Net Phytoplankton Research Articles

Flux of particulate organic material from the euphotic zone of oceans: Estimation from phytoplankton biomass

A major objective of modem biological oceanography is to quantify, model, and predict the downward flux of biogenic carbon. Previous papers have documented an empirical relationship between the sinking flux of particulate organic material from the euphotic zone (S) and phytoplankton biomass (B), in oceans, and suggested that S could be estimated directly from B. The present paper investigates, using a mathematical model and data from the literature, the relationship between S and B. Calculations show that S is determined mostly by B and also by p (instantaneous coefficient of net phytoplankton production minus euphotic zone heterotrophic community respiration and incorporation in the pelagic food web). The regression S = f(B) accounts for 90% of the variation in S. In remote sensing, B is derived from ocean color, phytoplankton production (P) is derived from B, and S is derived from P. Within that context, the biomass approach could be used as an additional means to constrain the estimates of S.

The net phytoplankton in Kongsfjorden, Svalbard, July 1988, with general remarks on species composition of Arctic phytoplankton

Examination of 17 samples collected by a 20 ?m meshed meshed net in Kongsfjorden, Svalbard, 8–19 July 1988, showed a dominance of dinoflagellates and the chrysophyte Dinobryon Balticum in the surface layers, whereas the diatom and the haptophyte Phaecystis pouchetii abundance increased with depth. The diatom Pseudo-nitzschia granii appeared together with P. pouchetii through the whole water column, and Actinocyclus curvatulus was one of the few diatoms present also in the surface samples. Two samples, from 15 and 50 m, respectively, were cleaned of organic material and mounted in Naphrax for a more critical identification of the diatoms. We were able to group the species according to habitats, especially types of ice. The planktonic Thalassiosira antarctica var. borealis, T. hyalina, T. nordenskioeldii, Bacterosira bathyomphaia, Chaetoceros furcellatus, C. socialis and Fragilariopsis oceanica were present mainly as resting stages representing a post-bloom situation. These species and T. gravida appear early in the season and may have started to grow already under the ice. Fragilariopsis cylindrus and F. oceanica seem to have a closer affinity to ice than Thalassiosira and Chaetoceros spp. although they are common in the plankton. Some Nitzschia species which are usually regarded as typical sea-ice diatoms and have thicker and older ice as the main habitat were present only in small cell numbers in the plankton samples. The last component, evidently introduced from Atlantic water in the Norwegian Sea, consisted of diatoms with a more oceanic distribution, e. g. Fragilariopsis pseudonana and a small form of Thalassiosira bioculata.

Limnocalanus macrurus (Copepoda: Calanoida) retains a marine arctic lipid and life cycle strategy in Lake Michigan

Limnocalanus macrurus, an omnivorous calanoid copepod with strong carnivorous tendencies that invaded fresh water during the Pleistocene glaciation, retains a marine arctic lipid and life cycle strategy in the hypolimnion of Lake Michigan. Its maximum lipid concentration of 67% of dry mass—consisting largely of wax esters—is the highest reported for freshwater zooplankton and is among the highest reported for marine polar species. The high lipid concentration of L.macrurus runs counter to the paradigm that high wax ester concentrations are found in herbivorous, but not in carnivorous, polar species. Lipids are drawn down to extremely low levels (10% of dry mass) during the reproductive period, November-May. Reproduction in this univoltine species appears to be timed so that the new generation develops during the high abundance of prey (crustacean microzooplankton and net phytoplankton) in spring. A high wax ester content may allow egg production to start in winter when the prey concentration is low. In contrast to many polar species, the new generation moves through all copepodid stages to adult without diapausing. Copepodid S females have a low lipid concentration, and lipids are slowly built up in new generation adults during summer and fall before reproduction commences. This lipid and life cycle strategy may have given L.macrurus an advantage over potential freshwater competitors, such as the arctic freshwater cyclopoid copepod Cyclops scutifer, in the hypolimnia of glaciated lakes.

