Strong Indirect Effects of a Submersed Aquatic Macrophyte, Vallisneria Americana, on Bacterioplankton Densities in a Mesotrophic Lake A.A
Total Page：16
File Type：pdf, Size：1020Kb
Strong Indirect Effects of a Submersed Aquatic Macrophyte, Vallisneria americana, on Bacterioplankton Densities in a Mesotrophic Lake A.A. Huss and J.D. Wehr Louis Calder Center—Biological Field Station and Department of Biological Sciences, Fordham University, P.O. Box 887, Armonk, NY 10504, USA Received: 11 March 2003 / Accepted: 1 July 2003 / Online publication: 25 March 2004 NH+-depleted conditions, the influence of Vallisneria Abstract 4 was negative or neutral. Effects of living macrophytes on Phytoplankton and allochthonous matter are important planktonic bacteria were apparently mediated by the sources of dissolved organic carbon (DOC) for plank- macrophytes use and/or release of nutrients, as well as tonic bacteria in aquatic ecosystems. But in small tem- through possible effects on phytoplankton production. perate lakes, aquatic macrophytes may also be an important source of DOC, as well as a source or sink for inorganic nutrients. We conducted micro- and meso- Introduction cosm studies to investigate the possible effects of an ac- tively growing macrophyte, Vallisneria americana,on Many studies in both marine and freshwater systems bacterial growth and water chemistry in mesotrophic support the idea of a tight coupling between phyto- Calder Lake. A first microcosm (1 L) study conducted plankton primary production and bacterial secondary + + under high ambient NH4 levels (NH4 ‡ 10 lM) dem- production through phytoplankton-produced dissolved onstrated that macrophytes had a positive effect on organic carbon (DOC) [2, 12, 25, 34]. However, unlike bacterial densities through release of DOC and P. A the open ocean, most temperate lakes are small and + second microcosm experiment, conducted under NH4 - shallow, with low pelagic to littoral zone ratios, sub- + depleted conditions (NH4 <10lM), examined inter- stantial allochthonous organic matter inputs, and an active effects of macrophytes and their sediments on abundance of submersed macrophytes [10, 20, 45, 46]. bacterial growth and water chemistry. Non-rooted mac- These properties may have profound effects on the fac- rophytes had negative effects on bacterial numbers, while tors regulating bacterioplankton in fresh waters. rooted macrophytes had no significant effects, despite In oligotrophic, eutrophic, and humic lakes, high significant increases in DOC and P. A 70-L mesocosm rates of respiration in relation to primary production experiment manipulated macrophytes, as well as N and P suggest the presence of other sources of organic carbon + + supply under surplus NH4 conditions (NH4 ‡ 10 lM), for bacteria in addition to phytoplankton-produced or- and measured effects on bacterial growth, Chl a con- ganic matter [13, 32, 37, 43]. In oligotrophic lakes with centrations, and water chemistry. Bacterial growth and large inputs of allochthonous (mainly terrestrial) carbon, Chl a concentrations declined with macrophyte addi- there is a greater supply of DOC for bacterial growth than tions, while bacterial densities increased with P addition in clear-water oligotrophic lakes less influenced by such (with or without N). Results suggest that the submersed sources [37, 40]. Further, because the littoral zone can macrophyte Vallisneria exerts a strong but indirect effect dominate over the pelagic zone on an area basis in many on bacteria by modifying nutrient conditions and/or small and shallow lakes, loading of DOC from littoral suppressing phytoplankton. Effects of living macrophytes communities can be significant and may be several times + differed with ambient nutrient conditions: under NH4 - greater than the amount produced by pelagic phyto- surplus conditions, submersed macrophytes stimulated plankton [45]. On a unit-area basis, typical annual net bacterioplankton through release of DOC or P, but in primary productivity of submersed macrophytes is esti- mated to equal or exceed that of phytoplankton in the majority of freshwater systems [46]. Aquatic macrophytes Correspondence to: A.A. Huss; E-mail: [email protected] release a portion of this fixed carbon into the water DOI: 10.1007/s00248-003-1034-7 d Volume 47, 305–315 (2004) d Ó Springer-Verlag New York, LLC 2004 305 306 A.A. HUSS, J.D. WEHR:MACROPHYTE EFFECTS ON BACTERIOPLANKTON column as DOC while living and also upon senescence. In To test the hypothesis that living macrophytes may marine [9, 33] and freshwater [20, 38, 44] systems, ac- have a significant effect on growth of bacterioplankton, tively growing macrophytes are estimated to release be- experiments were performed using the macrophyte tween 1 and 10% of their fixed C as DOC into the water Vallisneria americana in micro- and mesocosms with column. Furthermore, studies