Seasonal Cycle Of Chlorophyll Research Articles

Description and sensitivity analysis for the LESV model: Water quality variables and the balance of organisms in a fjordic region of restricted exchange

In this paper we describe a new ecological model for Regions of Restricted Exchange (RRE), such as fjords, estuaries, rias and lagoons. The model is intended to simulate the impact of external nutrient input on microplankton (phytoplankton plus pelagic microheterotrophs) in RREs. We have implemented the model with the practical purpose of finding a safe limit to the capacities of RRE to assimilate fish-farm waste. Sea-cage farming of fish is increasing in fjords in northern and southern hemispheres, and its external nutrient input can lead to environmental problems such as eutrophication and deoxygenation. The model includes a physical system of three layers with exchanges driven by tidal movement, freshwater input, wind stirring. The biological part includes two microplankton compartments, each parameterizing a microbial loop and each containing chlorophyll. The first compartment represents diatoms and associated heterotrophs, and the second compartment represents flagellates and associated heterotrophs. As well as the balance of these organisms, the model simulates concentrations of nutrient N, P, and Si, dissolved oxygen, and water transparency. Chlorophyll and nutrient change are linked by yields ( q). Losses of microplankton to grazing by mesozooplankton or benthos are simulated by a temperature-dependent grazing pressure acting on a mean loss ( L 0 ) . The model also includes the ability to simulate point source inputs of nutrients or organic matter and a generic tracer with first order decay. Sea-cage fish-farms exemplify such point sources. In order to explore model behaviour, we included inputs from a 1500 tonnes salmon farm multiplied by a factor ( γ ) . We carried out sensitivity analysis to identify the most influential model parameters and forcing variables in the case of the shallow Scottish fjord, Loch Creran, in 1975 before the introduction of salmon farming. We tested the model fit to this pristine state ( γ = 0 ) , using Major Axis Regression of simulated variables on observed variables. The model successfully follows the seasonal cycles of chlorophyll (summer over both microplanktons) and the limiting nutrients (P, N). The sensitivity analysis identified three sets of key parameters: ( γ ) and other fish-farm coefficients, which control farm waste effects on an RRE; ( L 0 ) parameters for each microplankton, which link these to the rest of the ecosystem and which have implications for future inclusion of shellfish farming in the model and, chlorophyll yields from nutrients ( q), which are crucial for the predication of eutrophication and the ecological understanding of the model.

Nutrient supply to anticyclonic meso-scale eddies off western Australia estimated with artificial tracers released in a circulation model

The phytoplankton distribution off western Australia in the period from April to October is unique in that high biomass is generally associated with anticyclonic eddies and not with cyclonic eddies. As the western Australian region is oligotrophic this anomalous feature must be related to differing nutrient supply pathways to the surface mixed layer of cyclonic and anticyclonic eddies. A suite of modelled abiotic tracers suggests that cyclonic eddies are predominantly supplied by diapycnal processes that remain relatively weak until June–July, when they rapidly increase because of deepening surface mixed layers, which start to tap into the nutrient-replete waters below the euphotic zone. To the contrary, we find that anticyclonic eddies are predominantly supplied by injection of shelf waters, which carry elevated levels of inorganic nutrients and biomass. These injections start with the formation of the eddies in April–May, continue well into the austral winter and reach as far as several hundred kilometers offshore. The diapycnal supply of nutrients is suppressed in anticyclonic eddies since the injection of warm, low-salinity shelf waters delays the erosion of the density gradient at the base of the mixed layer. Our results are consistent with the observed seasonal cycles of chlorophyll a and observation of particulate organic matter export out of the surface mixed layer of an anticyclonic eddy in the region.

Chlorophyll distribution in a temperate estuary: The Strait of Georgia and Juan de Fuca Strait

Data collected during 7 years of seasonal surveys are used to investigate the distribution of phytoplankton biomass within the estuarine waters of the Strait of Georgia and Juan de Fuca Strait. Variability of the chlorophyll distribution is examined in relation to the density stratification, light availability and nutrient concentration. In the Strait of Georgia, both the horizontal and vertical distribution of chlorophyll are found to be linked to the presence of a near-surface layer of increased density stratification. Despite important year-to-year variability, the seasonal cycle of chlorophyll in the Strait of Georgia is dominated every year by relatively large near-surface concentrations in the spring that are linked to the seasonal increase in solar radiation onto the stratified near-surface layer. In the vertical, a sub-surface peak is observed around 10 m depth, corresponding to the depth of maximum water column stability. Nutrients within the euphotic zone are in general abundant, with the exception of the Strait of Georgia in summer where phytoplankton growth is potentially limited by low nitrate concentration near the surface. The depth of the euphotic zone is estimated along the thalweg of the estuary from transmissometer profiles. It appears to vary relatively little within the estuary from a minimum of 20 m in spring, near the mouth of the Fraser River, to an autumnal maximum of about 30 m in the northern Strait of Georgia. Finally, the estimated self-shading contribution to light attenuation is shown to be generally significant (5–10%) in the surface waters of the Strait of Georgia, during spring and summer, reaching values as high as 35% during the spring bloom.

