Southeastern Naturalist 113 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 22001199 SOUTHEASTERN NATURALIST 1V8o(1l.) :1181,3 N–1o2. 91 Phytoplankton Prey Selection by Hypophthalmichthys molitrix Val. (Silver Carp) in a Lower Mississippi River Backwater Lake Clifford A. Ochs1,*, Orathai Pongruktham1,2, K. Jack Killgore3, and Jan Jeffrey Hoover3 Abstract - Hypophthalmichthys molitrix (Silver Carp) are invasive and abundant in the Mississippi River system, where they consume phytoplankton. There is concern that Silver Carp may influence phytoplankton community structure with cascading effects on other trophic levels. Information is needed regarding Silver Carp phytoplankton-consumption rates and prey selection to assess their potential impact on the food-web in the river. We investigated Silver Carp diets in a backwater lake of the Lower Mississippi River in order to quantify phytoplankton prey selectivity. We made measurements on 4 dates over a 2-y period, which spanned a range of hydrologic connectivity between the lake and the river and a variety of fish sizes. We quantified selection by comparing phytoplankton community composition in the lake to prey in foreguts of captured Silver Carp using Vanderploeg and Scavia’s (1979) relativized selection index. With a possible exception of diatoms on 1 date, there was no relationship of sample date or fish size on prey selection. However, there was a consistent pattern in prey selection: euglenoid algae were positively selected, selection of colonial algae and diatoms was variable, and flagellates and filamentous cyanobacteria were negatively selected. Results are discussed in the context of a conceptual model for Silver Carp phytoplanktivory that incorporates the roles of habitat selection, prey availability, prey capture and processing, and digestive physiology. Introduction Hypophthalmichthys molitrix (Valenciennes) (Silver Carp) invaded the Lower Mississippi River (LMR) system from aquaculture ponds in the 1970s (Chick and Pegg 2001, Kelly et al. 2011). Since then, this species has become common in the Mississippi River (Haupt and Phelps 2016), its tributaries (DeBoer et al. 2018, Fuller et al. 1999, Hayer et al. 2014, Williamson and Garvey 2005), and other river basins in the southeastern US (Slack et al. 2016). Silver Carp are suction filter-feeding, stomachless, obligate planktivores, able to consume seston particles down to at least 10 μm in size (Bitterlich 1985a, Dong et al. 1992, Vitál et al. 2015, Xie 1999). Even though Silver Carp are obligate phytoplanktivores during all life stages (Kolar et al. 2005), zooplankton is ingested incidentally (Sass et al. 2014). Native planktivorous fishes, in contrast, 1Department of Biology, University of Mississippi, University, MS 38677. 2UNESCO-IOC Regional Office for the Western Pacific, The Government Complex Building B 120 Moo 3, Chaengwattana Road, Lak Si, Bangkok 10210, Thailand. 3US Army Engineer Research and Development Center, Waterways Experiment Station, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199. *Corresponding author - byochs@olemiss.edu. Manuscript Editor: Kirsten Work Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 114 may consume phytoplankton but require zooplankton at 1 or more life stages. In the LMR, these native fish include adult Dorosoma cepedianum (Lesueur) (Gizzard Shad), Polyodon spathula (Walbaum in Artedi) (Paddlefish), and Ictiobus cyprinellus (Valenciennes) (Bigmouth Buffalo) (Radke and Kahl 2002, Sampson et al. 2009, Williamson and Garvey 2005), and juveniles of many species. Silver Carp may have negative impacts on native fishes (Collins and Wahl 2017, DeBoer et al. 2018, Irons et al. 2007, Pendleton et al. 2017, Varble et al. 2007) indirectly through bottom-up cascading effects of phytoplanktivory on the composition and/ or biomass of the zooplankton community, or by direct consumption of zooplankton prey. Results of mesocosm experiments (e.g., Collins and Wahl 2017, Collins et al. 2018, Nelson et al. 2017, Schrank et al. 2003), long-term monitoring (De- Boer et al. 2018, Phelps et al. 2017, Sass et al. 2014, Tumolo and Flinn 2017), and observations by anglers (Upholt 2017) showed that invasive carp may catalyze reorganization of river foodwebs. Silver Carp may alter riverine food webs by a trophic cascade (Carpenter et al. 1985) based on the quantity and particular taxa of phytoplankton they consume, and the subsequent influence on zooplankton community structure and potentially other trophic levels (Sommer 2008). Freshwater phytoplankton communities