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1.
Mangrove Lagoon, located on the island of St. Croix, US Virgin Islands (USVI), is one of few actively bioluminescent lagoons in a location experiencing significant anthropogenic impacts. The bioluminescence is due to an abundance of the dinoflagellate Pyrodinium bahamense in the water column. We recovered surface sediments and sediment cores from Mangrove Lagoon to analyze the spatial distribution and temporal variability of P. bahamense cysts in this system. Surface sediment P. bahamense cyst concentrations ranged from 0 to 466 cysts g?1 dry sediment, with higher abundances associated with elevated surface water nutrient concentrations and a mixed terrestrial–marine organic matter source regime. In combination with available bioassay data, we hypothesize that phytoplankton utilize nutrients rapidly and subsequent decay of organic matter makes nutrients available for dinoflagellates at the sediment–water interface in the eastern and northern quadrants of the lagoon. However, the nutrients are rapidly exhausted during counterclockwise lagoon circulation resulting in the decline of primary productivity and dinoflagellate abundance in the western quadrants. Downcore profiles suggest that P. bahamense blooms have been occurring for decades, declining in recent years. No cysts were present in sediments predating dredging activities of the 1960s that created Mangrove Lagoon. Recent reductions in cyst abundance may be the result of limited primary productivity caused by restricted water exchange with Salt River Bay due to shallowing of a sill at the mouth of the lagoon. This research highlights the need for more comprehensive geochemical and fossil analyses to better understand long-term ecological variability and inform conservation efforts of these unique habitats.  相似文献   

2.
Porewater (i.e., groundwater) samples were collected from multi-level piezometers across the freshwater-saltwater seepage face within the Indian River Lagoon subterranean estuary along Florida’s (USA) Atlantic coast for analysis of the rare earth elements (REE). Surface water samples for REE analysis were also collected from the water column of the Indian River Lagoon as well as two local rivers (Eau Gallie River, Crane Creek) that flow into the lagoon within the study area. Concentrations of REEs in porewaters from the subterranean estuary are 10-100 times higher than typical seawater values (e.g., Nd ranges from 217 to 2409 pmol kg−1), with submarine groundwater discharge (SGD) at the freshwater-saltwater seepage face exhibiting the highest REE concentrations. The elevated REE concentrations for SGD at the seepage face are too high to be the result of simple, binary mixing between a seawater end-member and local terrestrial SGD. Instead, the high REE concentrations indicate that geochemical reactions occurring within the subterranean estuary contribute substantially to the REE cycle. A simple mass balance model is used to investigate the cycling of REEs in the Indian River Lagoon and its underlying subterranean estuary. Mass balance modeling reveals that the Indian River Lagoon is approximately at steady-state with respect to the REE fluxes into and out of the lagoon. However, the subterranean estuary is not at steady-state with respect to the REE fluxes. Specifically, the model suggests that the SGD Nd flux, for example, exported from the subterranean estuary to the overlying lagoon waters exceeds the combined input to the subterranean estuary from terrestrial SGD and recirculating marine SGD by, on average, ∼100 mmol day−1. The mass balance model also reveals that the subterranean estuary is a net source of light REEs (LREE) and middle REEs (MREE) to the overlying lagoon waters, but acts as a sink for the heavy REEs (HREE). Geochemical modeling and statistical analysis further suggests that this fractionation occurs, in part, due to the coupling between REE cycling and iron redox cycling within the Indian River Lagoon subterranean estuary. The net SGD flux of Nd to the Indian River Lagoon is ∼7-fold larger than the local effective river flux to these coastal waters. This previously unrecognized source of Nd to the coastal ocean could conceivably be important to the global oceanic Nd budget, and help to resolve the oceanic “Nd paradox” by accounting for a substantial fraction of the hypothesized missing Nd flux to the ocean.  相似文献   