have demonstrated that water from a small mesotrophic lake (Calder Lake) lo- with experimental additions of macrophyte-derived DOC cated in New York, USA. Vallisneria is found mostly in to both Ogeechee River and Hudson River bacterial as- eastern North America [4, 24]; however, its distribution semblages, large increases in rates of bacterial production has been documented in western North America [27] are observed [17, 18]. In Calder Lake (NY), a small and as far south as the Gulf of Mexico [16]. Vallisneria temperate lake with a large assemblage of submersed was used because it is the most abundant macrophyte in aquatic macrophytes, DOC from non-phytoplankton Calder Lake and is widespread in many freshwater ec- sources may play an important role in supplying energy osystems. Objectives of this study were to determine (1) for bacterial production, although mainly as detritus in if there is an effect of Vallisneria on bacterioplankton the autumn [43]. densities and if so, whether this influence is positive or Rates of bacterial production can also be limited by negative; (2) whether Vallisneria rooted in sediment inorganic nutrients, particularly N and P. Several studies exert a different effect on bacterial densities than free- have demonstrated that organic substrate enrichment floating plants; and (3) how ambient nutrient condi- alone does not always increase bacterial growth, but ad- tions alter the effects Vallisneria may have on bacterio- ditions of inorganic phosphorus are often stimulatory plankton growth. [14, 26, 39]. Bacterial growth responses to nutrient en- richment (with and without phytoplankton) similarly Methods demonstrate that N and P can directly limit bacterial growth when either is in short supply [41, 42], including Study Site and Experimental Setup. Three experiments microbial communities in mesotrophic Calder Lake [26, were performed during July and December 1996 and 39]. Bacterioplankton production was significantly August 1997 at the Louis Calder Center, the biological stimulated by P additions as little as 0.05 lM [39]. This field station of Fordham University in Armonk, New phenomenon can cause an uncoupling of bacterial and York [43]. Lake water and bacterioplankton from Calder phytoplankton production [26] and alter rates of P and N Lake, a small (3.9 ha) mesotrophic, dimictic lake (mean retention within the epilimnion [42]. depth 2.9 m, max depth = 7 m), was used in all three Many investigators have shown that aquatic macro- experiments. Dense populations of submersed macro- phytes play an important role in nutrient cycling in many phytes, dominated by Vallisneria americana (termed freshwater systems [3, 10, 11, 46, 48]. Specifically, in- ‘‘Vallisneria’’ hereafter), cover >50% of the lake bottom teractions between submersed rooted macrophytes and from May through October. Although actively growing both sediments and overlying water may result in in- Vallisneria is not found in lakes during December, lake creases or decreases in nutrient levels within the water water was also collected during this month to add a column. Experiments with Scripus and Myriophyllum temporal component with regard to differences in water have shown that during active growth, the littoral zone chemistry. could act as a nitrogen sink [30]. Significantly, foliar Vallisneria plants for experiments were grown from uptake of N by Myriophyllum may dominate N uptake at turions (F & J Seed Service, Woodstock, IL) in sediment- + ambient concentrations >7 lmol NH4 -N/L and can filled pots (3 turions/pot) and placed in nursery tubs supply more N to the plants than do roots [31]. In filled with well water. In each experiment we used epi- general, the major route for P uptake by aquatic plants phyte-free plants and tried to match the size of the plant appears to occur from sediments via their roots [3, 11]. with the size of the container; therefore, the average plant These and other studies [28, 29] have suggested that P is biomass varied between all three experiments (plant translocated from roots to shoots where it may be re- biomass 3.8 g/L, 1.2 g/L, and 0.6 g/L, respectively). Sed- leased by actively growing macrophytes. However, in iment was collected from a macrophyte-free (shaded) many of these systems including our study lake, macr- pond at the field station, as pilot studies using Calder ophytes may become dislodged but continue growing in Lake sediment resulted in contamination by other a free-floating condition for several weeks, where they macrophytes (e.g., Najas flexilis, Chara spp.). In the first presumably use water-column nutrients exclusively. Be- two experiments, plants were used when they reached cause bacterial growth can be limited by the availability lengths >25 cm. Half were rinsed and rerooted into 30- of N, P, and/or DOC, aquatic macrophytes may exert an mL plastic bottles containing