# FINAL REPORT

Fe br uar y , 2004 

# Quantitative assessment of benthic food resources for juvenile Gulf sturgeon,

Acipenser oxyrinchus desotoi in the Suwannee River estuary, Florida, USA.

P r ep ared by : R. Allen Brooks (Principal Investigator)

Project Sup ervisor: Kenneth Sulak

Coastal Ecology & Conservation R e search Group US GS F l orid a In tegrated S c ience Cen t er Ce nte r f o r Aquatic Res o ur ce St udi e s 7920 NW 71st St., Gainesville, FL, 32953 U.S. Depart ment of t h e Interior U.S. Ge ol ogical Sur v ey

Coast a l Ecology & Conserv ati o n Researc h Gr oup Investi gati o n Re po rt CEC 2004- 02

### U.S. Geological Survey Ea st ern Reg i on F l or i d a Inte grate d Sc ience Center

QUANTITATIVE ASSESSMENT O F BENTHIC FO O D RESO URCES FOR JUVENILE GULF STURGEON, ACIPENSER OXYRINCHUS DESOTOI, IN THE SUWANNEE RIVE R ESTUARY, FLORIDA, USA.

Final Report 16 February 2004

Prep a r ed by: R. Allen Broo ks* and Kenneth J. Sulak

U S GS F l or ida Inte gr ated Science Center, Cente r for Aquatic Resource Studies, 7920 NW 71st St., Gainesv ille, FL, 32653, 352-378-8181

C o ll aborators : Kenneth Lit z enberger, U.S. Fish and Wildlife Service, Lower Su w a nnee N a tional W i ldlife Re fuge

*phone: (352) 378-8181 ext. 3478, e m a i l : a l l e n

_ b r o o k s @ u s g s
. g o vFigure Legends

Figure 1. The Suwannee River, Florida, with a circle inse t which encloses the estuarine river mouth area (rkm 0-2.5) u tilized as trophic habitat by juvenile Gulf sturgeon.

Figure 2. Five different sect ors of the Suwannee River Estuar y based upon salinity regimes.

Figure 3. The location of the ra ndom sampling sites used in this study. Th e triangles indicate June-July, 2002, samples (n=1 56) and the circles indicate February-April, 2003, samples (n=103).

Figure 4. A) Comparative mean abun dance (number of individuals/m 2 ± 95% C.I.) of benthic macrofauna (> 0.5 mm ) from core samples in the Suwannee River estuary. B) Comparative mean biomass (g dry wt./m 2 ±s.e.) of benthic macr ofauna (> 0.5 mm) from core samples in the Su wannee River estuary. Location ab breviations are as follows: WDP – Wadley Pass, EP – East Pass, NS – North So und, SS – South Sound, and WP – West Pass.

Figure 5. Interpolated ma p of macrofaunal density (number of individuals/m 2 ) in the Suwannee River estuary during the summer of 2002 (A) and wint er of 2003 (B).

Figure 6. Interpolated map of benthic biomass (g dry wt./m 2 ) of benthic macrofauna in the Suwannee River estuary during the summer of 2002 (A) and wint er of 2003 (B).

Figure 7. Relative composition of benthic macrofau nal taxa from core samples in the Suwannee River estuary.

Figure 8. Interpolated map of amphipod density (number of individuals/m 2 ) in the Suwannee River estuary during the summer of 2002 (A) and wint er of 2003 (B).

Figure 9. Interpolated map of polyc haete density (number of individuals/m 2 ) in the Suwannee River estuary during the summer of 2002 (A) and wint er of 2003 (B).

Figure 10. Interpolated map of brachiopod density (number of individuals/m 2 ) in the Suwannee River estuary during the summer of 2002 (A) and wint er of 2003 (B).

Figure 11. Interpolated map of th e density (number of individuals/m 2 ) of macrofauna which fall into the Principal Foods category in the Suwa nnee River estuary during the summer of 2002 (A) and winter of 2003 (B).

Figure 12. Interpolated map of the Biased Food Density (number of individuals/m 2 ) estimate based upon core samples taken in the Suwannee River estuary during the summer of 2002 (A) and winter of 2003 (B).