3.
Seasonal variation in the standing crop of the seagrassSyringodium filiforme and its associated macrophytes was studied in a northern basin of the Indian River, a large mesohaline lagoon in central Florida, near the northern distributional limit ofS. filiforme. The minimum standing crop occurred from February through April and the maximum in September. Two other seagrasses,Halodule wrightii andHalophila engelmannii, together with a drift algal community, occurred in the study quadrat, but were not major components of the macrophytic system. The formation of sizeable sandy patches within Indian River seagrass beds is partially due to the burrowing activities ofLimulus polyphemus. Thermal stresses associated with the northern geographicalS. filiforme range may contribute to this phenomenon by restricting annual production, hence limiting patch regrowth.  相似文献   

4.
The effect of a 7-mo drought (La Niña 1988) was evaluated on pelagic properties in the large Patos Lagoon (30°12′–32°12′S, 50°40′–52°15′W). From December 1987 to December 1988, surface water was sampled along the longitudinal axis of the lagoon for temperature (10–29°C), salinity (0–31.4), dissolved inorganic phosphate (0.02–4.73 μM), nitrate (0.05–66.25 μM), nitrite (0.01–3.54 μM), ammonium (0.09–33.19 μM), silicate (1.11–359.20 μM), phytoplankton chlorophylla (chl; 0.4–41.2 mg m?3), primary production (gross PP 1.72–161.82 mg C m3 h?1; net PP 0.04–126.19 mg C m3 h?1), and species composition and abundance (42–4,961 ind ml?1). In the wet season the whole system acted as a river and light availability limited phytoplankton growth. During the drought from February to August monthly freshwater runoff was low and the inflow of marine water to the southern sector generated spatial variability of the analyzed properties and five functional areas were recognized. The northernmost Guaíba River (1) presented low light availability and phytoplankton chl concentration compared to the northern limnetic area (2) (chl mean 13.3 μg I?1; max 41.2 μg I?1; gross PP mean 52.6 mg C m3 h?1), which acted as a biological filter removing dissolved inorganic nutrients. Silicate concentration was strongly diminished in this area due to diatom uptake (Aulacoseira granulata, 9,330 cells ml?1). In the northern limnetic and central oligohaline (3) areas, phytoplankton biomass was controlled by light but nitrogen also played a limiting role. In the southern area (4) that is under marine influence, low chl concentration (mean 4.5 μg I?1) and gross PP (mean 28.1 mg C m3 h?1) coincided with co-limitation of nitrogen and light while the channel to the ocean (5) was strongly light limited. This study demonstrated that low light and high silicate input had a buffer effect at Patos Lagoon, hampering negative expression of cultural eutrophication. The main effect during the drought period occurred in the northern limnetic region, where low silicate values due to diatom uptake led to higher cyanobacteria abundance, and enhanced mineralization occurred in the central oligohaline lagoon. Increased rainfall resulted in light limitation and decreasing primary production in the entire freshwater lagoon, and the adjacent coastal region benefited from nutrient enrichment.  相似文献   

5.
Field data of tidal current speeds collected January 9–31, 1990, in Sebastian Inlet, which connects the Atlantic Ocean and the Indian River Lagoon on the east coast of central Florida, show that the average Eulerian and Stokes residual currents are both lagoonward. This pattern can be used to explain the long-term trend of accumulations of marine sediments on the flood tidal delta adjacent to the lagoon end of the inlet. Numerical model results indicate that the long-term Stokes residual current is mainly determined by the tidal characteristics of the lagoon and ocean, and subsequently, are less variable. The long-term lagoonward Eulerian current, on the other hand, is interrupted by episodic weather events such as frontal storms. Storms can cause the abrupt superelevation of instantaneous water-levels on the lagoon side of the inlet. The short-lived pulses of freshwater inflow into the lagoon associated with storms could be discharged through the inlet instantaneously. Both the instantaneous superelevation of lagoon water levels and freshwater outflow can cause temporary reversal of Eulerian residual current in the inlet. Therefore, the general residual flow pattern in Sebastian Inlet is not only determined by the tidal characteristics of the Atlantic Ocean and Indian River Lagoon but also by the wind and precipitation associated with episodic storms, and by the long-term mean sea-level difference between the lagoon and the ocean.  相似文献   