A numerical study of the effect of periodic nutrient supply on pathways of carbon in a coastal upwelling regime

A size-based ecosystem model was modified to include periodic upwelling events and used to evaluate the effect of episodic nutrient supply on the standing stock, carbon uptake, and carbon flow into mesozooplankton grazing and sinking flux in a coastal upwelling regime. Two ecosystem configurations were compared: a single food chain made up of net phytoplankton and mesozooplankton (one autotroph and one heterotroph, A1H1), and three interconnected food chains plus bacteria (three autotrophs and four heterotrophs, A3H4). The carbon pathways in the A1H1 simulations were under stronger physical control than those of the A3H4 runs, where the small size classes are not affected by frequent upwelling events. In the more complex food web simulations, the microbial pathway determines the total carbon uptake and grazing rates, and regenerated nitrogen accounts for more than half of the total primary production for periods of 20 days or longer between events. By contrast, new production, export of carbon through sinking and mesozooplankton grazing are more important in the A1H1 simulations. In the A3H4 simulations, the turnover time scale of the autotroph biomass increases as the period between upwelling events increases, because of the larger contribution of slow-growing net phytoplankton. The upwelling period was characterized for three upwelling sites from the alongshore wind speed measured by the NASA Scatterometer (NSCAT) and the corresponding model output compared with literature data. This validation exercise for three upwelling sites and a downstream embayment suggests that standing stock, carbon uptake and size fractionation were best supported by the A3H4 simulations, while the simulated sinking fluxes are not distinguishable in the two configurations.

Nitrogen, Silicate and Zooplankton Control of the Planktonic Food-web in Spring

The control of silicate, nitrogen and zooplankton was studied with a special emphasis on the planktonic food web on spring bloom on the coastal area of the Baltic Sea in a laboratory experiment. Samples of 150l of surface seawater were manipulated in transparent plastic laboratory tanks for 3 weeks atin situtemperature (+2°C) with additions of nitrate and silicate (NO3–N, SiO4–Si), as well as removal of mesozooplankton by filtration through a 100μm mesh-sized plankton net. A diatom bloom developed in the experimental tanks within 7 days together with maxima in both picoplanktonic eukaryote and heterotrophic nanoflagellate abundances, during which time inorganic nitrogen and phosphorus were exhausted. Despite the clear response of phytoplankton to the silicate and nitrogen additions, the remainder of the food web was not stimulated by the nutrients. The production and abundance of picoplanktonic algae and bacteria were low and probably controlled by low temperature and bacterial substrates. Maximum growth rates of the organisms varied between 0·1 and 1·2 day−1, being the highest among diatoms and the lowest amongAcartiacopepods. The removal of mesozooplankton through prefiltration enhanced the growth of ciliates with a weak cascading effect through the microbial food web. Most of the biomass and production in the community was concentrated in the net phytoplankton, clearly indicating that in a diatom-dominated spring bloom community in the Baltic sea, only a minor fraction of carbon flow is channelled through the heterotrophic food web. Moreover, increased inputs of silicate and nitrogen to the coastal area before the spring bloom will be channelled to the production of net phytoplankton, therefore not stimulating the heterotrophic food web.

Does the Sverdrup critical depth model explain bloom dynamics in estuaries?