Figure 13. Interpolated map of the Adjusted Food (g dry wt./m 2 ) estimate based upon core samples taken in the Suwannee River estuary duri ng the summer of 2002 (A) and winter of 2003 (B).Figure 14. The location of the places within the Suwann ee River Estuary that juvenile (,1 m) Gulf sturgeon and the number which have been caught, USGS sampling 1988-1995.Abstract

Gulf sturgeon, Acipenser oxyrinchus desotoi , forage extensivel y in the Suwannee River estu ar y f o llowing em igration out of the Suwannee River. W h ile in the estuary, juvenile sturgeon prim arily feed on benthic infauna. In June-July, 2002, and Feb-A p ril, 2003, random sites within the estuary were sam p led for benthic macrofauna (2002, n=156; 2003, n=103). A m ean abunda nce of 2,562 individuals/m 2 ( s

. e

. + 204) was found in the summ er, with significantly reduced m a c r ofaunal abundance in the winter (m ean density of 1,044 individuals/m 2

, s

. e

. + 117). Benthic biom ass wa s significantly higher in the summ er with an average summ er sam p l e d r y w t

. o f 5

. 9 2 ( s

. e

. + 0.82) g/m 2 com p ared
t o 3

. 9 1 ( s

. e

. + 0.67) g/m 2 in the winter. Am phi pods and polychaetes w e re the dominant taxa collected during both sam p ling periods. A Pr incipal Foo d estim ate indicated that the potential food resource value for juvenile Gulf sturgeon is spatially heterogeneous within the Suwannee River estuary.

Introductio n

The Gulf sturgeon, Acipenser oxyrinchus desotoi , was designated as threatened by the U.S. Fish and W ildlife Service in 1991, under the U.S. Endangered Species Act of 1991. Subsequently, there has been a need to acquire inform a tion re lated to locating and characterizing their critical h a bitats (U SFWS and Gulf States Marine Fisheries Comm ission, 1995). The largest population of Gul f Sturgeon utilizes the Suwannee River, Florida, and estuary (Sulak & Clugston, 1998). Prior to spring spaw ning, sturgeon from offshore begin pre-m i gr atory staging around m i d-Febru a ry with m o vem e nt into the lower riv e r f r om the estuary. Af ter spawning up-river in March-April, Gulf sturgeon reside in the river, fastin g until Octo be r-Novem b er (Mason & Clugston 1993). W h en t h ey

em igrate back to the estuary, Gulf sturgeon begin active foraging to replace lost energy reserves. Sturgeon over the ag e of five/six spend the wi nter beyond the estuary, but juveniles re m a in and f eed in the e s tuary during the winter (F ox & Hightower, 1998; Sulak & Clugston, 199 8). Recent analysis ind i cates that so m e juveniles age 1-6 rem a in and feed in the estuary year round (USG S 17 year Suwannee River Gulf Sturgeon Database).

1 Three passes (inlets), tw o w ith previously dredged navigation channels exist with in the m outh of the Suwannee Rive r. None of the channels are currently

m a intained f o r naviga tion all hav e becom e sedim e nt-f ille d over tim e. The U.S. Arm y Corps of Engineers (USACE) has proposed to dr edge one inlet, W a dley Pass, in an area probably used as feeding ha bitat by juvenile sturgeon (F&W Coordination Act Report: GEC, 2002) . The channel is planned to be approxim a tely 25 m wide by 3 m deep extending for approximately 2.5 m iles (F igure 1). It has been proposed that the dredge

m a terial be placed subtidally adjacent to Cat, Little Bradfo rd, and No-Nam e Island to aid in shoreline stabilization (F &W Coordination Act Report: GEC, 2002). Of concern is how m a intenance dredging and/or spoil dis posal will im pact th e threaten ed and endangered species which are know n to inhab it the Suwannee Estuary (e.g., W e st Indian Manatee, Gulf sturgeon).Juvenile sturgeon have been found to cons um e prim arily benthic infauna while in the estuary (Huff, 1975; Mason & Clugston 1993). Thus, any alterations within the estuary that changes eith er sed i m e nt grain s i ze /so r ting o r ch e m istry m a y im pact juve nile food resources (Kenny & Rees, 1996; Seiderer & Newell, 1999). In order to prevent disruption of sturgeon m i gration patterns , the date of dredging has been restricted to between May 15 th and October 15 th while the m a jority of the stu r geon are s t ill up river

. However, benthic m acrofauna recovery from dredging m a y require a longer tim e fram e (De Grave & W h itaker, 1999) limiting availa ble food resources for juvenile G u lf sturgeon within the estuary.