6.
The flushing of Florida’s Indian River Lagoon is investigated as a response to tidal and low-frequency lagoon-shelf exchanges in the presence of freshwater gains and losses. A one-dimensional computer model uses the continuity equation to convert water-level variations into both advective transport within the lagoon and lagoon-shelf exchanges. The model also incorporates transport by longitudinal diffusion. Flushing is quantified by calculating the 50% renewal time, R50, for each of 16 segments. R50 is calculated for tidal exchanges enhanced by 0–30 cm nontidal fluctuations in coastal sea level, then for a range of rainfall rates. In both series of simulations, results suggest that in the northern sub-basin, R50 increases dramatically with distance from the inlet due to relatively weak tidal and nontidal exchanges. A 50% renewal occurs in about one tidal cycle just inside Sebastian Inlet; at the northern end of the northern sub-basin, R50 is over 230 d, and only coastal sea-level variations on the order of 30 cm and/or dry season rainfall rates decrease R50 to less than 1 yr. R50 is 1 wk or less throughout the central and southern sub-basins, where lagoon-shelf exchanges occur through two inlets. Simulations involving seasonal variations in precipitation and evaporation indicate that maximum and minimum rates of freshwater input lead minimum and maximum salinities by time periods on the order of 2–3 wk for the lagoon as a whole and in the northern sub-basin. The central and southern sub-basins respond in 1–2 wk.  相似文献   

7.
Estuarine species with wide geographic distributions often experience tidal regimes that vary significantly throughout their range. Plasticity in behaviors associated with the tide is expected to enable synchronization with local tides. The American horseshoe crabLimulus polyphemus typically inhabits estuaries and coastal areas with pronounced semi-diurnal tides that play a role in synchronizing the timing of spawning and larval hatching, but also lives in areas that lack significant tides and associated synchronization cues. We investigated the spatial and temporal pattern of adult spawning and larval hatching ofL. polyphemus in a microtidal coastal lagoon (Indian River Lagoon, Florida, USA). Spawning activity and larval abundance were monitored weekly February 1998–August 2000 at sites spanning 100 km of the lagoon. To identify possible synchronization cues for spawning and hatching success, the presence of adult and larvalL. polyphemus were related to environmental and hydrologic variables using logistic regression. The presence of spawning adults varied significantly among the sub-basins of the lagoon, with the highest densities occurring in the Banana River. Large spawning aggregations were not observed and densities never exceeded 6 m−2. Spawning occurred year-round but varied seasonally with episodes of increased mating activity in the early spring. The occurrence of mating pairs was episodic and was not synchronized among sites. Larval densities were low (4 m−3) and larvae were present at only 12 of the 21 sites. Hatching success was decoupled temporally from spawning activity, with peaks in larval abundance occurring approximately 8 wk after peaks in spawning. Larval abundance was associated with periods of high water. Reproductive activity of horseshoe crabs in the lagoon differs significantly from populations inhabiting areas with semi-diurnal and diurnal tides. These differences are likely due to the lack of periodic tidally-related synchronization cues and regular beach inundation.  相似文献   

8.
Venice Lagoon, Italy, rests on a series of aquifers that are 1,000 m thick. Measurements of submarine groundwater discharge (SGD) were made in Venice Lagoon using benthic chambers vented to a plastic collection bag. Two hundred measurements taken in a pristine northern lagoon site (Isola la Cura) revealed flow rates as high as 200 cm d−1 with an average of 30 cm d−1. Over 100 measurements taken adjacent to a bulkhead shoreline in the Porto Marghera industrial zone (Fusina) showed flow rates as high as 30 cm d−1 and averaging 6 cm d−1. These flow rates, if representative of even a fraction of the lagoon floor, are easily able to account for the 15% deficit previously calculated between precipitation and runoff for the entire Venice Lagoon drainage basin. Land elevation surrounding the Venice Lagoon is < 10 m within 20 km of the shoreline and is unable to support any substantial onshore water table. Submarine groundwater discharge most likely represents upward artesian discharge from deeper partially confined aquifers. Over 60 samples were collected in total from both sites for nutrient analysis. Ammonium concentration was found to be 2–8 fold higher in the device water than in the lagoon water at the northern site depending on season, and 10–30 times higher at the industrial zone site. These numbers suggest that SGD may be the primary pathway for nutrients and perhaps other contaminants to enter Venice Lagoon.  相似文献   

