Resuspension of benthic protists at subtidal coastal sites with differing sediment composition

Abstract

Mechanisms and consequences of sediment resuspension are well studied in coastal environments, but the associated resuspension of micro-organisms is poorly understood. Using an in situ flume, and by sampling the benthic boundary-layer during tidal cycles, we measured resuspension of protists and sediment at a subtidal, sandy site in Buzzards Bay, Massachusetts, USA, and compared it to previous studies at a silty site in the bay with similar current magnitude. At the sandy site, where the bottom shear stress typically is greater on flooding than on ebbing tides, resuspension occurred only with the flooding current, whereas at the silty site, where the bottom stress is relatively similar on flooding and ebbing tides, resuspension occurred during both flood and ebb. For sediment and most protistan taxa at the sandy site, erosion thresholds were higher and the total material resuspended in strong flow was lower than at the silty site. At both sites, organic-rich, flocculent surficial sediment resuspended in weak flow, but the composition of the protistan community associated with it differed. At the sandy site it included heterotrophic nanoflagellates (HNan), euglenoid flagellates, oligotrich ciliates, and scuticociliates, whereas at the silty site it included HNan, oligotrich ciliates, and the diatom Navicula distans. Differences in the resuspension thresholds of ciliate groups between sites were due largely to differences in taxonomic composition, whereas differences for diatoms were linked to sediment dynamics. At both sites, protistan taxa resuspended in sequence as flow accelerated, implying that the assemblage of species exchanged between benthic and planktonic communities depends on the maximal bottom stress. At the silty site, however, the composition of this group should be more sensitive to variations in tidal strength (e.g. during the spring-neap cycle) than at the sandy site. Resuspension of protists may have strong impacts on microbial food-web ecology in the water column and sediments by linking the communities and by creating strong fluctuations of cell concentrations (measured up to 24 × in the boundary-layer in the case of oligotrichs). Tidal resuspension of protists is common in the subtidal of Buzzards Bay, but its dynamics depend on local sediment properties, hydrography, and taxonomic composition. KEY WORDS: Resuspension · Protists · Benthic flume · Erosion threshold · Buzzards BayResale or republication not permitted without written consent of the publisher

Community structure of marine sedimentary protists in relation to flow and grain size

“…Bacteria were separated from sediment by sonication, and protists were extracted by Percoll-gradient centrifugation, after which cells were stained with DAPI, and the bacteria and nanoflagellates were counted using epifluorescence microscopy (Shimeta & Sisson 1999, Shimeta et al 2003. Nanoflagellates were classified as pigmented if chlorophyll fluorescence was visible under blue excitation.…”

“…Resuspension stimulates growth and trophic interactions of unicellular sedimentary protists (Garstecki & Wickham 2001, Garstecki et al 2002, and the fact that benthic diatoms and ciliates each show species-specific resuspension dynamics (Lucas et al 2001, Shimeta et al 2002 also suggests that flow regime can influence populations differentially and possibly impact community structure. Furthermore, resuspension dynamics can differ between sites in ways suggesting an interaction between flow regime and grain size (Shimeta et al 2003). Detailed analyses of community structure have not been conducted, however, in relation to flow regime at subtidal habitats.…”

“…For example, it is possible that Thalassiosira could have sedimented from the plankton and been part of the phytodetritus. Other protists, including ciliates and nanoflagellates (Table 2) and likely amoebae, were also present at reasonable cell concentrations, but DGGE analysis of their diversities will likely require the use of PCR primers with sequences specific to these taxa in order to avoid amplifying an overwhelming amount of 18S rDNA from diatoms or metazoans.By comparing the sedimentary chlorophyll a concentrations (Table 2) with data from these sites in previous summers when we counted pennate diatoms microscopically (Shimeta et al 2002(Shimeta et al , 2003. data), we estimated the concentrations of pennates in the July samples of the present study to be 1.68 × 10 4 cells ml -1 at WF, 8.73 × 10 3 cells ml -1 at RH, and 1.19 × 10 4 cells ml -1 at WI.…”

“…Sediment resuspension, another flowinduced process, has been shown experimentally to enhance growth rates and intensify trophic interactions throughout the sedimentary protistan community, including diatoms, flagellates, ciliates, and rhizopods (Garstecki & Wickham 2001, Garstecki et al 2002. Shimeta et al (2002Shimeta et al ( , 2003 documented periodic resuspension at Stations RH and WI, with various taxa of ciliates and diatoms displaying different thresholds of flow strength for resuspension. Thus, community structure could be affected by flow regime, as various taxa spend differing amounts of time in resuspension.…”

“…Except as noted below, replicate samples were from separate cores collected on the same dive within several meters of one another, and samples were processed immediately. Cores of 3.8 cm diameter were used to determine grain-size distributions in the top 2 mm by mass fractions after wet sieving (Shimeta et al 2003). On 12 July 2004, cores of 2.6 cm diameter were collected from all 3 stations, and the top 4 mm of sediment was used for the following analyses.…”