Gulf sturgeon feed only in the marine environm ent (Mason & Clugston, 1993; Gu et al. , 2001), with adults and juveniles foraging extensively in the estuary during winter (Novem b er to February-March) (Mason & Clugs ton 1993). T h erefore, it is im perative to acquire food resource infor m ation for this cri tical area. Analysis of the current benthos status will allow for inform ed m a na gem e nt of future a c tion s in the estu ary, and can be used to m i nim i ze i m pact to sturgeon trophic habitat. Most environm ental im pact studies do not have a pre-im pact baseline and rely upon what appear to be relatively sim ilar sites to represent “control areas”. This study al low e d acquisition of quantitative “before”

sam p les which can be used to c o m p arativ ely evaluate post-dre dging changes. Two objectives regarding m acroinfauna within th e S u wannee River Estuary are addressed in this study: 1) Determ ination of the spatia l distribution, abundance, and biom a ss of potential food resources for juvenile Gulf st urgeon. 2) Evaluation of quantitative m e thods for predicting which estuarine areas are po te ntially m o st im portant in providing food resources for juvenile sturgeon.

Methods A) Study Area

The Suwannee River is one of Florida’ s largest rivers and has an average flo w of 299.6 m 3 s -1 (Mattson, 1992) (Figure 1). The river is not dammed, and the m outh of the river receives a substantia l freshwater plum e (Mattson, 1992; Mason et al., 1994). The Suwannee River Estuary is separa ted f

r om the Gulf of Mexic o by a series of oyster reefs. It can be divided up into five sectors differi ng in salinity (F igure 2). The river has three

m a in exits into the Gulf of Mexico – W e st Pass and East Pa ss that are n a tura l pass es , and Wadley Pass which is m a n m a de. West Pass rece ives the m a in outflow of freshwater. The estuarine sound can be separa ted into two parts, Nort h and South Sound. The South Sound receives substantial freshwater input di rectly from West Pass. However, the North Sound generally does not receive as m u ch sa me freshwater input and usually m a intains more saline conditions. W e hypothesized that the five sectors of the estuary, W a dley Pass, W e st Pass, East Pass, North Sound, and South Sound, would differ in benthic community param e ters based upon salinity a nd freshwater input (Kimmerer, 2002).

B) Benthos Sampling

2 A virtual grid of 100 m x 100 m cells was projected ove r the entire Suwannee Estuary.In June-July, 2002 156 random sites and in Feb-April, 2003 103 random sites, were selected for sam p ling ( F igure 3). Random s ites were allocated equally a m ong the five different sectors of the estuary (treated as five strata) disc ussed above and located in the field using a Garm in GPSMAP 76S. At each sa mpling site, a benthic core was tak e n to

sam p le m a c r ofauna (> 0.5mm in siz e ). The core taken was 15 c m in diam eter and 15 c m deep. Each core sam p le was sieved through a 0.5 mm m e sh in the field. For each sample subsurface salin ity (0.5 m depth), water tem p erature, d i ssolv ed oxygen content, and pH

were m easured using a YSI 6000 handheld m u ltim eter. Upon return to the laboratory, all

m acrofauna were identified to lowest possi ble taxon. Dry weight (to the nearest 0.001 g) of each sample was determ ined after drying for 24 hours at 60° C. All bivalv e and gastropod shell m a terial was discarded prior to dry weight determ ination. Polychaetes were iden tif ied by specialis ts a t the Atlantic Ref e rence Cent re of the Huntsm an Marine Science Centre, St. Andrews, New Brunswi ck, Canada. Am phipods were identified accord ing to Bousfield (1973).

