Loss, growth and transport dynamics of Chaetomorpha aerea and Ulva rigida in the Lagoon of Venice during an early summer field campaign

Abstract

The growth, the losses by grazing and sporulation and the advective transport of benthic macrophytes, dissolved and particulate nutrients were studied in the Lagoon of Venice during May–June 1995. The growth rate of Ulva rigida was 0.043 d −1 and the grazing rate found to be 0.01 d −1. No sporulation was observed in the study period. The advective transport of macrophytes was measured by catches in vertically exposed nets during several tidal excursions and varied from 0 to 11 kg Chaetomorpha aerea, 10 kg Zostera sp. and 9 kg Ulva sp. wwt. hour −1 per 2 m net length normalized to effective length perpendicular to the current direction. The transport of Chaetomorpha and Ulva was linearily correlated to the current velocity ( r 2 = 0.80 and 0.88 respectively). Vertically separated nets revealed that 89% of the Zostera sp. was transported in the top 30 cm of the water column and 65% of the Chaetomorpha sp. was transported in the deepest 30 cm of the column and the transport of Ulva sp. was dispersed over the 1 m water column. Automatically taken water samples were analysed for dissolved inorganic nutrients (carbon, nitrogen and phosphorus) and particulate nutrients during the campaign periods together with nutrients bound in macroalgae and seagrasses. The advective transport of dissolved and non-macrophyte bound, particulate nutrients were calculated by a 2-dimensional, hydrodynamic model. Less than 1% of the nutrients were transported as dissolved inorganic nitrogen and phosphate, 3–4% was transported as non-macrophyte, particulate nitrogen and phosphorus and more than 90% of the nutrients were transported as macrophyte bound nutrients. It is therefore obvious that mass balance calculations for shallow estuarine systems should include advective transport of dead and alive macrophytes.