Primary productivity of biofilm in farmed shrimp Farfantepenaeus paulensis and Litopenaeus vannamei
Cultivation of animals in aquatic environments has long used primary production of biofilm, or peripheryton (community of microorganisms adhered to natural or artificial submerged substrates), as strategies for improving water quality, or as a complementary food source for organisms grown. Despite the importance of natural productivity for farming systems, little is known about the primary productivity of the periphery, specifically in shrimp farming. Thus, this work sought to evaluate the levels of primary production of the peripheryton in culture systems with shrimp Litopenaeus vannamei and Farfantepenaeus paulensis and to determine the effects of abiotic factors and shrimp predation on the microalgae of the peripheryton.Two experiments were carried out during 38 days with each species of shrimp, in November 2006 and April 2007 at the Marine Aquaculture Station - FURG. In each experiment, three treatments containing the addition of artificial substrates (nylon screens with 1 mm mesh) were placed in 150 L plastic tanks as follows: 1) CR treatment, shrimp plus feed addition; 2) R treatment, just adding feed and 3) Control treatment, containing only the screens. During this period, samples were collected for primary production incubations, analysis of chlorophyll a, dry weight and abundance of microorganisms. The experiment with L. vannamei was characterized by having high temperatures and salinities, which correlated positively with the levels of primary production.These prawns effectively preyed on small nematodes and small diatoms (Cymbella less than 20 µm), causing a replacement of dominant diatoms by r-strategist species. Such succession probably promoted the highest levels of primary production in the CR treatment (0.0022 to 0.117 mgO2 cm; 2.h; 1), compared to the other treatments. Chlorophyll ranged between (0.52 and 1.8 & g cm # 2). The experiment with F.paulensis showed 8 CR maximum biomasses in chlorophyll a (0.19 to 8.25; g cm; 2) and primary production values equal to (0.0015 to 0.050 mgO2cm; 2.h; 1), and these low values may have been the result of shading due to microalgae overlap.These prawns consumed mainly larger diatoms and nematodes. Knowing the net production of the systems and the metabolic rates (respiration) of L. vannamei and F. paulensis it was possible to estimate how much the primary production of the periphyton in the culture systems could meet the metabolic demand of these shrimp. In the case of L. vannamei, the net production of the entire system could meet 8 to 150% of the requirements of this shrimp, whereas for F. paulensis, which has a lower metabolic activity, the primary production of the periphyton in the tanks could supply 137 287% of the metabolic needs of this shrimp. The results of this work demonstrate that the primary productivity of biofilm can supply a large part of the nutritional demand of shrimp.The increase in submerged surface in cultivation systems can represent an increase in the availability of food for cultivated shrimp, which could result in an important reduction in costs, since the purchase of feed represents more than 50% of the total production cost.