Interannual variability in the seasonal cycle of chlorophyll in the Leeuwin Current off the southwest Western Australian coast

The seasonal cycle of surface chlorophyll in the Leeuwin Current off the southwest Western Australian coast is characterized by low austral summer concentrations that increase during the austral autumn and winter. Results from a one-dimensional numerical model show that a combination of solar heating and nutrient poor surface waters during the summer can restrict plankton growth to the base of the euphotic zone resulting in low surface chlorophyll concentrations that are consistent with observations. The model also suggests that increased vertical mixing caused by surface cooling can explain some of the observed increases in surface chlorophyll that occur during the winter. However, the magnitude of the autumn chlorophyll signal varies between years, and sometimes the bloom precedes the onset of surface cooling at the end of the summer. Two different mechanisms are proposed to explain this result. During years when the Leeuwin Current is weaker, southward transport of heat is reduced, enhancing the influence of surface cooling during autumn, which results in convective overturning and a moderate increase in surface chlorophyll. By contrast, during years when the Leeuwin Current is stronger, surface cooling is less effective and convective mixing is delayed by up to two months during the autumn. Instead, nonlocal forcing associated with increased Leeuwin Current flow is proposed to be responsible for relatively large autumn increases in these years. The autumn surface chlorophyll signal increases with the strength of the Leeuwin Current suggesting a link between climate variability and primary production in this region.

Seasonal variation in abundance and species composition of choanoflagellates (Acanthoecideae) at Antarctic coastal sites

The abundance of loricate choanoflagellates at three shallow inshore sites (< 30 m) near Davis, Antarctica from March 1982 to February 1983 showed marked variation which followed the seasonal cycle of chlorophyll a concentration. In summer the concentration of choanoflagellates ranged from 104–106 cells 1−1 while in winter they were in the order of 102 cells 1−1. Species seasonal succession was observed with Bicosta spinifera, Crinolina aperta and Parvicorbicula socialis, organisms which dominated the summer population, being absent in winter whereas Cosmoeca ventricosa and Saepicula leadbeateri were found only in winter. Acanthocorbis unguiculata, Calliacantha simplex and Diaphanoeca grandis were present throughout the year.

Phosphorus and Nitrogen Budgets of Barton Broad and Predicted Effects of a Reduction in Nutrient Loading on Phytoplankton Biomass in Barton, Sutton and Stalham Broads, Norfolk, United Kingdom

AbstractBarton, Sutton and Stalham Broads are shallow, man‐made lakes formed in mediaeval times when peat excavations were flooded. Recently, a once diverse submerged macrophyte flora has, in most broads, been replaced by large phytoplankton populations. This change has been attributed to increased nutrient loadings. The seasonal cycles of chlorophyll α, nitrogen and phosphorus in the three broads are described and the inputs of nitrogen and phosphorus to Barton Broad are budgeted. A reduction in the phosphorus loading is recommended as the best method of restricting phytoplankton populations in these broads. An equation relating phosphorus loading, flushing rate, mean depth and retention coefficient to mean standing phosphorus concentration is used to predict the reduction in phosphorus loading required to limit phytoplankton populations and permit the re‐establishment of submerged macrophytes.

Pigment cycle in a north German lake

SummaryThe vertical distribution of photosynthetic pigments and their degradation products in a north German lake was studied fluorometrically over a period of one year. The maximum chlorophyll concentration was usually in the euphotic zone and at times coincided with that of the phaeo-pigments. Phaeo-pigments maximum was usually in the thermocline, near the base of the euphotic zone. The phaeo-pigments maximum occurred above the portion of the water column where nutrients increased in concentration. Investigations at greater depths revealed that there was a percentage increase in the degradation of chlorophyll with depth. The seasonal cycle of chlorophyll and their degradation products showed a bimodal pattern. Chlorophyll degradation products in the Pluss See constitute a significant fraction of the chlorophylls. From these observations it is concluded that post-lytic photochemical oxidation could be responsible to a large extent for chlorophyll decomposition products in situ in lakes. Chlorophyll derivatives, especially phaeophytins and phaeophorbides, have been detected chromatographically in Pluss See and Gerbiener See.