Growth of the seaweed Ulva rigida C. Agardh in relation to biomass densities, internal nutrient pools and external nutrient supply in the Sacca di Goro lagoon (Northern Italy)

Abstract

Growth of the seaweed Ulva rigida C . Agardh was investigated in relation to biomass densities, internal nutrient pools and external nutrient supply . Research was carried out from 23 March to 5 July 1994 in the Sacca di Goro (Po Delta, Northern Italy), whose south-eastern part was covered by extensive mats of Ulva rigida . Two types of field experiments were conducted by incubating Ulva thalli inside large cages . In the first experiment, beginning on 23 March, 100 g of wet thalli were placed into the cages, allowed to grow for two weeks, then collected and replaced. This procedure was repeated 8 times over the study period. In the second experiment, Ulva thalli were left inside the cages and collected at selected time intervals (14, 27, 41, 64 and 76 days) in order to simulate the effects of increased density on growth and nutrient storage .We recorded specific growth rates (NGR) ranging from 0 .025 to 0 .081 d -' for a period up to two months in the repeated short-term experiments performed at relatively low initial algal densities (300-500 g AFDW m -3 ) . These NGR resulted significantly related to dissolved inorganic nitrogen (DIN) in the water column . Tissue concentrations of total Kjeldahl nitrogen (TN) were almost constant, while extractable nitrate decreased in a similar manner to DIN in the water column . Total phosphorus showed considerable variation, probably linked to pulsed freshwater inflow.In the long-term incubation experiment, NGR of Ulva was inversely related to density. Internal concentrations of both total P and TN reached maximum values after one month ; thereafter P concentration remained almost constant, while TN decreased below 2% w/w (by dry weight) . The TN decrease was also accompanied by an abrupt decrease in nitrate tissue concentration . The biomass incubated over the two month period suffered a progressive N limitation as shown by a decreasing N :P ratio (49 .4 to 14 .6) . The reciprocal control of Ulva against biogeochemical environment and viceversa is a key factor in explaining both resource competition and successional stages in primary producer communities dominated by Ulva . However, when the biomass exceeds a critical threshold level, approximately 1 kg AFDW m --', the macroalgal community switches from active production to rapid decomposition, probably as a result of selfshading, biomass density and development of anaerobic conditions within the macroalgal beds . IntroductionThe wide spread distribution of large opportunistic seaweeds has been recognized to depend on their growth In recent years there has been an increased production strategy, viz . the species-specific growth rate and its of large seaweeds in shallow coastal waters . These seasonal timing (Rosenberg & Ramus, 1981 ; Luning, macroalgae are mostly ephemeral Chlorophyceans, 1993) . Moreover, macroalgal growth is modulated by able to survive in fluctuating environments because factors such as light, nutrient availability, and temof their ability to rapidly take-up and store nutrients .perature, although in ...

The response of primary producer assemblages to mitigation measures to reduce eutrophication in a temperate estuary

“…This difference may be due to the nitrate's negative charge, which makes the uptake energy-dependent and thus slower (Runcie et al, 2003). After the uptake, nitrate must be converted into ammonium in a rate-limiting step catalyzed by nitrate reductase (NR) before being accumulated in vacuoles (Viaroli et al, 1996;Cohen and Fong, 2004;Lartigue and Sherman, 2005). The level of NR in Ulva intestinalis appears to be determined by the presence or absence of NO 3 À in the water column conferring macroalgae the ability to use a greater proportion of this nitrogen form.…”

“…Phytoplankton and macroalgae are capable of taking advantage of the available resources in transient environments (Viaroli et al, 1996;Raven and Taylor, 2003;Cohen and Fong, 2004). Their high surface area to volume ratio and high affinity for nutrients, especially N and P, favor a rapid nutrient uptake and high growth and production rates leading to very large biomass values (Rosenberg and Ramus, 1984;Hernández et al, 1997;Raffaelli et al, 1998;Raven and Taylor, 2003).…”

Regulation of spatial and seasonal variation of macroalgal biomass in a brackish, eutrophic lake

“…at Middelplaten were close to or below the critical values at the end of the build-up phase, it is concluded that nitrogen limitation may have occurred during this period, causing the transition from the build-up period to a more stationary period. The increase in tissue nitrogen during the stationary and the decomposing phase is probably mainly due to the leakage of nitrogen-free compounds from decomposing thalli (Hanisak 1993;Viaroli et al 1996). However, self-shadowing, leading to strong light limitation and luxury uptake of nitrogen, may also increase the average tissue N of a dense algal mat, as suggested by Vergara et al (1998).…”

“…for luxury uptake and storage of nutrients. When nutrient concentrations in the water decrease, the algae can then grow on their internal reserves (Fujita 1985;Viaroli et al 1996;Pedersen and Borum 1997). However, the results should be interpreted with care, as intercorrelation of the independent variables can greatly influence the outcome of a multiple regression analysis (Robertson et al 1993;Sokal and Rohlf 1995).…”

Benthic primary production and nutrient cycling in sediments with benthic microalgae and transient accumulation of macroalgae

“…Macroalgae have a high capacity for growth and nutrient uptake (Viaroli et al 1996a), and because of their position on top of the sediment, they are, like the microphytobenthos, able to control the exchange of nutrients between the sediment and the water column (McGlathery et al 1997;Krause-Jensen et al 1999). In eutrophied coastal environments in southern Europe, macroalgal mats develop to huge densities, very often followed by a sudden collapse of the whole mat, with sulfidic and anoxic water as the result (Bartoli et al 1996;Viaroli et al 1996b;Sfriso and Marcomini 1999). In Danish coastal waters, the macroalgal mats generally do not reach such high densities, and the collapses of macroalgal mats are often only of local importance.…”