contain multiple taxonomic, chemical, and morphological forms, spanning a size range of at least 2 orders of magnitude (≤2 μm to ≥ 200 μm). When a heterogeneous pool of potential prey is available, a useful approach to quantify selective-foraging activity are prey selection (electivity) indices. These indices quantify predator (or grazer) selective consumption of particular prey from the pool (Lechowicz 1982). Selective consumption can be quantified by comparing the relative abundance of particular prey items in the environment to their relative abundance in the predator mouth or gut. The index assumes that potential prey are randomly distributed in the portion of the environment in which foraging is occurring, and are identifiable pre- and post-consumption. It does not evaluate whether selection is by a passive process (e.g., limited by feeding-apparatus morphology) or active process (e.g., by prey discrimination). Selection indices for analysis of feeding by planktivorous fish have been used, for example, by O’Brien and Vinyard (1974), Strauss (1979), Cremer and Smitherman (1980), Gras and Saint-Jean (1982), Spataru and Gophen (1985), Xie (1999), and Minder and Pyron (2017). In this study, we determined prey-selection indices of Silver Carp feeding on phytoplankton in a floodplain lake of the LMR. Such lakes are common along the LMR and, because they are much more autotrophically productive by area than the main river channel (Pongruktham and Ochs 2015), they are critical areas for feeding and growth of visiting river fishes (Baker et al. 1991, Fremling et al. 1989). Our objectives were to temporally assess selection of phytoplankton prey by Silver Carp over varying connectivity levels, and as a function of fish size, directly from field samples of plankton and fish-gut contents, without confinement, stocking, fertilization, or other manipulation. Southeastern Naturalist 115 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 Materials and Methods Study site and sample dates The lake in which we conducted this study, Forest Home Chute (FHC), is a former meander channel of the LMR located in Warren County, MS (32°45.340'N; 91°01.440'W). FHC is 9–10 km in length and is about 170 m wide. It is sub-divided by low, dirt berms into 3 sections: an upper (northwest), middle, and lower (southeast) section. When the Mississippi River is above 12 m at the Vicksburg USGS gage, usually in spring, river water enters FHC at the upper and lower ends, and fishes can move easily between the river and lake. At lower stages, hydrologic connection with the river is severed. Silver Carp and Hypophthalmichthys nobilis Richardson (Bighead Carp) have been observed in FHC since at least 2005 (Varble et al. 2007). Water and plankton samples We collected samples of water, phytoplankton, and fish from FHC during 4 sample events: 25–26 June 2009, 27–28 September 2009, 29–30 March 2010, and 8–9 September 2010. In June 2009 and March 2010, the entire lake was connected to the main channel, but by September of both years the lake was hydrologically isolated. Sample sites were located near the middle (by length and width) portion of each FHC section. At each site, we conducted a vertical profile for water temperature, dissolved oxygen concentration, turbidity, pH, and depth using a Yellow Springs Instrument ProDSS multiparameter system (YSI, Yellow Springs, OH). We collected water samples in triplicate using a 1.5-m length of polyvinyl chloride pipe connected by a hose to an on-board Whale Gulper 220 diaphragm pump (Whale Marine, Bangor, Northern Ireland). For each sample, we pumped water at 0.5-m intervals from the surface down to the depth where oxygen was ~5 mg L-1, mixed the water from each depth in a 20-L bucket, and then removed the integrated sample from the bucket. For analysis of the phytoplankton community, we preserved lake-water samples in 1% Lugol’s solution immediately after collection (APHA 1995). We filtered unpreserved water samples through Whatman GF/C filters and then froze them for subsequent analysis (see below) of chlorophyll a concentration, a routine proxy for phytoplankton biomass. Fish and fish foregut contents samples We collected Silver Carp used for gut-content analysis by gill nets (91 m in length, 8–12-cm bar-mesh) positioned near the center of each FHC section. We processed 9–15 Silver Carp per sample date, a sample size that enabled rapid onboard processing of the fish. Upon collection of specimens, we measured for total length (nearest mm on a fishboard), weighed (nearest 0.1 kg on a top-loading