In this paper we use numerical models of coupled biological-hydrodynamic processes to search for general principles of bloom regulation in estuarine waters. We address three questions: What are the dynamics of stratie cation in coastal systems as ine uenced by variable freshwater input and tidal stirring? How does phytoplankton growth respond to these dynamics? Can the classical Sverdrup Critical Depth Model (SCDM) be used to predict the timing of bloom events in shallow coastal domains such as estuaries? We present results of simulation experiments which assume that vertical transport and net phytoplankton growth rates are horizontally homogeneous. In the present approach the temporally and spatially varying turbulent diffusivities for various stratie cation scenarios are calculated using a hydrodynamic code that includes the Mellor-Yamada 2.5 turbulence closure model. These diffusivities are then used in a time- and depth-dependent advection-diffusion equation, incorporating sources and sinks, for the phytoplankton biomass. Our modeling results show that, whereas persistent stratie cation greatly increases the probability of a bloom, semidiurnal periodic stratie cation does not increase the likelihood of a phytoplankton bloom over that of a constantly unstratie ed water column. Thus, for phytoplankton blooms, the physical regime of periodic stratie cation is closer to complete mixing than to persistent stratie cation. Furthermore, the details of persistent stratie cation are important: surface layer depth, thickness of the pycnocline, vertical density difference, and tidal current speed all weigh heavily in producing conditions which promote the onset of phytoplankton blooms. Our model results for shallow tidal systems do not conform to the classical concepts of stratie cation and blooms in deep pelagic systems. First, earlier studies (Riley, 1942, for example) suggest a monotonic increase in surface layer production as the surface layer shallows. Our model results suggest, however, a nonmonotonic relationship between phytoplankton population growth and surface layer depth, which results from a balance between several ‘ ‘ competing’ ’ processes, including the interaction of sinking with turbulent mixing and average net growth occurring within the surface layer. Second, we show that the traditional SCDM must be ree ned for application to energetic shallow systems or for systems in which surface layer mixing is not strong enough to counteract the sinking loss of phytoplankton. This need for ree nement arises because of the leakage of phytoplankton from the surface layer by turbulent diffusion and sinking, processes not considered in the classical SCDM. Our model shows that, even for low sinking rates and small turbulent

Plankton ecology and the Proterozoic-Phanerozoic transition

Most modern marine ecology is ultimately based on unicellular phytoplankton, yet most large animals are unable to graze directly on even relatively large net phytoplankton; the repackaging effected by herbivorous mesozooplankton thus represents a key link in marine metazoan food chains. Despite the deep taphonomic biases affecting plankton fossilization, there is a clear record of phytoplankton from at least 1800 m.y ago. Proterozoic plankton are represented by small-to medium-sized sphaeromorphic acritarchs and probably do not include many/most of the unusually large acritarchs that characterize the Neoproterozoic. The first significant shift in phytoplankton diversity was therefore the rapid radiation of small acanthomorphic acritarchs in the Early Cambrian. The coincidence of phytoplankton diversification with the Cambrian radiation of large animals points compellingly to an ecological linkage between the two, particularly in light of recently discovered filter-feeding mesozooplankton in the Early Cambrian. The introduction of planktic filter feeders would have established the second tier of the Eltonian pyramid, potentially setting off the “self-propagating mutual feedback system of diversification” now recognized as the Cambrian explosion (Stanley 1973, 1976).By consuming significant percentages of net phytoplankton and suspending it as animal biomass and non-aggregating fecal pellets, mesozooplankton cause a netreductionin export production; a general introduction of zooplankton would therefore have reduced carbon burial and moderated the bloom and bust cycle that must have characterized Proterozoic populations of net phytoplankton. The effect of added trophic levels in Early Cambrian ecosystems can be viewed as a serial application of the trophic cascade process observed in modern lakes, whereby the introduction of higher trophic levels determines the accumulation of plant biomass at the base of the system. As such, the major biogeochemical perturbations that mark the onset of the Phanerozoic might be considered a consequence, rather than a cause, of the Cambrian explosion; reduced C export due to zooplankton expansion explains the otherwise anomalous drop in δ13C at the base of the Tommotian.Cambrian acanthomorphic acritarchs likely derived from planktic leiosphaerids exposed to mesozooplanktic grazing pressure, the ornamentation effectively increasing vesicle size without compromising buoyancy or surface-area:volume ratios. Alternatively, they may represent an escape into the plankton through a miniaturization of the much larger Neoproterozoic acanthomorphs. An invasion of small benthic herbivores into the water column to exploit the phytoplankton accounts for the origin of the mesozooplankton and may have been the key innovation in the Cambrian explosion.

