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- Fecundity, spore recruitment and size in Gelidium sesquipedale (Gelidiales,Rhodophyta)Publication . Santos, R.; Duarte, PedroGelidium sesquipedale fecundity was quantified by counting tetrasporangial sori and cystocarps per meter squared and by estimating the number of spores contained inside them . These were obtained by regression on a size metric of reproductive structures . Tetrasporangial sori length and cystocarp thickness were the best estimators of spore number. To assess spore recruitment, 12 pottery tiles were fixed to the bottom, and the appearance of small fronds was monitored. No clear seasonal pattern of reproduction was found . Tetraspore production peaked in March 1990 with 10.4 x 106 spores m-2, whereas the carpospore peak was lower, 4.9 x 10 5 spores m-2 in July 1989. Recruitment followed tetraspore peaks . The probability of a G. sesquipedale tetraspore making the transition to a recruit was 4.7 x 10-5. Frond length was significantly related to tetrasporangial sori number, while cystocarp number was only related to frond branching order. Minimum size for reproduction was 6.9 cm for gametophytes and 5.4 cm for tetrasporophytes; very rarely were cystocarpic fronds smaller than 9 cm, while tetrasporic fronds were often longer than 15 cm . Cystocarpic fronds were significantly shorter and had more branches than tetrasporic fronds .
- A methodology for parameter estimation in seaweed productivity modellingPublication . Duarte, Pedro; Ferreira, JoãoThis paper presents a combined approach for parameter estimation in models of primary production. The focus is on gross primary production and nutrient assimilation by seaweeds. A database of productivity determinations, biomass and mortality measurements and nutrient uptake rates obtained over one year for Gelidium sesquipedale in the Atlantic Ocean off Portugal has been used. Annual productivity was estimated by harvesting methods, and empirical relationships using mortality/ wave energy and respiration rates have been derived to correct for losses and to convert the estimates to gross production. In situ determinations of productivity have been combined with data on the light climate (radiation periods, intensity, mean turbidity) to give daily and annual productivity estimates. The theoretical nutrient uptake calculated using a 'Redfield ratio' approach and determinations of in situ N and P consumption by the algae during incubation periods have also been compared. The results of the biomass difference and incubation approaches are discussed in order to assess the utility of coefficients determined in situ for parameter estimation in seaweed production models.
- Trophic capacity of Carlingford Lough for oyster culture – analysis by ecological modellingPublication . Ferreira, João; Duarte, Pedro; Ball, B.A one-dimensional ecosystem box model is presented for carrying capacity assessment.The model includes physical and biological processes. The physical processes are the transport of nutrients, suspended matter and phytoplankton through the system boundaries and between model boxes. The biological processes are primary production and oyster (Crassostrea gigas) population dynamics and physiology. The model was implemented using an object oriented approach. The model was employed to estimate the carrying capacity of Carlingford Lough (Ireland) for oyster culture. In the Lough, low water temperatures prevent the oysters from reproducing. Therefore, recruitment is human dependent. Small oyster spat is seeded every year during spring and harvested after the summer of the next year. During this period oysters reach commercially harvestable weight. The results obtained indicate that the carrying capacity of this system is approximately 0.45 g oysters (AFDW) m-3, determined more by the availability of particulate matter than by phytoplankton. It is suggested that a fivefold increase in oyster seeding may optimise harvest yield.
- Assessment and comparison of the Marennes-Oléron Bay (France) and Carlingford Lough (Ireland) carrying capacity with ecosystem modelsPublication . Bacher, C.; Duarte, Pedro; Ferreira, João; Héral, M.; Raillard, O.Based on the individual growth, food limitation, population renewal through seeding, and individual marketable size, a theoretical model of the cultured species population dynamics was used to assess the carrying capacity of an ecosystem. It gave a domeshape curve relating the annual production and the standing stock under the assumption of individual growth limited by the available food in an ecosystem. It also showed the influence of mortality rate and marketable size on this curve and was introduced as a means to explore the global properties resulting from the interactions between the ecophysiology of the reared species and the environment at the ecosystem level. In a second step, an ecosystem model was built to assess the carrying capacity of MarennesOl ´eron bay, the most important shellfish culture site in France, with a standing stock of Crassostrea gigas around 100 000 tonnes fresh weight (FW) and an annual production of 30 000 tonnes FW. The ecosystem model focused on the oyster growth rate and considered the interaction between food availability, residence time of the water, oyster ecophysiology and number of individuals. It included a spatial discretization of the bay (box design) based on a hydrodynamicmodel, and the nitrogen or carbon cycling between phytoplankton, cultured oysters, and detritus. From simulations of the oyster growth with different seeding values, a curve relating the total annual production and the standing stock was obtained. This curve exhibited a dome shape with a maximum production corresponding to an optimum standing stock. The model predicted amaximum annual production of 45 000 tonnes FWfor a standing stock around 115 000 tonnes FW. The prediction confirmed some results obtained empirically in the case of MarennesOl ´eron bay and the results of the theoretical model. Results were compared with those obtained in Carlingford Lough (Ireland) using a similar ecosystem model. Carlingford Lough is a small intertidal bay where the same species is cultured at a reduced scale, with current biomass less than 500 tonnes FW. The model showed that the standing stock can be increased from 200 tonnes FW to approximately 1500 tonnes FW before any decrease of the production.
- A mechanistic model of the effects of light and temperature on algal primary productivityPublication . Duarte, PedroIn this work a model of algal primary productivity combining a mechanistic light function with a temperature Arrhenius function is presented. Data on primary productivity obtained with algae acclimated to different environmental conditions was used to test the model. A simple method for model parameter estimation based on regression analysis is described. The parameter estimates can be improved by a non-linear least-squares method (e.g. the Gauss-Newton method) resulting in a significant fit to the observed data as tested by regression analysis. According to the present model, the initial slope of the productivity/light curves is temperature dependent whilst the optimal light intensity is temperature independent. These model predictions were validated by the obtained experimental results.
- A model for the simulation of macroalgal population dynamics and productivityPublication . Duarte, Pedro; Ferreira, JoãoA mathematical model to simulate the population dynamics and productivity of macroalgae is described. The model calculates the biomass variation of a population divided into size-classes. Biomass variation in each class is estimated from the mass balance of carbon fixation, carbon release and demographic processes such as mortality and frond breakage. The transitions between the different classes are calculated in biomass and density units as a function of algal growth. Growth is computed from biomass variations using an allometric relationship between weight and length. Gross and net primary productivity is calculated from biomass production and losses over the period of simulation. The model allows the simulation of different harvesting strategies of commercially important species. The cutting size and harvesting period may be changed in order to optimise the calculated yields. The model was used with the agarophyte Gelidium sesquipedale (Clem.) Born. et Thur. This species was chosen because of its economic importance as a the main raw material for the agar industry. Net primary productivity calculated with it and from biomass variations over a yearly period, gave similar results. The results obtained suggest that biomass dynamics and productivity are more sensitive to the light extinction coefficient than to the initial biomass conditions for the model. Model results also suggest that biomass losses due to respiration and exudation are comparable to those resulting from mortality and frond breakage. During winter, a significant part of the simulated population has a negative net productivity. The importance of considering different parameters in the productivity light relationships in order to account for their seasonal variability is demonstrated with the model results. The model was implemented following an object oriented programming approach. The programming methodology allows a fast adaptation of the model to other species without major software development.