Geochemical changes in white seabream (Diplodus sargus) earth ponds during a production cycle

The knowledge of geochemical processes in fishponds is important in defining farming strategies and the carrying capacity of these systems, and is therefore essential for the management and sustainability of semi-intensive aquaculture in earth ponds. The main purpose of the present work, developed in the Aquaculture Research Station located in Ria Formosa, was to study the geochemical changes in semi-intensive earth ponds of white seabream Diplodus sargus L. during a production cycle, and relate it to farming conditions (fish biomass and feeding rate). Settled material and sediment samples were collected in a fish production pond and in a non-fish production pond during 2 years. The results obtained showed that particle-settling rates (S, g m⁻² day⁻¹) increased linearly with time (t, days): S=0.7t-34, in the fishpond. Increasing deposition of particulate material increased the organic matter content of bottom sediments, particularly during the second production year. Organic matter mineralization, during periods of high temperatures, led to high nutrient concentrations in porewater (NH₄⁺, 965 μM; NO₃⁻, 40 μM; HPO₄²⁻, 39 μM) and subsequently to an increase in benthic primary production in the fishpond. The geochemical similarities between fishpond sediments and shallow coastal system's sediments, along with the high fish survival rate (94%), suggest that for the assayed farming conditions there were no environmental constraints within the pond. However, some impact on bottom sediments, namely, an increase in settled material, organic matter deposition, nutrients in porewater and microphytobentos production, was evident above a fish biomass of 500 g m⁻³ and a feeding rate of 150 kg month⁻¹, indicating that pond environmental conditions should be carefully monitored from this point on.