balance), euthanized, dissected, and removed the entire gut from pharynx to anus of each fish. We placed the intact gut in a sealed plastic bag on ice in a cooler until it was returned to the lab. After measuring the length of each gut, we removed the lumen contents from the first 10-cm section of the foregut, just behind the pharynx, and placed these in a sterile container with 100 ml of water and preservative that Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 116 had been strained through a Whatman GF/F filter. We preserved gut contents with 2% formaldehyde (2009) or 2% Lugol’s solution (2010). We processed a total of 50 Silver Carp over the 4 dates. Analyses of chlorophyll, plankton, and gut-contents We measured chlorophyll a after extraction in alkaline acetone using the spectrophotometric method (Wetzel and Likens 2000). For determination of composition of the phytoplankton community (water column) and prey consumed by Silver Carp (foregut contents), we concentrated samples in 5-mL Hydro-Bios settling chambers (Kiel-Altenholz, Germany). From the count data and sample volumes concentrated and analyzed, we determined abundances of phytoplankton or prey in the natural environment or gut contents, respectively. For selection measurements, we sorted phytoplankton into 5 groups based on a combination of taxonomic identity, cell or colony size, and growth form (unicellular, non-filamentous colonial, filamentous). The 5 groups were (1) euglenoid algae (Division Eugleophyta); (2) non-euglenoid unicellular flagellates (Divisions Chlorophyta, Chrysophyta, and Cryptophyta); (3) filamentous cyanobacteria (Division Cyanophyta); (4) diatoms (Division Bacillariophyta, all forms), and (5) other colonial forms including chlorophytes and non-filamentous cyanobacteria. For colonial organisms, we did not attempt to count individual cells, and therefore, refer to all counts, whether as single cells or colonies, as phytoplankton units. We counted at least 400 units per sample. We enumerated desmids and dinoflagellates, but these taxa were uncommon (less than 1% of abundance in plankton), and were not included in selection measurements. Phytoplankton prey smaller than ~5 μm in cross-section were not enumerated because they were difficult to discern in gut contents. BSA Consulting Services, Inc. (Beachwood, OH) provided validation of major taxa from multiple dates. We employed light microscopy to determine sizes of randomly selected phytoplankton units from all groups (n ≥ 25 per group) and on multiple dates. We based size comparisons on the longest linear dimension of the cell or colony. For filamentous organisms, the longest dimension is the length of the filament, but we also measured filament widths. Selection of phytoplankton prey We quantified phytoplankton prey selection by Vanderploeg and Scavia’s relativized electivity E* (Vanderploeg and Scavia 1979): Ei * = (Wi - [1 / n]) / (Wi + [1 / n]), where Ei * = relativized electivity (selectivity) index for prey source i; n = the number of kinds of prey items; and Wi = Chesson’s prey selectivity index = (ri / pi) / Σ (ri / pi), where ri = a prey type in proportion to its abundance in the plankton (pi) (Chesson 1978, Lechowicz 1982). In a comparison of commonly used feeding selectivity coefficients, E* is generally considered reliable and most appropriate when numbers of prey types or proportions vary among experiments (Confer and Moore 1987, Horn 1985, Lechowicz 1982). The index value for a particular prey type or group can vary between -1 (strongly avoided relative to its proportion in the environment or not consumed), and +1 (strongly preferred and consumed). E* = 0 Southeastern Naturalist 117 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 when feeding is random and prey items are selected according to their proportion in the environment. Statistical analyses We compared phytoplankton potentially available in the water column for selection by group median-unit sizes. For analysis of possible differences among groups, we used non-parametric Kruskal–Wallis ANOVA by ranks (H-test). Where the overall test result was significant, we employed a Dunn’s multiple comparisons test to identify significant differences (Siegel and Castellan 1988). For analysis of phytoplankton prey selectivity by Silver Carp, we calculated values of E* for each phytoplankton group for every processed fish. E* is not subject to parametric analysis, but it can be ranked for selectivity of prey groups (Lechowicz 1982). Therefore, we also tested for statistical significance by Kruskal–Wallis