Longwaves and primary productivity variations in the equatorial pacific at 0°, 140dgW

High temporal resolution measurements of physical and bio-optical variables were made in the upper ocean using a mooring located at 0°, 140°W from 9 February 1992 to 15 March 1993 as part of the equatorial Pacific Ocean (EgPac) study. Chlorophyll and primary productivity time-series records were generated using the mooring data. Primary productivity varied by about 50% around the mean on time scales of weeks and by over a factor of four within our observational period. The mooring observations encompassed both El Niho and cool conditions. Kelvin waves were evident during the El Nifio phase, and tropical instability waves (TIWs) were dominant during the cool phase. The two extreme conditions also were observed concurrently with complementary ship-based measurements. In addition, bio-optical drifters provided simultaneous spatial data concerning net phytoplankton growth rates during passage of a TIW. The collective data sets have been used to examine the causes of the observed variability in phytoplankton biomass and productivity. Our joint results and analyses appear to support the hypothesis that the vertical transport of iron into the upper layer and primary production rates are modulated by variability of the depth of the Equatorial Undercurrent and by equatorial longwaves. In particular, our results are consonant with the suggestion of Barber et al. (1996) that passage of a TIW may be considered to be a natural analog of a small iron enrichment experiment. Predicting primary productivity and, thus, carbon flux in the equatorial Pacific requires continuous, long-term observations of a few physical, biological, and optical properties that can be used to parameterize the biological variability.

Using non-linear analysis to compare the spatial structure of chlorophyll with passive tracers

A form of non-linear analysis, termed the near-neighbour algorithm, was applied to transects of chlorophyll and salinity collected off eastern Antarctica in the Austral summer of 1995/96. The near-neighbour algorithm was initially developed to detect chaos in time series, but was applied here to compare the spatial structure of chlorophyll, a non-conservative tracer, with that of a conservative tracer, salinity. The validity of such an application is discussed in the context of the literature and as a complementary approach to traditional methods such as autocorrelation and spectral analysis. The results indicate that the spatial structure of salinity could be classified as non-linear in nature. The structure of chlorophyll at corresponding spatial scales contains a stochastic component, and it is postulated that this is caused by biological factors, specifically net phytoplankton growth. The potential for expanded, more detailed analyses is discussed, and parallels are drawn between the current state of non-linear analysis in biological oceanography and the development of spectral analysis over the last three decades.

Sedimentation and buoyancy of Aphanizomenon cf. flos-aquae (Nostocales, Cyanophyta) in a nutrient-replete and nutrient-depleted coastal area of the Baltic Sea

Abstract Daily variation of Aphanizomenon cf. f1s-aquae Ralfs ex Bornet et Flahaut (Nostocales, Cyanophyta) biomass and sedimentation was studied in the southern Gulf of Riga (GoR) and in the coastal area of the Gulf of Finland (GoF), Baltic Sea, during July and August 1994. In the GoR, variation in the vertical distribution of Aphanizomenon biomass was investigated during two diurnal cycles. In both regions, net phytoplankton biomass was dominated by Aphanizomenon, although no massive surface blooms were observed. Nutrient-replete conditions in the GoR resulted from deep mixing as well as remineralization in midwater. The subsequent increase in nutrients in the euphotic zone (mainly ammonium) and decrease of the mixed layer depth was favourable for Aphanizomenon, which was able to maintain positive buoyancy in the upper water column and increase its biomass. The biomass and sedimentation rates of Aphanizomenon were c. four times higher in the GoR than in the GoF. However, the daily loss rates related to ...