9.
Fifteen species of elasmobranchs, eight sharks and seven rays, have been recorded with reasonable certainty from the Indian River lagoon system on the central east coast of Florida. We collected four shark and six ray species during a three and one-half year study of the northern portion of the lagoon system. Five of these appear to be year-round residents, and the remainder utilize the area only at restricted times of the year or as a nursery ground. The most abundant resident species areDasyatis sayi, D. sabina, andCarcharhinus leucas. Pristis pectinata, once a common resident species, has been extirpated from the lagoons. The distribution of ocean inlets and salinity appear to be major factors affecting diversity and numbers of elasmobranchs in the Indian River system.  相似文献   

10.
Between August 14 and September 26, 2004, four tropical weather systems (Charley, Frances, Ivan, and Jeanne) affected the central Indian River Lagoon (IRL). The central IRL received a prodigious amount of rainfall for the 2 mo, between 72 and 83 cm, which is a once-in-50-yr rainfall event. High stream discharges were generated that, combined with wind-suspended sediments, significantly reduced salinities and water transparency. In September, salinities among central IRL segments dropped from 30 psu or more to ≤15 psu, color increased from a low of 10 pcu to ≥100 pcu, and turbidity increased from ≤3 NTU up to 14 NTU. Evidence of the hurricanes' physical effects on seagrasses (burial, no scour) was limited to just one of the more than 25 sites inspected. Within 2 to 3 mo following the hurricane period, most parameters related to water transparency returned to or showed improvement over their prehurricane (February–July 2004) levels. Unseasonably low salinities (<20 psu) and moderately high color (>20 pcu) were observed through spring 2005, largely attributable to a relatively long residence time and a wetter-than-average spring season in 2005. By the end of the study period (July 2006), the central IRL generally showed a continuation of two opposite seagrass trends—an increase in depthlimit coverage but a decline in coverage density—that began before 2004. Also, within a limited reach of the central IRL, there was a temporary shift in species composition in summer 2005 (Ruppia maritima increased asHalodule wrightü decreased). It is likely that the persistently low salinities (not color) in 2004–2005 affected the species composition and coverage density. This study reveals that seagrasses are resilient to the acute effects of hurricanes and underscores the need to reduce chronic, an thropogenic effects on seagrasses.  相似文献   

11.
Pore and surface water sulfide variation near artificial ditches and a natural creek are examined in salt marshes bordering the Indian River Lagoon in east-central Florida. Pore water sulfide concentrations ranged from 0 μg-at I?1 to 1,640 μg-at I?1. On average, the natural creek had the lowest sulfide concentrations (mean <1.0 μ-at I?1) and the perimeter ditch of a managed salt marsh impoundment the highest (436.5 μg-at I?1). There was a trend of increasing sulfide concentration in the summer, and sharp peaks in late fall-early winter which correspond with peak litter input into the sediments. Significant differences in sulfide concentration between sites are attributed to differences in water flow and in organic matter content. Delaying the seasonal opening of culverts (which connect impounded marshes with the lagoon) until lagoon water levels rise in fall may prevent massive fish kills that have been associated with high sulfide levels in the impoundment perimeter ditches.  相似文献   