ANOVA and Dunn’s post-hoc test differences among phytoplankton groups in E* values by date. Statistical analyses were performed in Statistica 8 (Statsoft, Inc., Tulsa, OK). Results Mean depths of sites varied from 1.7 m to 6.0 m, and were most shallow late in the year after prolonged disconnection from the river (Table 1). For all sample dates, mean surface-water temperature varied from 11 °C (March) to 30 °C (June), turbidity varied widely, and pH was circumneutral. There was variation among sample dates in phytoplankton group proportions in lake water (Fig. 1). Diatoms were most abundant in spring (March) and early summer (June) when there was strong connection to the river. By September of both years, diatoms were a reduced component of the community, replaced in dominance by filamentous cyanobacteria and non-euglenoid unicellular flagellates. In each group, the most abundant phytoplankton genera included Euglenophyta (Lepocinclis spp., Phacus spp. and Trachelemonas spp.), filamentous cyanobacteria (Planktolyngbya spp. and Pseudanabaena spp.), small unicellular flagellates (Chroomonas spp., Chrysococcus spp., Chalamydomonas spp.), diatoms (Cyclotella spp., Skeletonema spp., Aulacoseira spp.), and, among other non-filamentous colonial forms, chlorophytes (Crucigenia spp., Scenedesmus spp.) and coccoid cyanobacteria (Chroococcus spp., Snowella spp., Aphanocapsa spp.). Table 1. Means (SE) for hydrologic parameters, and water-quality parameters in surface (0.5 m) water (n = 3; derived from means of 3 sample sites). Days post-connection is the period of time since last hydrologic connection of Forest Home Chute with the Lower Mississippi River. Parameter June 2009 September 2009 March 2010 September 2010 Vicksburg gage (m) 10.15 5.84 11.71 4.91 Days post-connection 15 94 36 94 Depth (m) 4.4 (0.4) 1.8 (0.6) 6.0 (0.5) 1.7 (0.4) Temperature (°C) 30.3 (0.7) 27.0 (0.6) 11.3 (0.7) 29.7 (0.3) Turbidity (NTU) 9.2 (0.9) 51.0 (18.0) 37.0 (16.0) 46.0 (10.0) O2 (mg L-1) 11.3 (1.8) 5.7 (0.8) 10.7 (0.6) 7.0 (0.8) pH 8.1 (0.4) 6.7 (0.2) 7.9 (0.1) 7.5 (0.1) Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 118 Phytoplankton groups overlapped in median size, but could be separated clearly into 2 groups (Fig. 2). Larger phytoplankton were non-filamentous colonial phytoplankton (median size = 26 μm), euglenoid algae (21.5 μm), and filamentous cyanobacteria (21.0 μm). These phytoplankton groups did not differ significantly in median unit size (P = 1.0 in any 2-way comparisons). However, each of these groups was significantly larger (P ≤ 0.001 for all comparisons) than the similarly sized (P ≤ 0.38) diatoms (median size = 10 μm) and unicellular flagellates (9.3 μm). Captured Silver Carp varied from 330 mm to 961 mm total length and from 0.4 kg to over 10 kg in wet weight. They were longer later in the year (September 2009: Figure 1. Composition (means [SE]) of 5 dominant phytoplankton groups in the water column and in the Silver Carp foregut by sample date. Groups are euglenoid algae (E), diatoms (D), non-filamentous colonial algae (CA), non-euglenoid unicellular flagellates (UF), and filamentous cyanobacteria (FC). Note that for the June 2009 graph, the scale of the vertical axis is different than for the graphs for other dates. Southeastern Naturalist 119 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 750 ± 39 mm SE; September 2010: 732 ± 11 mm SE) than in spring (March 2010: 586 ± 45 mm SE) (P = 0.025 and P = 0.042, respectively). There was a strong relationship between carp length and weight: log10weight = -4.83 + 2.96(log10length) (slope 95% CI = 2.85 to 3.06; intercept 95% CI = -5.13 to -4.53) (r2 = 0.98, F1, 56 = 3139, P < 0.0001; n = 59), with regression coefficients similar to those obtained for carp from Missouri River tributaries (Hayer et al. 2014). Hybrids of Silver and Bighead Carp are identifiable based on a distinct gill-raker morphology (Lamar et al. 2010), but we observed none during our study. Chlorophyll a concentration and phytoplankton densities varied by sample date, as did abundances of phytoplankton prey in carp foreguts (Table 2). Chlorophyll a concentration in the upper guts of carp collected in 2010 (n = 32 fish) was up to 1400 times (mean = 359) higher than in the water column. For 4 of the 5 phytoplankton prey groups, there was no clear relationship of E* with fish weight or date of collection (Fig. 3). For diatoms, selectivity was mostly negative for fish weighing between about 3.5 kg and 6 kg, but positive for smaller and larger fish (Fig. 3B). This weight range of carp for which