferred changes in biological exposure to UV radiation were at least two orders of magnitude greater than those associated with moderate (30%) ozone depletion. Our findings indicate that freshwater ecosystems at present located across vegetation gradients will experience significant shifts in underwater spectral irradiance through the effects of climate change on catchment vegetation and the export of coloured dissolved organic material.

Aquatic macrophyte distribution in relation to water and sediment conditions in the Itaipu Reservoir, Brazil.

Aquatic macrophyte community distribution along the eastern shoreline of the Itaipu Reservoir (one of the South America’s largest impoundments) is described in relation to limnological and sedimentological factors. The central body of the reservoir is mesotrophic, while the arms (flooded influent river valleys) along the eastern shore may be oligo-mesotrophic to eutrophic, depending on time of year and sub-catchment characteristics. Macrophyte community composition and species cover were surveyed at 30 sites in four arms, in relation to sediment total P and organic matter; underwater light regime; and water total P and Kjeldahl N concentration, alkalinity, conductivity, depth and pH. Seventeen euhydrophyte and six emergent macrophyte species were recorded. Large stands of Egeria najas dominated the euhydrophyte vegetation, together with free-floating weed species (Pistia stratiotes Linn., Salvinia auriculata Aublet and Eichhornia crassipes (Mart.) Solms.). Canonical Correspondence Analysis of the data showed that two sets of variables were important predictors of aquatic macrophyte community structure. Floating macrophyte assemblage was closely related to concentration of nutrients in both water and sediment, while light penetration was the strongest predictor of submerged species occurrence. Although a large number of potential nuisance species were present, dense growths were restricted to shallower areas of the Itaipu Reservoir, causing localised problems. The possibility of increasing interference by these plants with fisheries, recreational use, transport and hydroelectricity generation suggests a need for continued monitoring of weed distribution and abundance, and investigation of appropriate management measures.

Seagrasses, Dredging and Light in Laguna Madre, Texas, U.S.A.

Light reduction resulting from maintenance dredging was the suspected cause of large-scale loss of seagrass cover in deep parts of Laguna Madre between surveys conducted in 1965 and 1974. Additional changes to 1988, together with an analysis of dredging frequency and intensity for different parts of the laguna, were consistent with this interpretation. Intensive monitoring of the underwater light regime and compilation of detailed environmental data for 3 months before and 15 months after a dredging project in 1988 revealed reduced light attributable to dredging in four of eight subdivisions of the study area, including the most extensive seagrass meadow in the study area. Dredging effects were strongest close to disposal areas used during this project but still were detectable on transects >1·2 km from the nearest dredge disposal area. In the subdivision of the study area where most of the dredge disposal occurred, light attenuation was increased throughout the 15 months of observation after dredging. In the seagrass meadow and the transition zone at the outer edge of the meadow, effects were evident up to 10 months after dredging. Resuspension and dispersion events caused by wind-generated waves are responsible for the propagation of dredge-related turbidity over space and time in this system.

SEASONAL PRODUCTIVITY OF TWO RED ALGAE IN A CENTRAL CALIFORNIA KELP FOREST1

ABSTRACTThe in situ productivity of Botryocladia pseudodichotoma (Farl.) Kyl. and Rhodymenia californica var. californica Kyl., two common understory macrophytes in Macrocystis pyrifera (L.) C. Ag. forests, was determined once per month for one year. Gross productivity for B. pseudodichotoma was highest in October (1.23 mgC · g dry wt.−1· h−1), but rates were generally greater in spring and lower in summer. Respiration was variable throughout the year, especially in proportion to gross productivity. Gross productivity for R. californica var. californica was also highest in October (4.62 mgC · g dry wt.−1· h−1), and lowest in summer. Respiration was highly variable throughout the year. Deep (11 m) populations o/B. pseudodichotoma had 50% higher productivity than shallow (3 m) populations when incubated in shallow water, and 45% higher productivity when incubated in deep water. Populations of B. pseudodichotoma growing in shade at 3 m had 77% higher productivity than populations growing in sun at the same depth when incubated in a sun exposed location at 3 m. Respiration of the shade‐adapted plants was only one‐half that of the sun‐adapted plants. In comparison with similar studies in other sub‐tidal communities, net productivity of these two understory red algae is somewhat lower. Lack of strong seasonal productivity patterns and the highly variable underwater light regime suggests that understory algae may be adapted for rapid growth during short periods of high light, regardless of the time of year.