Carlos Sanz-Lázaro

Effect of temperature on biogeochemistry of marine organic‐enriched systems: implications in a global warming scenario

Coastal biogeochemical cycles are expected to be affected by global warming. By means of a mesocosm experiment, the effect of increased water temperature on the biogeochemical cycles of coastal sediments affected by organic-matter enrichment was tested, focusing on the carbon, sulfur, and iron cycles. Nereis diversicolor was used as a model species to simulate macrofaunal bioirrigation activity in natural sediments. Although bioirrigation rates of N. diversicolor were not temperature dependent, temperature did have a major effect on the sediment metabolism. Under organic-enrichment conditions, the increase in sediment metabolism was greater than expected and occurred through the enhancement of anaerobic metabolic pathway rates, mainly sulfate reduction. There was a twofold increase in sediment metabolism and the accumulation of reduced sulfur. The increase in the benthic metabolism was maintained by the supply of electron acceptors through bioirrigation and as a result of the availability of iron in the sediment. As long as the sediment buffering capacity toward sulfides is not surpassed, an increase in temperature might promote the recovery of organic-enriched sediments by decreasing the time for mineralization of excess organic matter.

Effect of sediment grain size and bioturbation on decomposition of organic matter from aquaculture

Sediment grain size plays a major role in sediment biogeochemistry and sediments with different grain size are expected to react differently to organic enrichment. Through a mesocosm approach we tested the behavior of sediments with two types of predominant grain size (sandy and muddy sediments) under two levels of organic enrichment, related to mussel and fish farming. The polychaete Hediste diversicolor was used to simulate macrofauna bioturbation and bioirrigation. H. diversicolor stimulated organic matter (OM) mineralization and nutrient recycling. Muddy sediments had more OM from nature, resulting in higher sulfate reduction rates than sandy sediments. Under low levels of organic enrichment grain size did not have any effect on benthic fluxes (sediment oxygen uptake, total CO2, ammonium, nitrate, nitrite and phosphate). However, at high levels of organic enrichment, sandy sediments accumulated less OM, less sulfide and less ammonium than muddy sediments, while sediment oxygen uptake and total CO2 showed similar levels between sandy and muddy sediments. Thus, grain size should be considered a key parameter for site selection of fish farming facilities when aiming for a sustainable aquaculture industry.

Depth matters for bivalve culture in integrated multitrophic aquaculture (IMTA) and other polyculture strategies under non-eutrophic conditions

Bivalve cultivation, in single cultivation or in polyculture (including integrated multitrophic aquaculture; IMTA), is generally limited to eutrophic waters. We carried out a modeling study to test if, under meso- and oligotrophic conditions, depth could be a key factor for bivalve productivity associated to IMTA and other polyculture strategies. We applied the model Farm Aquaculture Resource Management (FARM) at three strata of the water column in two coastal fish farm areas in the Mediterranean Sea, using water column variables sampled seasonally to estimate the potential mussel production. According to FARM, mussel production was high in both areas and, in some cases, almost doubled when mussels were cultured below 25-m depth compared to shallower levels. Phytoplankton abundance is expected to notably influence mussel production compared to particulate organic matter. Thus, in meso- and oligotrophic stratified waters, where chlorophyll maximum is relatively deep, depth can be a key factor for the productivity of mussel cultivation. The obtained results could help to maximize the production of suspension-feeding bivalve cultivation and, therefore, the expansion and development of sustainable aquaculture in non-eutrophic marine waters.