Statistical Analysis

A) Fauna Abundance and Distribution Due to a lack of norm a lity and hom ogeneity of variances am ong t h e data, all data

were com p a r ed using nonparam e tric statis tical com p arison techniques. Mann-W h itney Rank Sum Test was used to te st for si gni fi cant di fferences in m acrofaunal density and biom ass bas e d upon season, summ e r (July) vers us winter (F ebruary-March). The summer and winter sam p ling intervals also corresponded to dry (low flow) versus wet (high flow) tim es of the year, respec tively. Kru s k a l- W a llis A NOVA on Ranks com p arisons was u s ed to test for significant dif f erences in m acrof aunal density, m a crofaunal biom ass, a m phipod density, and polychaete density among the five estuarine sectors (Fig ure 2) for each
season sepa rate ly. If Kruskal-W a llis results in dicated a signif i cant tr eatm e nt ef fect, Dunn’s Method m u ltiple com p arison tests were subsequen tly performed to determ ine significant differences (Glantz, 1997). The point values (i.e., m acrofauna density, m acrofauna bi om ass, amphipod density) for each sample were used to create interpo l ated m a ps of the study area, using an Inv e rse Distance Weighting (ID W ) m e thod in the Spat ial Analyst extension of ArcView 3.2. An edit m a sk was used to exclude all land cover and constrain the interpolation to only the

sam p led area. IDW is an interpolation techni que in which estim ates are m a de based on values at nearby locatio ns we ighted only by distance from the interpo l ation lo cation. IDW does not m a ke assum p tions about spatia l r e lationship s except the b a sic assum p tio n that nearby points are more cl osely related than m o re dist ant points. Twelve neighbors were used for the interpolation (power of two and cell size of 100 m )

.

B) Principal Foods Analysis

3 Two studies have exam ined gut conten ts of juvenile Gulf sturgeon. Huff (1975) found that guts of juveniles in the Suwannee River Estuary feed upon non-tube building

am phipods, isopods, insect larvae and m ud shrimps. Mason & Clugston (1993) found sim ilar pre y in juven i le Gulf stu r geon gut contents, including non-tube buildingcrustaceans and insect larvae, with relatively few organisms that possess either a hard carapace or shell. Heard et al. (2002), did not exam ine gut contents, but stated that juvenile Gulf sturgeon consum e non-tube building am phipods. After review of these studies, three different principal food categor ies (Principal, Seconda ry, and Minor) were established (Table 1). Using these three food categories, thr ee ways to predict potential food resources for juvenile Gulf sturgeon were exam ined: 1) Principal Food Density – This estim at e is simply the de nsity (individuals/m 2 ) of m acrofauna which fa ll into the P r incipa l Foods category. Using only the density of Principal Food item s , however, ignores th e Secondary and Minor Food categories. 2) Biased Food Density – The density of m acrofauna (individuals/m 2 ) was weighted by food category: B i ased F ood Density = [(1.0*Principal Food item s /m 2 )+(0.66*Secondary Food item s / m 2 )+(0.33*Minor Food item s /m 2 )]”. The Biased Food Density estim a te does not take benthic biom ass ( i.e. , energy) into account. 3) Adjusted Biom ass – The Adjusted Bi om ass e s tim ate was derived to take food catego r ies and biom ass into accou n t. First, th e Biased Food D e nsity estimate was used to derive a Food Index for each sam p le: “Food In dex = Biased Density / Total Density”. The Food Index retu rns a value b e tween 0 a nd 1 for each sam p le. Higher index values indicate a greater abundance of foods in th e Principal Food category. The food index for each sam p le was then com b ined with the sam p le biom ass (g/m 2 ) to derive an Adjusted Biom ass estim a te: “Adjusted Biom ass = Overall Biom ass* Food Index”.