selection was mostly negative was confounded with sample date, September 2010, during a period of low water. We calculated E* for each of 50 fish from data summarized in Figure 1 that compare relative proportions of phytoplankton groups in the water and foreguts. Figure 2. Size distribution of phytoplankton units (cells or colonies) by group. Groups are euglenoid algae (E), diatoms (D), non-filamentous colonial algae (CA), non-euglenoid unicellular flagellates (UF), and filamentous cyanobacteria (FC). Median values are indicated by small squares, large rectangles indicate the interquartile (25–75%) spread, and whiskers indicate the non-outlier range (± 1.5 times the height of the b ox). Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 120 On each sample date, there were significant differences in mean ranks of E* values, and the ranked order for E* of prey was consistent (Fig. 4). On all dates, euglenoid phytoplankton ranked first or second in prey selection, non-euglenoid unicellular flagellates ranked fourth, and filamentous cyanobacteria ranked last. Diatoms and colonial algae were usually intermediate in ranking, with the exception of September 2009 when the diatom median E* was positive (0.52). In every case, E* did not differ in the 2 most-preferred groups and 2 least-preferred groups. Generally, results for euglenoid algae indicated positive selectivity (median E* > 0), while for small flagellates and filamentous cyanobacteria selectivity was generally low (median E* < 0). Mixed colonial algae and diatoms varied by date across the E* = 0 boundary, indicating overall neutral selection. For all sample dates combined, euglenoid algae (median E* = 0.28) and diatoms (median E* = 0.06) had positive ranks that were not significantly different (P = 0.44), euglenoids ranked higher than colonial algae (median E* = -0.15), and colonial algae ranked higher than non-euglenoid unicellular flagellates (median E* = -0.68), which ranked higher than filamentous cyanobacteria (median E* = -0.98). Discussion Large, alluvial river systems are commonly turbid, with limited phytoplankton production (Cole et al. 1992, Ochs et al. 2013); their organic-matter pool is dominated by terrestrially derived detritus (Vannote et al. 1980). Terrestrial detritus is less nutritious than phytoplankton, making the latter a particularly valuable resource in these ecosystems (Brett et al. 2017, Thorp and Delong 2002). Additionally, as different phytoplankton taxa vary in nutritional value, phytoplankton community composition influences energy and nutrient flow through the foodweb (Strandberg et al. 2015, Thorp and Bowes 2017). Whatever factors drive phytoplankton community biomass and structure are likely to have cascading ecosystem-level effects, from the benthos to the pelagic zone (Collins and Wahl 2017, 2018; Phelps et al. 2017; Strayer et al. 1999). Table 2. Chlorophyll a concentrations and phytoplankton densities in the water column and Silver Carp foregut by date. Values shown are means (SE). NA = not available. For chlorophyll and phytoplankton units in water column, n = 3 (derived from means of 3 sample sites). Parameter June 2009 September 2009 March 2010 September 2010 Chlorophyll in water column 31.5 (4.4) 56.0 (8.4) 19.9 (6.4) 98.3 (41.4) (μg L-1) Chlorophyll in foregut NA NA 6621 (2715) 31,907 (5445) (μg L-1) n =14 fish n = 18 fish Phytoplankton in water 2004 (287) 13,850 (5245) 2338 (761) 19,230 (5349) column (units mL-1) Phytoplankton prey in 755,060 12,503,210 1,542,480 1,549,580 foregut (units mL-1) (333,540) (9,522,500) (761,940) (371,945) n = 9 fish n = 14 fish n = 14 fish n = 16 fish Southeastern Naturalist 121 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 An abundant invasive animal such as Silver Carp may shape the native plankton community at any step in the feeding process, from habitat selection to prey capture to defecation (e.g., Collins and Wahl 2018, Collins et al. 2018, Görgényi et al. 2016, Sass et al. 2014, Tumolo and Flinn 2017). Thus, for this discussion, we adopt an integrated perspective, placing results in the context of a step-wise conceptual model for Silver Carp feeding that includes their behavior, prey selection, feedingapparatus morphology, and digestive physiology (Fig. 5). Figure 3. E* values for 5 groups of phytoplankton with respect to body weight of individual Silver Carp and sample date. Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 122 Habitat selection by Silver Carp is antecedent to food-resource use, with possible impacts on phytoplankton community structure. Phytoplankton production in the LMR main channel is strongly light-limited (Ochs et al. 2013), but can be very high where flow is reduced such as seasonally connected lentic backwater sites like FHC (Pongruktham and Ochs 2015). Taking advantage of high backwaters production, Silver Carp migrate into backwaters for feeding (Calkins et al. 2012), returning to the main channel for spawning (Cooke 2016, Kolar et al. 2007). Thus, impacts of Silver Carp on phytoplankton community structure, and subsequent cascading effects, may be amplified in riverine backwaters compared to the main channel. Figure 4. E* values for 5 groups of phytoplankton by sample date. Median values are indicated by the small squares. Large rectangles indicates the interquartile (25–75%) spread for all sampled carp for each potential phytoplankton prey group. Whiskers indicate the nonoutlier range ± 1.5 times the height of the box). Groups are euglenoid algae (E), filamentous cyanobacteria (FC), non-euglenoid unicellular flagellates (UF), diatoms (D), and non-filamentous colonial algae (CA). For each date, superscripts having the same letter are not significantly different (P ≥ 0.05). Southeastern Naturalist 123 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 Upon locating food, prey capture and selectivity ensues. Silver Carp are filter feeders; thus, capture is dependent partly on gill-raker morphology (Smith 1989, Vörös et al. 1997). Distances between gill-raker processes of Silver Carp vary from 12 μm to 26 μm (Hampl et al. 1983), suggesting a lower size-limit for efficient prey retention. In FHC Silver Carp, euglenoid algae had high selectivities, while capture efficiencies of colonial algae and diatoms were inconsistent, and varied by sample date. For small flagellated phytoplankton, E* consistently indicated poor capture by Silver Carp, possibly due to their small size (median = 9.3 μm) or another property (e.g., lack of a rigid cell wall as found in similarly sized diatoms). A lower limit for prey capture of about 10 μm is supported by laboratory (Smith 1989) and mesocosm experiments (Laws and Weisburd 1990, Radke and Kahl 2002, Vörös et al. 1997), and may explain the transition of the phytoplankton community to smaller cells upon Silver Carp introduction to mesocosms (Vörös et al. 1997). This is not to suggest that it is impossible for Silver Carp to consume smaller particles (Esmaeili and Johal 2015, Görgényi et al. 2016, Kolar et al. 2007, Xie 1999), only that prey size, and possibly shape and composition, influence capture effic iency. In this study, plankton selectivity was not generally a function of fish size across the range of sizes sampled. An exception occurred for diatoms on 1 sample date where E* was negative. We propose alternative explanations for this observation. Perhaps, on that date, diatoms were particularly small, but we found no difference in diatom size compared to other dates, with the centric genus Cyclotella spp. always in highest abundance. Alternatively, perhaps, on the sample date in question, diatoms were concentrated at a depth where they were more effectively sampled by us than by Silver Carp. Regardless, the ranking of prey selection in September 2010 was similar to other days. Figure 5. Conceptual overview of steps in Silver Carp phytoplanktivory. The upper row of boxes illustrates main steps in the feeding process. The row of ovals indicates the primary factor involved in each step (behavioral, morphological, physiological), and the direct influence via prey availability (p) or prey consumption (r) on E*. The lower row of 3 boxes describes the impact on plankton community structure by size-selective feeding, palatability, and digestibility. Arrows indicate if the net effects at each step are to directly increase (↑) or decrease (↓) the described form in the water column relative to the entire phytoplankton community. Southeastern Naturalist C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 124 Filamentous cyanobacteria, considering their mean length, were an exception to the proposition of an ~10-μm lower-size limit for efficient capture of prey. However, these organisms are generally less than 2 μm in width, and perhaps this size facilitates escape from capture. This explanation was made previously for Silver Carp (Xie 1999) and Bighead Carp (Opuszynski and Shireman 1993), and is consistent with field observations in Kentucky Lake, on the Tennessee River, that cyanobacteria are less susceptible to suppression by Silver Carp than chlorophytes or diatoms (Tumolo and Flinn 2017). In