Abstract
The number of successful missions with autonomous underwater vehicles (AUVs) has been increasing steadily during the last few years, validating this emerging technology as an e cient tool for underwater sampling. At the same time it has served to envisage their operation in more demanding scenarios. This paper presents results from environment monitoring missions with an AUV and new mission concepts for the operation of one or multiple AUVs. These new concepts are the natural evolution of previous work and also the response to the requirements of some of the identi ed scenarios. For each type of mission, solutions for navigation, control, coordination, and communications are presented. © 2004 IFAC

Abstract
This work presents a hybrid maneuver for gradient search with multiple AUV's. The mission consists in following a gradient field in order to locate the source of a hydrothermal vent or underwater freshwater source. The formation gradient search exploits the environment structuring by the phenomena to be studied. The ingredients for coordination are the payload data collected by each vehicle and their knowledge of the behaviour of other vehicles and detected formation distortions. Copyright © 2004 IFAC. Copyright © 2004 IFAC

Abstract
Mean sea level is a variable of considerable interest in meteorological and oceanographic studies, particularly long-term sea level variation and its relation to climate changes. This study concerns the analysis of monthly mean sea level data from tide gauge stations in the Northeast Atlantic with long and continuous records. Much research effort on mean sea level studies has been focused on identifying long-term linear trends, usually estimated through least-squares fitting of a deterministic function. Here, we estimate nonparametric and robust trends using lowess, a robust smoothing procedure based on locally weighted regression. This approach is more flexible than a linear trend to describe the deterministic part of the variation in tide gauge records, which has a complex structure. A common trend pattern of reduced sea levels around 1975 is found in all the analysed records and interpreted as the result of hydrological and atmospheric forcing associated with drought conditions at the tide gauge sites. This feature is overlooked by a linear regression model. Moreover, nonlinear deterministic behaviour in the time series, such as the one identified, introduces a bias in linear trends determined from short and noisy records.

Abstract
An evaluation of the influence of environmental factors on the normal growth of young Anodonta cygnea was undertaken in semi-controlled conditions in the natural habitat. Organic and inorganic compounds and the microalgal population composition were analysed for I year in the lagoon habitat and correlated with bivalve length and weight variations. The results from the water samples showed a significant increase in calcium and magnesium in the spring-summer period, suggesting an influence on shell growth that is apparent at this time. Sodium potassium and chloride show no significant variations. The significantly higher lipid concentration in spring, albeit still at a low level, may be related to the increase in body weight during this season, whilst the stable protein and carbohydrate contents provide the bases for growth and metabolism, respectively. An appropriate combination of microalgae, including diatoms, chlorophytes and cyanophytes may be an important factor in the increase in length and weight of young bivalves during spring and autumn periods. This influence seems to be reinforced by association with mild temperatures (12-15degreesC). In contrast, a low rate of growth in summer and its virtual absence in winter are due to the presence, in substantial numbers, of only two kinds of microalgae and high or low temperatures. A high level of survival (85%) of young A. cygnea was observed during the 3-year period of culture. The correlation coefficient between length and weight during the early adult growth period was estimated to be 95%.

Abstract
Selenastrum, Chlorella and the Cyanophyta groups, important components of the phytoplankton in the natural habitat of Anodonta cygnea in Mira Lagoon, were used separately as experimental diet for Anodonta cygnea. To assess the effects of the different diets, several organic and inorganic compounds of the haemolymph and extrapallial fluid were measured. Protein content was significantly reduced with experimental Non Feed, and carbohydrate content with Non Feed, Selenastrum, Chlorella, and Cyanophyta diets compared to the control in the lagoon. Lipid contents in body fluids were similar among the different diet groups, with the exception of the Chlorella diet, which showed a higher value. In all diets, the inorganic elements, magnesium and chloride, presented a non-significant variation profile. The concentration of sodium and potassium was significantly affected in Selenastrum and Chlorella diets, observing in the Non Feed diet a minimum value for potassium. The non-significant variation of osmolality suggests the presence of a specific osmotic regulatory mechanism independent of the diet. A particular regulation mechanism seems to exist for both calcium and manganese ion inducing similar profiles for all experimental diets with higher contents in Selenastrum and Cyanophyta diets, respectively. The higher calcium ion concentration appears associated with lower protein-carbohydrate contents, probably due to weak calcium aggregation and deposition mechanism, except for the control and Mixture diets. In general, the concentrations of organic and inorganic compounds in the body fluids of Mixture-fed A. cygnea, were more similar to those found in the control diet group. This suggests that an equal mixture of the three algae, Selenastrum, Chlorella, and Cyanophyta, is more appropriate diet to provide the necessary nutrients for a normal maintenance of A. cygnea in the wild.

Abstract
In this paper we provide a different way to estimate matrices B and D, in subspace identification algorithms. The starting point was the method proposed by Van Overschee and De Moor [10] - the only one applying subspace ideas to the estimation of those matrices. We have derived new (and simpler) expressions and we found that the method proposed by Van Overschee and De Moor [10] can be rewritten as a weighted least squares problem, involving the future outputs and inputs.