Results

A) Faunal Abundance and Distribution

In the summer sam p les, 6,618 individuals were collected with an average of 2,562

m acrofauna/m 2 ( 9 5 % C

. I

. + 403). Macrofauna abundance wa s significantly (P=0.001, Mann-W h itney Rank Sum Test) reduced in th e winter with 1,900 individuals collected for an average density of 1,044 macrofauna/m 2 ( 9 5 % C

. I

. + 232). Benthic biom ass was significantly higher in the summ er (P= 0.015, Mann-W h itney Rank Sum Test) w ith an
average sum m e r d r y w t

. o f 5

. 9 2 ( 9 5 % C

. I

. + 1.3) g/m 2 com p a r e d t o 3

. 9 1 ( 9 5 % C

. I

. + 1.3) g/m 2 in the winter. Macrofaunal density was not significantly different am ong the five estuarine sectors in the summ er (P=0.388, ANOVA on Ranks, Figure 4a ). However, from an interpolated

m a p of m a crofauna density, it is apparent that density is not hom o genous but rather patches of high density exist (Figure 5a). Sp ecifically, areas of hi gh density can be found throughout Suwannee Sound and in part of the West Pass sector. A significant difference in m acrofaunal density was found am ong the five sectors during the winter (P=0.001, ANOVA on Ranks, Figure 4a). Both the North Sound and South Sound sectors had significantly higher infaunal dens ity than th e Ea st Pass s ector. An inte r polated m a p f o r the winter also shows areas of relativel y high density in the Suwannee Sound (both sectors) and parts of the We st Pass sector (Figure 5b).

4 A significant difference in benthic bi om ass was found between the five differentTable 1. Principal food categories for j uvenile Gulf Sturgeon erected from the macrofauna found in the Suwanee River Estuary. PRINCIPAL FOODS SECONDARY FOODS MINOR FOODS Brachiopods Anthozoans Bivalves Free-Living Amphipods Cumaceans Decapods Insect Larvae Nematodes Gastropods Isopods Nemerteans Ophiuroids Oligochaetes Ostracods Shrimp Polychaetes Tube-Building Amphipods HirudineaMa crofaun a De nsityAvg. Densitym-2

0 1000 2000 3000 4000 5000

Sum m e r 2002 W i nter 20 03

Macr ofau na B i om ass

Location DI E P NS S S W P O v er al lAvg.DryWt.(g) m-2

0 2 4 6 8 10 12 14 A.

B.estuarine sectors for both summ er and wint er samples (P=0.007 and 0.001, respectively, ANOVA on Ranks). In both summ e r and winter, biom ass was si gn if ican tly g r ea ter in the North Soun d sec t or co mpared to the W e st Pass sec t or (Fig ure 4b ). Ad dition a lly, b i om ass in both the North and South Sound sector was si gnificantly greater than that of W e s t or East Pass sectors in the winter. Patches of high biom ass can be found throughout both Suwannee Sound sectors during both sam p ling tim e s and in the summ er at the m outh of West Pass (Figures 6a,b).

Am phipods and polychaetes were the num erically dom inant f a una colle cted in this study (Figure 7). Although ANOVA on ranks results indicate a significant difference in

am phipod density am ong estuarine regions in the summer (P=0.023), results of a Dunn’s Multip le Co mparisons T e st indicated no sign ificant differences. Areas of high am phipod abundance in the summer can be found in bot h the South S ound and West Pass (Figure 8a) which both along with the North Sound ha d a significantly greater amphipod density than East P a ss in the winter (P=0.001, ANOVA on Ranks). Overall, amphipod density

was m o re hom ogeneous throughout the study area in the winter with sm aller patches of high am phipod density (Figure 8b). Am phipod sp ecies richness was hi gher in the summer (n=10) com p ared to winter (n =7) (Table 2). One am phipod species, A m pelisca ab dita

