Effects of artificial disturbance on quantity and biochemical composition of organic matter in sediments of a coastal lagoon

The eutrophication of the coastal lagoon of Burano (Tuscany, Italy) produces periodic toxic-anoxic events. The possibility of mitigating eutrophication of a lagoon by resuspension of sediment was tested in a three-year field experiment conducted in 2008–2009. An unreplicated before-after control-impact (BACI) study design was used to ascertain variations in the quantity and biochemical composition of organic matter in sediment artificially disturbed by a specially equipped boat. In October 2008, before beginning disturbance, January 2009, half way through the disturbance period, and May 2009, at the end of disturbance, sediment was sampled in a disturbed area and an undisturbed control area to determine chlorophyll-a (Chl-a), phaeopigments (PHAE), proteins (PRT), carbohydrates (CHO), lipids (LIP), labile organic matter (LOM), refractory organic matter (ROM), total organic carbon (TOC) and total nitrogen (TN). The disturbed area, measuring 44 ha, was divided into nine subareas, eight of which were subjected to four different frequencies of disturbance (from 2 to 5) in the period October 2008 – April 2009. Sediment was sampled in each sub-area in the three months mentioned above to determine labile and total organic matter. The results were processed by univariate and multivariate analysis using Primer 6.0, Permanova + and Prism 5.0 software. The findings were as follows: (1) a large proportion of the biopolymeric carbon consisted of labile matter throughout the lagoon; (2) higher abatement of labile organic matter was recorded in the disturbed area; (3) a lower protein:carbohydrate ratio was found in the disturbed than in the undisturbed area; (4) the C:N ratio of sediment was much lower in the undisturbed area than in the disturbed area at the end of the study period; (5) sediment and macroalgal C:N ratios did not significantly change in response to disturbance; (6) initial accumulation of organic matter from phytoplankton was greater in the disturbed area than the undisturbed area; (7) chlorophyll-a, phaeopigments and the pigment diversity index did not reflect any decline in photosynthesis in the disturbed area; (8) treatment effects increased with increasing frequency of disturbance. These findings indicate less availability of labile organic matter and therefore lower risk of extended anoxic events in the disturbed area and support our hypothesis that artificial disturbance of sediment can be used to manage lagoon environments and prevent the consequences of eutrophication