12.
The distilling effect of evaporation and the diluting effect of precipitation on salinity at two estuarine sites in the humid subtropical setting of the Indian River Lagoon, Florida, were evaluated based on daily evaporation computed with an energy-budget method and measured precipitation. Despite the larger magnitude of evaporation (about 1,58 mm yr−1) compared to precipitation (about 1,180 mm yr−1) between February 2002 and January 2004, the variability of monthly precipitation induced salinity changes was more than twice the variability of evaporation induced changes. Use of a constant, mean value of evaporation, along with measured values of daily precipitation, were sufficient to produce simulated salinity changes that contained little monthly (root-mean-square error = 0.33‰ mo−1 and 0.52‰ mo−1 at the two sites) or cumulative error (<1‰ yr−1) compared to simulations that used computed daily values of evaporation. This result indicates that measuring the temporal variability in evaporation may not be critical to simulation of salinity within the lagoon. Comparison of evaporation and precipitation induced salinity changes with measured salinity changes indicates that evaporation and precipitation explained only 4% of the changes in salinity within a flow-through area of the lagoon; surface water and ocean inflows probably accounted for most of the variability in salinity at this site. Evaporation and precipitation induced salinity changes explained 61% of the variability in salinity at a flow-restricted part of the lagoon.  相似文献   

13.
The St. Lucie Estuary, located on the southeast coast of Florida, provides an example of a subtropical ecosystem where seasonal changes in temperature are modest, but summer storms alter rainfall regimes and external inputs to the estuary from the watershed and Atlantic Ocean. The focus of this study was the response of the phytoplankton community to spatial and temporal shifts in salinity, nutrient concentration, watershed discharges, and water residence times, within the context of temporal patterns in rainfall. From a temporal perspective, both drought and flood conditions negatively impacted phytoplankton biomass potential. Prolonged drought periods were associated with reduced nutrient loads and phytoplankton inputs from the watershed and increased influence of water exchange with the Atlantic Ocean, all of which restrict biomass potential. Conversely, under flood conditions, nutrient loads were elevated, but high freshwater flushing rates in the estuary diminished water residence times and increase salinity variation, thereby restricting the buildup of phytoplankton biomass. An exception to the latter pattern was a large incursion of a cyanobacteria bloom from Lake Okeechobee via the St. Lucie Canal observed in the summer of 2005. From a spatial perspective, regional differences in water residence times, sources of watershed inputs, and the proximity to the Atlantic Ocean influenced the composition and biomass of the phytoplankton community. Long water residence times in the North Fork region of the St. Lucie Estuary provided an environment conducive to the development of blooms of autochthonous origin. Conversely, shorter residence times in the mid-estuary limit autochthonous increases in biomass, but allochthonous sources of biomass can result in bloom concentrations of phytoplankton.  相似文献   

14.
Ten years (1985–1994) of data were analyzed to investigate general patterns of phytoplankton and nutrient dynamics, and to identify major factors controlling those dynamics in the York River Estuary, Virginia. Algal blooms were observed during winter-spring followed by smaller summer blooms. Peak phytoplankton biomass during the winter-spring blooms occurred in the mid reach of the mesohaline zone whereas peak phytoplankton biomass during the summer bloom occurred in the tidal fresh-mesohaline transition zone. River discharge appears to be the major factor controlling the location and timing of the winter-spring blooms and the relative degree of potential N and P limitation. Phytoplankton biomass in tidal fresh water regions was limited by high flushing rates. Water residence time was less than cell doubling time during high flow seasons. Positive correlations between PAR at 1 m depth and chlorophylla suggested light limitation of phytoplankton in the tidal fresh-mesohaline transition zone. Relationships of salinity difference between surface and bottom water with chlorophylla distribution suggested the importance of tidal mixing for phytoplankton dynamics in the mesohaline zone. Accumulation of phytoplankton biomass in the mesohaline zone was generally controlled by N with the nutrient supply provided by benthic or bottom water remineralization.  相似文献   