, dom i nated in both the w i nter and summer acc ounting for 58% of all am phipods collected. Parahaustorius spp. (14.4% of all amphipods collect ed, summer and winter) was also commonly collected. Cerapus tubularis (15.3% of all am phipods collected, summ er and winter) was relatively d o m i nant in the summ er sam p les while Gammarus mucronatus (1.8% of all a m phipods collected, summ er and winter) was relatively common in the winter sam p les. Polychaete density w a s higher in the Nort h Sound compared to all other estuarine sectors in the summer (P=0.001, ANOVA on Ra nks). An interpolated m a p of summer polychaete density indicates a large patch of high density in the North Sound with sm a ller patches in West Pass (Figure 9a). Polychaet e density in the winter was significantly greater in both Sound sectors com p ared to W e st Pass and significantly greater in the North Sound than in E a st Pass (P=0.001, ANOVA on Ranks). An interpolated m a p of polychaete density in the wint er ind i cates a larg e area of high density in both the North and South Sound (Figure 9b). Melinna ( c f. cr is tata) dom i nated in both the winter and summer accounting for 39% of all polychaete s p ecies (Tab le 3). Tharyx sp ., Parandalia americana, Glycera americana, Leitoscolop l os (cf. robustu s)

, and Aricidea philb in ae were also relatively abu ndant rep r es enti ng 14%, 10%, 6%, 5%, and 4%, respectively, of all po lychae tes co llec t ed in both the summer and winter com b ined.

The stalked brachiopod, Glottidia pyramidata , and isopods were also relatively im portant taxa, representing between 2-5% and 3%, respectively, of the

m acroinvertebrates collected, respectivel y. The stalked brachiopod is a common food resource for adult Suwannee River Gulf sturge on (Harris, 2003). Br achiopods were found only in the outer part of Suwannee sound, with higher densities in the summ er (Figures 10a,b).

B) Principal Foods

5 Interpolated m a ps for only the Prin cipal Foods category are shown in Figure 11a,b.SUM M ER WINTER

Amphipo ds Polychaetes Brachiop ods Isopo ds Bivalves Oligo c ha ete s Ne m e rte ans OtherTable 2. Percent representation of amphipod taxa collected in summer, winter and overall samples.

Species Percent of Amphipods Collected in the Summer Percent of Amphipods Collected in the Winter Overall Percent of All

Amphipods

Collected Ampelisca abdita 60.5 48.5 58.9 Cerapus tubularis 17.2 3.3 15.3 Haustoriidae 10.5 39.1 14.4 Ampithoe longimana 5.5 1.3 4.9 Corophium simile 4.2 1.2 3.7 Gammarus mucronatus 1.1 6.1 1.8 Cymadusa compta <1.0 0 <1.0 Melita nita <1.0 0 <1.0 Stenothoe minuta <1.0 <1.0 <1.0 Luecothoe spinicarpa <1.0 0 <1.0Table 3. Percent representation of the dominant polychaete taxa collected in summer, winter, and overall samples.

Species Percent of Polychaetes Collected in the Summer Percent of Polychaetes Collected in the Winter Overall Percent of All

Polychaetes

Collected Melinna (cristata?) 25.1 51.9 39.0 Tharyx 17.9 9.6 13.6 Parandalia americana 15.2 5.8 10.3 Glycera americana 7.0 5.8 6.4 Leitoscoloplos (robustus?) 9.0 1.6 5.2 Aricidea philbinae <1.0 7.2 3.8 Kinbergonuphis 6.2 <1.0 3.4 Paraspionospio pinnata 1.9 4.6 3.3 Websterinereis 4.6 1.8 3.2 Anaitides mucosa 3.5 <1.0 1.8 Malanidae 3.7 0 1.8 Pectinaria gouldii 2.4 <1.0 1.5 Unidentified sp. 0 2.6 1.4 Neanthes succinea 1.2 <1.0 1.0 Other 1.3 5.4 4.3High densities of Principal Food items are f ound in W e st Pass and the outer Sound in the summer. A sim ilar pattern is found for the wint er with a high density of principal food item s throughout W e st Pass and in th e Sound by the m outh of West Pass. The Biased Food De nsity m easure indicates high summer values in West Pass and the near-shore area of the South Sound (Figure 12a ). A few s m al l patche s of high density, are also observed in the outer Sound. Only a few s pots stand out in the winter (Figure 12b), but a pattern is still pres e n t tha t is s i m ilar to the summer. The Adjusted Food measure, which com b ines both density and biom as s, indicated that the m a in areas of potential high food value we re located in the outer Sound in both the summer and winter (Figures 13a & 13b). The density of brachiopods was m a them atically driving this patte rn in the summer, but the patte rn pe rsis ted in the winter, when brachiopod abundance was reduced.