PHOTOSYNTHESIS AND REGULATION OF RUBISCO ACTIVITY IN NET PHYTOPLANKTON FROM DELAWARE BAY1

ABSTRACTNet phytoplankton (> 20 μm) comprised 51 ± 9% of the total chlorophyll (Chl) in a Skeletonema costatum– dominated spring bloom in Delaware Bay. The net phytoplankton had low C:N and high protein: carbohydrate ratios, indicating that their growth was nutrient‐replete. Their photosynthetic responses were characterized by low specific absorption, low light‐limited and light‐saturated rates of photosynthesis, and high quantum yields, indicative of acclimation to low irradiance and internal self‐shading. High fucoxanthin: Chi ratios also indicated low light acclimation, but high photoprotective xanthophyll: Chi ratios suggested a high capacity for photoprotective energy dissipation. Ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) could be activated and deactivated in response to changes in irradiance and was fully activated at the surface of the water column and fully deactivated in aphotic deep water. Maximum Rubisco activity was correlated with Rubisco content and bulk protein content of the phytoplankton and with light‐saturated rates of photosynthesis measured in short (< 20‐min) incubations. Long (60‐min) incubations caused a decrease in the light‐saturated rate of photosynthesis, possibly because of feedback limitation. While feedback limitation is unlikely to occur in the water column, it should be considered when estimating productivity in well‐mixed waters from fixed light‐depth incubations.

Preliminary investigation of the contribution of fast-ice algae to the spring phytoplankton bloom in Ellis Fjord, eastern Antarctica

Algae released from fast-ice in Ellis Fjord, eastern Antarctica, made little contribution to subsequent phytoplankton growth. Dominant taxa in the interior ice community included Nitzschia cylindrus (Grun) Hasle, Navicula glaciei V.H. and a dinoflagellate cyst. Diatom mortality within the ice was high. The algal contribution to the phytoplankton from the fast ice was estimated by calculating the difference between algal biomass in ice cores taken on 14 November with those taken on 18 December 1992. The biomass of sedimenting phytoplankton was estimated using sediment traps; weekly cell counts of water were used to monitor net phytoplankton growth. The low contribution from the fast-ice of Ellis Fjord to the phytoplankton is similar to results from other Antarctic fast-ice communities but is not necessarily reflective of processes occurring within either Antarctic or Arctic pack ice communities. An algal mat growing on the base of the fast-ice had a carbon standing crop of between 0.231 gC m-2 and 0.022 gC m-2. Much of this was delivered to the water column as the ice melted while the remainder was exported.

Summer maxima of phytoplankton in the Římov Reservoir in relation to hydrologic parameters and phosphorus loading

The Rimov Reservoir (Czech Republic) is a eutrophic, stratified, deep-valley reservoir with large inter-seasonal fluctuations in the occurrence of summer algal blooms. Its phytoplankton is moderately to strongly limited by phosphorus during summer period. The results of ten-year monitoring of phytoplankton, phosphorus loading, and related hydrologic and climatic factors were evaluated in order to elucidate principles of phytoplankton growth in this reservoir. The study revealed that average summer concentrations of chlorophyll-a and fresh mass in the lacustrine part were linearly and positively correlated with the dissolved phosphorus load into the epilimnion, caused by surface discharges. This type of load did not result in a significant increase of total phosphorus concentration in the lacustrine part of the reservoir. However, it caused shifts in the structure of the phytoplankton community towards the prevalence of net phytoplankton species > 40 μm which were able to accumulate in large biomasses at low demands for phosphorus. The dominant large-sized species were represented mainly by cyanobacteria (Microcystis aeruginosa, Aphanizomenon flos-aquae), except for 1993, when a green alga (Staurastrum planctonicum) dominated.

Development and Testing of a Nitrogen Model for Onondaga Lake

ABSTRACT A dynamic two-layer mass balance model for total nitrogen (N) and various species of N is developed and tested for Onondaga Lake, NY, a system in which high concentrations of total ammonia (T-NH3) prevail. The model simulates concentrations of T-NH3, nitrate plus nitrite, particulate organic N (PON), dissolved organic N, total Kjeldahl N, and total N. Processes of the N cycle accommodated include net phytoplankton growth, nitrification, denitrification, ammonification, decay of PON, volatilization of free ammonia, sediment release of T-NH3, settling of PON, and vertical mixing-based exchange between the layers. Model testing is supported by a comprehensive monitoring database of lake concentrations of the N species, and tributary and waste discharge concentrations for calculation of loads. Model credibility is enhanced by the independent determination of several important model coefficients based on field and laboratory experiments, which greatly reduces the role of calibration. Model verificatio...