15.
Al-Kharrar Lagoon is a fossil back-reef basin with hypersaline waters, situated 10 km northwest of Rabigh city, central of the eastern Red Sea coast, Saudi Arabia. About 130 stations were selected for measurements of the lagoon’s water temperature, salinity, pH, dissolved oxygen, and water depths during March 2014. The common macro-algae, flora, and fauna were also sampled and identified. The present study aims to investigate the prevailing environmental parameters and their impact on the macro-fauna/flora of the lagoon. The average water depth of the lagoon was around 5 m and reached maximum values of 8 and 16 at the lagoon centre and inlet, respectively. The results showed that the lagoon’s surface water temperature and salinity have mean values of 25 °C and 40‰, but with extreme values of 30 °C and 45‰ that occurred only at the enclosed intertidal areas, respectively. Their dissolved oxygen (DO) and pH were 6.5 mg/l and 8.3, respectively and the latter showing the highest values up to 8.5 in the intertidal areas dominated by the green cyanobacteria. These physicochemical conditions make the lagoon as a favorite place for the mangrove Avicennia marina, macro-algae, seagrasses (Halophila stipulacea and Cymodocea rotundata), and algal mats (Cyanobacteria) which dominate the intertidal and supratidal areas of the lagoon, tolerating extremely high-salinity and high-temperature conditions. On the other hand, corals were observed alive at the southern part of the lagoon, immediately south of the Al-Ultah Islet. Vertical profiles of temperature, salinity, and density in the lagoon’s water indicated that the water column consists of two layers throughout the year.  相似文献   

16.
We investigated the dissolved oxygen metabolism of the Curonian Lagoon (Baltic Sea) to assess the relative contributions of pelagic and benthic processes to the development of transient hypoxic conditions in shallow water habitats. Metabolism measurements along with the remote sensing-derived estimates of spatial variability in chlorophyll a were used to evaluate the risk of hypoxia at the whole lagoon level. Our data demonstrate that cyanobacterial blooms strongly inhibit light penetration, resulting in net heterotrophic conditions in which pelagic oxygen demand exceeds benthic oxygen demand by an order of magnitude. The combination of bloom conditions and reduced vertical mixing during calm periods resulted in oxygen depletion of bottom waters and greater sediment nutrient release. The peak of reactive P regeneration (nearly 30 μmol m?2 h?1) coincided with oxygen depletion in the water column, and resulted in a marked drop of the inorganic N:P ratio (from >40 to <5, as molar). Our results suggest a strong link between cyanobacterial blooms, pelagic respiration, hypoxia, and P regeneration, which acts as a feedback in sustaining algal blooms through internal nutrient cycling. Meteorological data and satellite-derived maps of chlorophyll a were used to show that nearly 70 % of the lagoon surface (approximately 1,000 km2) is prone to transient hypoxia development when blooms coincide with low wind speed conditions.  相似文献   

17.
The availability of reactive phosphorus (P) may promote cyanobacterial blooms, a worldwide increasing phenomenon. Cyanobacteria may also regulate benthic P cycling through labile organic input to sediments, favouring reduced conditions and P release, ultimately acting as self-sustainment mechanism for the phytoplankton blooms. To analyse P–cyanobacteria feedbacks and compare external versus internal loads, we investigated P cycling in the Curonian Lagoon, a freshwater estuary with recurrent summer blooms. At two sites representing the dominant sediment types, we characterised P pools and mobility, via combined pore water analysis, calculation of diffusive exchanges and flux measurements via sediment core incubations. Annual P budgets were also calculated, to analyse the whole lagoon role as net sink or source. Muddy sediments, representing nearly 50 % of the lagoon surface, displayed higher P content if compared with sandy sediments, and most of this pool was reactive. The muddy site had consequently higher pore water dissolved inorganic phosphorus (DIP) concentrations maintaining high diffusive gradients. However, measured fluxes suggested that both sediment types were mostly P sinks except for a large DIP regeneration (nearly 30 μmol m?2 h?1) recorded at the muddy site during an intense cyanobacteria bloom. Such internal regeneration had the same order of magnitude as the annual external P load and may offset the net annual DIP sink role of the estuary. It may also prolong the duration of the bloom. Our results suggest that positive feedbacks can regulate N-fixing cyanobacteria blooms and internal P recycling, through either diffusive fluxes or sediment settling and resuspension.  相似文献   