Discussion

Macrofaunal abundance in the S u wann ee Estuary was w ithin the m i d range found for other estuaries around the Gulf of Mexico. Macrofaunal densities ranged from 0-13,701 individuals/m 2 with an average de n s i t y o f 1
, 9 3 5 ( s

. e

. + 137) individuals/m 2 . Present estim ates of benthos density in the Suwann ee Estuary are higher th an those found for Mobile Bay, AL (TechCon, 1980); Neches River Estuary, TX (Harrel & Sm ith, 2002); Nueces Bay, TX (Mannino & Montagna, 1997); and Santa Rosa Sound, FL (EPA, 1975). But Suwannee estuary estim ates are not as hi gh as values reporte d for Mississippi Sound, MS (USACE, 1982); Tam p a Bay, FL (Culter, 1995); and Rookery Bay, FL (Sheridan, 1997). Although m acrofaunal population density differences am ong the five estuarine sectors were only found for the winter sam p li ng, the distribution of m acrofauna was very patchy. Areas of high population density were f ound throughout the Sound and in part of West Pass, regardless of sa m p ling season. In contrast to these results, no significant differences were found for m acrofa unal densit ies am ong salinity region s in Nueces Bay, TX. Seasonal differences in benthos densit y w e re also found in the S u wannee Estuary with abundances over two tim es greater in the summer com p ared to winter.

Benthic biom ass (dry weight) ranged from 0-41 g/m 2 w i t h a n a v e r a g e o f 4

. 8 ( s

. e

. + 0.5) g/m 2 . Biom ass estim a tes were alm o st 50% hi gher in the summ er than in winte r

. Patches of high biom ass were found in areas of higher salinity (e.g., Suwannee Sound), sim ilar to results from Nueces Bay, Texas (Mannino & M ontagna, 19 97). Overall, the range of biom ass values found in the Suwanne e Estuary were substa ntially greater than the range reported from Nueces Bay (<1-9.23 g/m 2 ). In the Suwannee estuary, average benthic biom ass was higher than above the salt wedge (R KMS 52-64, 0.5 g/m 2 + 0.1) (Sulak et al., in press).

6 Am phipods were the dom i nant taxa in the summer with most species being tubicolous. The spatial distribution of a m phipod density displays a clea r spatial correlation between high density and freshwater flow. W e st Pass and parts of the Sout h Sound, which predom inantly receive the freshwater plum e, contained relative hot spots of am phipod density compared to the rest of the st udy area. Am phipod distribution m a y not beresponding solely to the low salinity regim e . Sedim e nt deposition, particulate organic content, and/or high oxygen c onten t as well as f r e shwater in flow stim ulated prim ary and secondary production may also influence am phipod distribution (Mannino & Montagna, 1997; Kimm erer, 2002). Ampelis ca , by far the most common am phipod genus collected, are filter feeders and hav e been noted for thei r vulnerability to sedi m e nt quality. They are often used as indicator species for h eavy m e tals and P AH organic com pounds in toxicology studies (ASTM, 2003; G o m ez Gest eira & Dauvin, 2000). A high density of

am peliscans in the Suwannee River Estuary sug g ests low sedim e nt contam ination levels. Polychaetes were the dom i nant taxa coll ec ted in the winte r . The spatia l distribu tion of polychaetes was opposite that of amphipods. Po lychaete density was highest in the North Sound away from the fr eshwater inf l uence of the river. Tharyx , a tube building deposit feeder, was comm only found in both the winter and summ er. Free burrowing Leitoscoloplos and Parandalia were typical genera co llected in the summ er. Leitoscoloplos is a deposit f eeder, while Parandalia is s u spected to be a carnivore. Melinna , a tube builder and surface deposit feeder was the do m i nant p o lychae te ta xon collected in the winter (Fauchald & Jum a rs, 1979). Brachiopod distribution within the Su wannee Estuary is restricted to outer Sou n d areas and ap pears to be associated with hard substratum provided by oyster shell m a terial. Brachiopods can be found in patches of high density (> 6,000 individuals m -2 ) in the summer but have reduced abundance in winter. It is unknown if juvenile sturgeon utilize brachiopods as an im portant prey item althou gh adults utilize them heavily (Harris, 2003).