Modelling impact of urban and upstream nonpoint sources on eutrophication of the Milwaukee River

This paper describes a study conducted to evaluate the impact of the absence vs. the presence of the North Avenue Damin Milwaukee, Wisconsin, on water quality in the Milwaukee River in terms of conventional constituents, such as DO, chlorophyll a, BOD, and nutrients (N and P). Phytoplanktonic primary production had been recognized earlier as a link between mostly nonpoint nutrient loads to the Milwaukee River and degradation of water quality in the River. A mathematical model of a relevant section of the River was used in the study to estimate the effect of the removal of the Dam on this primary production link. The authors established the model using WASP, which is USEPA's widely used modelling package. The modelled extent of the River included both shallow, productive upstream reaches as well as impounded downstream sections. In the impounded reach of the River where the Dam is located, the simulated removal of the Dam (1) replaced net phytoplankton decay with net phytoplankton growth because of improved light conditions and limited settling; (2) substantially increased the concentration of DO because of the increased photosynthetic production and reduced benthic consumption; and (3) somewhat increased bottle BOD5 because of the elevated phytoplankton levels. The simulated removal of the Dam eliminated the negative effects of eutrophication at the site while possibly shifting the potential for these effects further downstream towards Lake Michigan.

Modelling impact of urban and upstream nonpoint sources on eutrophicationof the Milwaukee River

This paper describes a study conducted to evaluate the impact of the absence vs. the presence of the North Avenue Damin Milwaukee, Wisconsin, on water quality in the Milwaukee River in terms of conventional constituents, such as DO, chlorophyll a, BOD, and nutrients (N and P). Phytoplanktonic primary production had been recognized earlier as a link between mostly nonpoint nutrient loads to the Milwaukee River and degradation of water quality in the River. A mathematical model of a relevant section of the River was used in the study to estimate the effect of the removal of the Dam on this primary production link. The authors established the model using WASP, which is USEPA's widely used modelling package. The modelled extent of the River included both shallow, productive upstream reaches as well as impounded downstream sections. In the impounded reach of the River where the Dam is located, the simulated removal of the Dam (1) replaced net phytoplankton decay with net phytoplankton growth because of improved light conditions and limited settling; (2) substantially increased the concentration of DO because of the increased photosynthetic production and reduced benthic consumption; and (3) somewhat increased bottle BODS because of the elevated phytoplankton levels. The simulated removal of the Dam eliminated the negative effects of eutrophication at the site while possibly shifting the potential for these effects further downstream towards Lake Michigan.

Using ratios of stable nitrogen and carbon isotopes to characterize the biomagnification of DDE, mirex, and PCB in a Lake Ontario pelagic food web

Stable isotopes of nitrogen (δ15N) and carbon (δ13C) were used to describe the trophic status and interactions of biota characteristic of a Lake Ontario pelagic food web. Stable isotopes of nitrogen were further used to characterize the relationship between an organism's trophic position and the biomagnification of specific hydrophobic contaminants through this food web. The δ15N defines the relative trophic status as (i) the top predator, lake trout (Saivelinus namaycush); (ii) prey species, alewife (Alosa pseudoharengus), rainbow smelt (Osmerus mordax), and slimy sculpin (Cottus cognatus); (iii) macroinvertebrates, mysids (Mysis relicta), and amphipods (Diporeia hoyi); (iv) net zooplankton, dominated by cyclopoids (Diacyclops thomasi) and cladocerans (Bosmina longirostris); and (v) net phytoplankton, dominated by diatoms (Melosira spp.). The separation of the four fish species, lake trout and associated prey items (alewife, rainbow smelt, and slimy sculpin), on the basis of their mean δ13C signatures complements what is known about the preferred diet of these fishes. The enrichment of15N through this food web indicates that there is a strong correlation between the biomagnification of persistent lipophilic contaminants (p,p′-DDE, mirex, and PCB) and the relative trophic status of an organism as described by stable isotopes of nitrogen.