18.
Estuarine and coastal systems represent a challenge when it comes to determining the causes of ecological change because human and natural perturbations often interact. Phytoplankton biomass (chlorophyll a) and group-specific photopigment indicators were examined from 1994 to 2007 to assess community responses to nutrient and climatic perturbations in the Neuse River Estuary, NC. This system experienced nutrient enrichment and hydrologic variability, including droughts, and an increase in hurricanes. Freshwater input strongly interacted with supplies of the limiting nutrient nitrogen (N) and temperature to determine the location, magnitude, and composition of phytoplankton biomass. Multi-annual, seasonal, and episodic hydrologic perturbations, including changes in the frequency and intensity of tropical storms, hurricanes and droughts, caused significant shifts in phytoplankton community structure. Climatic oscillations can at times overwhelm anthropogenic nutrient inputs in terms of controlling algal bloom thresholds, duration, and spatial extent. Eutrophication models should incorporate climatically driven changes to better predict phytoplankton community responses to nutrient inputs and other anthropogenic perturbations.  相似文献   

19.
In the shallow microtidal Patos Lagoon estuary, southern Brazil (32° 07′ S–52° 06′ W), chlorophyll a (Chl a) variability was studied at different time scales during the last 25 years (hourly–daily sampling in 1984/1985; weekly sampling in 1986 and from 1988 to 1990; monthly sampling from 1993 to 2008). Phytoplankton biomass variation seems to be most influenced by hydrology, which is primarily driven by meteorological factors like wind, rainfall, and evaporation. However, it was observed that the hydrological driving forces play different roles at different time scales. For instance, short-term Chl a variability is mainly controlled by winds, while long-term changes are related to the freshwater input by rainfall. Significant correlation was found between the total amount of rain in the year and the mean annual value of Chl a, though this relationship was linear until 1,500 mm of rain per year. After this threshold, mean annual Chl a values dropped significantly, probably due to a washout of the produced biomass from the estuary. Similarly, low rainfall levels and drought years lead to small phytoplankton biomass due to scarcity of nutrient, mainly silicate, or a possible inhibitory effect generated by high ammonium concentration. In this sense, large-scale Chl a variability would be related to the El Ni?o-Southern Oscillation climatic anomaly, which influences the rainfall levels in Southern Brazil, though sampling periodicity has also great influence on this relationship. No Chl a or nutrient enrichment was observed in the estuarine region along the last years, indicating that this estuary is not subject to an eutrophication process. In contrast, signals of an ongoing oligotrophication are observed, possibly a remote effect of the eutrophication in the Northern area of the lagoon where the phytoplankton nutrients uptake may act as a biological filter mechanism.  相似文献   

20.
Opening and closure of a marginal Southern California lagoon inlet   总被引：1，自引：0，他引：1  
Over the past 50 yr, direct observations of the inlet status (open or closed) of San Dieguito Lagoon, a typical southern California lagoon located in Del Mar, California, have shown that river flooding is the major natural determinant of inlet conditions on time scales longer than a few years. River flooding is strongly dependent on rainfall in the San Dieguito River watershed and on the influences of two water storage reservoirs in the area. Rainfall fluctuates on yearly and longer time scales and undergoes cycles of wet and dry periods. Over short time periods, ranging from a few months to several years, inlet status is primarily determined by the available tidal prism and littoral sand transport. Recognition of these factors is crucial in order to correctly evaluate the probability that a small lagoon will remain open naturally. A probability approach is essential because the variables controlling inlet conditions are random in nature. The results of our study show that the inlet will remain open naturally 34% of the time. The tendency to remain open is vastly smaller during years of dry weather (12%) versus times of above-average rainfall (66%).  相似文献   

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