Tubiculous am phipods and polychaetes , although consum ed by juvenile sturgeon, do not appear to be principal food items base d upon gut content analysis (Huff, 1975; M a son & Clugston, 1993). Brachiopods were included in the principal foods category for this analysis based upon adult gut contents (Harris, 2003), but their inclusion needs to be reevaluated as m o re diet infor m ation becom e s available. Other than brachiopods, isopods and oligochaetes are also im portant poten tial food resources in the estuary. Isopod density is highest near the lower parts of the river passes, while oligochaetes have a pocket of high density in the North Sound. Depending upon which quantitative m easure is used to evaluate potential resource value for juvenile Gulf sturgeon foraging, diffe rent locations within the estuary stand out. When exa m ining the distribution of only those taxa which were assigned to the Principal Food category, high densities are found through out W e st Pass and in the South Sound by the m outh of W e st Pass. The pattern is consis tent regardless of sa m p ling date. The Biased Food Density m easure highlights not only areas of W e st Pass as cont aining high potential resource value but the outer Sound areas a nd the near-shore area of the South Sound as well. In the winter, the dist ribution of high reso urce areas based upon the Biased F ood estim ate is fairly hom ogeneous with relativel y few hot spots. Valu e s of the Adju sted Food m easure are highest in the outer Sound areas away from the river m o uth.

7 Interestingly, the Adjusted Food m e asure does not highlight West Pass as an area of high resource value for juvenile sturgeon and the Biased Food Density estim ate is only high in W e st Pass during the summer. Historic al sam p ling within W e st Pass however has indicated that juvenile sturge on are consistently found within this area, as docum ented inthe U. S. Geological Survey (USGS) Suwa nnee River Gulf sturgeon database (Figure 14). Therefore, without any addition a l inform a tion on juvenile habitat us e it appears that the Principal Food Density estim a te is the be st predictor of potenti al food resources for juvenile Gulf sturgeon. Adu lt sturgeon have been found to f o llow what appears to be a Levy search pattern when foraging such that an initia l se arch direc t ion is chose n at random and followed linearly until a patch of prey is located, after w h ich m ove ments becom e random within a confined area (Edwar ds et al., 2003). Therefore, prey density and prey patch size are possibly more im porta nt than prey biom ass for juvenile Gulf sturgeon. Estim ates which incorporate an indica tor of biom ass such as the Adjusted Food

m easure m a ybe m o re important for other bent hic fishes (e.g., Spot, Cr oaker, Flounder) with alterna t ive f o ra ging strategies.

The m easures calculated in this st udy provide a static assessm ent of potential food resources (density and biom ass) that m a y need t o be adjusted if benthic feeding fishes such as the Gulf sturgeon continuously feed with in the sam e area. In this regard, we lack data on benthic productivity. A secto r m a y initia lly have relatively high b e nthic resou r ce s before it is foraged, but recovery time is unknown. An area with a lo w recolonization rate

m a y not be as valuable for sturgeon over th e winter feeding season compared with an area with a higher recolonizat ion rate (i.e., populated by fast reproducing am phipods). Thus, there m a y be a trade-off between im m e diate and long-term res ource v a lue in a given area.

The quantitative measures used in this study have identi fied areas which are potentially sensitive to disturbance by dre dging and/or spoil disposal. The Principal Food Density e s tim a te is hig h in W a dley Pa ss only during the summ er when sturgeon residen ce within th is ar ea is poten tially low. Thus, dredging W a dley Pa ss in either the winter when there is a low food density or summer would seem to present a low potential for long-term i m pact on critical ju venile sturgeon trophic habitat.

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