Impact of fishstock manipulation on the composition of net phytoplankton in the rímov reservoir (Czech Republic)

A ten-year period of monitoring of the Rímov reservoir was chosen to demonstrate the impact of fishstock manipulation on phytoplankton biomass and on species composition of the phytoplankton assemblage. Biomass of planktivorous fish decreased substantially under various forms of biomanipulative pressure. The proportion of large individuals of Daphnia galeata (retained on the 0.71 mm mesh sieve) increased from nearly 0 to 9% of the total zooplankton biomass during the period of 1985-1988 and remained nearly constant during subsequent years. Change in zooplankton structure had no significant response in the abundance of phytoplankton. Species composition of phytoplankton assemblage, however, changed considerably after the large daphnids had reached a biomass of more than 10 mg.m−2 of protein-N. Aphanizomenon flos-aquae var. flos-aquae formed about 8% of phytoplankton biomass before 1985, while its share increased to 40-65% during 1988-1990. Later, Aphanizomenon was replaced by other large forms of algae. The factors possibly influencing Aphanizomenon blooms are discussed with respect to the situation in other water bodies.

Variations in the seasonal cycle of biological production in subarctic oceans: A model sensitivity analysis

The various hypotheses that have been proposed to explain the absence of a spring bloom at OWS P in the North Pacific have been evaluated by means of a sensitivity analysis of two seasonal mixed layer ecosystem models. The main model consisted of six compartments (phytoplankton, herbivorous and carnivorous zooplankton, nitrate, ammonium and detritus) forced by climatological mixed layer depths, photosynthetically available radiation and deep nitrate concentrations. It was concluded that the absence of a spring bloom at OWS P can be attributed to the limiting effect of herbivore grazing. In contrast at OWS I the deeper winter mixed layer depths means that net phytoplankton production in winter is insufficient to allow herbivore populations to reach high enough levels to exert a limiting grazing pressure at the time of the spring bloom. The high levels of summer nitrate at OWS P are mainly due to the limitation of new production by grazing coupled with a supply of nitrate from below the mixed layer; ammonium limitation of nitrate uptake is a secondary factor. The simulations at OWS P showed that small variations in the parameter values had only a marginal effect on phytoplankton biomass and this might explain the low degree of inter-annual variability of phytoplankton at OWS P. The possible role that iron deficiency might play in limiting production is discussed.

Parameter optimization and analysis of ecosystem models using simulated annealing: A case study at Station P

A simulated annealing optimization algorithm is formulated to optimize parameters of ecosystem models. The optimization is used to directly determine the model parameters required to reproduce the observed data. The optimization routine is formulated in a general manner and is easily modified to include additional information on both the desired model output and the model parameters. From the optimization routine, error analysis of the optimal parameters is provided by the error-covariance matrix which gives both the sensitivity of the model to each model parameter and the correlation coefficients between all pairs of model parameters. In addition, the optimization analysis provides a means of assessing the necessary model complexity required to model the available data. To demonstrate the technique, optimal parameters of three different ecosystem model configurations are determined from nitrate, phytoplankton, mesozooplankton and net phytoplankton productivity measurements at Station P. At Station P, error analysis of the optimal parameters indicates that the data are able to resolve up to 10 independent model parameters. This is always less than the number of unknown model parameters indicating that the optimal solutions are not unique. A simple nitrate-phosphate-zooplankton ecosystem is successful at reproducing the observations. To justify the use of a more complicated model at Station P requires additional data to constrain the optimization routine. Although there is evidence supporting the importance of the microbial loop at Station P, without additional ammonium and bacteria measurements one cannot validate a more complicated model that includes these processes.