contrast, when cyanobacteria are concentrated in dense blooms or floating mats, they are readily consumed by Silver Carp (Fukushima et al. 1999, Radke and Kahl 2002) and Bighead Carp (Collins and Wahl 2017). An alternative explanation for the low proportion of filamentous cyanobacteria detected in foreguts relates to the third step in the feeding process—behavioral food processing (Vitál et al. 2015). Asian Carp possess pharyngeal teeth by which disruption of food items can occur (Kolar et al. 2005; Xie 1999, 2001), however, our selection measurements would be unaffected unless potential losses were taxonspecific. Alternatively, there may be rapid digestion of cyanobacteria in the carp foregut (Vörös et al. 1997). To minimize this possibility, we sacrificed and quickly dissected fish on-board after capture, sampled only from the first 10 cm of the gut (1.0–4.4% of the total length), and chemically preserved samples immediately. The next steps in the process of feeding physiology are digestion and defecation, processes with potentially profound consequences for the resulting phytoplankton community composition. Some phytoplankton can survive the long passage through an adult carp gut (Bitterlich 1985b, Görgényi et al. 2016, Pongruktham et al. 2010, Spataru 1977), but we do not know if there is selective digestion of algae, or even uptake of nutrients by algae. If so, Silver Carp could facilitate changes in phytoplankton community structure, not just by selective ingestion, but also by selective digestion. For example, Porter (1973) and Smith et al. (1998) demonstrated the effect of differential digestion for planktivorous invertebrates on phytoplankton community structure. By re-structuring the phytoplankton community of floodplain lakes, Silver Carp may affect the community structure of zooplankton and other consumers (DeBoer et al. 2018, Irons et al 2007, Phelps et al. 2017, Solomon et al. 2016). For zooplankton, this phenomenon would presumably occur along the lines of taxonomic group and feeding traits (Mitra et al. 2014). For example, compared to cladocerans, rotifers consume a narrower spectrum of prey size (0.5–40 μm vs. 4–18 μm, respectively), feed more selectively, and ingest larger cells (Bogdan and Gilbert 1984, 1987; Rothhaupt 1990). Hence, in addition to consuming rotifers, Silver Carp may impact rotifers indirectly and negatively by biasing the phytoplankton community to smaller cells (Vörös et al. 1997). However, the net effect of Silver Carp on plankton community structure must also account for consumption of zooplankton that prey on rotifers (Collins et al. 2018, DeBoer et al. 2018, Sass et al. 2014). Ultimately, for a comprehensive, mechanistic model of the influence of Silver Carp on the LMR foodweb, we must consider their upward cascading impacts via selective phytoplanktivory, their downward cascading impacts via zooplanktivory, and their effects on nutrient fluxes (Collins and Wahl 2017, Kaemingk et al. 2016) in Southeastern Naturalist 125 C.A. Ochs, O. Pongruktham, K.J. Killgore, and J.J. Hoover 2019 Vol. 18, No. 1 a hydrological- and habitat-specific context (Haupt and Phelps 2016, Tumolo and Flinn 2017). Acknowledgments J. Beard, S. George, W. Lancaster, B. Lewis, B. Munxayaphom, C. Murphy, and K. Boysen provided assistance in the field. L. Brooks, S. Morgan, B. Munxayaphom, J. Pemment, J. Sackreiter, N. Vera, and W. Wagner assisted in the laboratory and with microscope analyses. We thank W. and B. Creekmore, M. Parker, and M. Bowen and Tara Wildlife for allowing access to Forest Home Chute via private property. Zanethia Barnett and Audrey Harrison provided internal reviews of the manuscript. We also thank several anonymous reviewers. Primary funding was provided by the US Army Corps of Engineers Aquatic Nuisance Species Research Program, with additional support provided by the Department of Biology and the Office of Research, University of Mississippi. Permission to publish was granted by the Chief of Engineers. Literature Cited American Public Health Association (APHA). 1995. Standard Methods for the Examination of Water and Wastewater. Washington, DC. Baker, J.A., K.J. Killgore, and R.L. Kasul. 1991. Aquatic habitats and fish communities in the Lower Mississippi River. Reviews in Aquatic Science 3:313–356. Bitterlich, G. 1985a. Digestive enzyme pattern of stomachless filter feeders, Silver Carp, Hypophthalmichthys molitrix Val., and Bighead Carp, Aristchthys nobilis Rich. Journal of Fish Biology 27:103–112. Bitterlich, G. 1985b. 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