Kreeger ACOE 22 DE Bay Seston Report Final

Concentration of particulate matter averaged ± SE for all years and months at 22 stations in the Delaware Estuary, sampled 2009-2011.. Concentration of particulate organic matter average

Analysis of Particulate Nutrients and Seston Weights

from 2009 to 2011 at Delaware Bay Oyster Stations

Danielle Kreeger, Ph.D.

Academy of Natural Sciences of Drexel University

August 28, 2013

A final report prepared for Rutgers University

as part of the U.S Army Corps of Engineers Section 22

Delaware River and Bay Monitoring Study

Introduction

Population dynamics of oysters, Crassostrea virginica, in the Delaware Estuary are governed by

diverse physical and biological factors including salinity, disease prevalence and virulence, predation, recruitment, and food availability and quality Sea level rise associated with climate change has the potential when combined with physical alterations to the ecosystem (e.g., channel deepening), has the potential to affect oysters and other natural resources in various ways These factors, singly or together, could increase the bay’s volume and salinity (which could promote disease) or alter hydrology and associated food regimes (which could affect nutrition and production) As part of a 3-year study to assess oyster and sturgeon conditions in the Delaware River and Bay, the goal of this portion of the study was to characterize seasonal, inter-annual and spatial variability in food conditions for oysters in representative growing areas

of Delaware Bay

The importance of food supply for larval and adult bivalves is widely recognized; however, little direct evidence exists for food limitation within estuaries, either for adult or larval stages The influence of food quantity and quality on larvae is strongly influenced by the recognized

importance of lipids in larval diets to permit successful growth and metamorphosis (e.g.,

Gallager and Mann, 1986; Pernet et al., 2003; Nevejan et al., 2003; Fern ndez-Reiriz et al., aa2006) On the other hand, other studies have pointed to the importance of dietary protein for regulating growth of post-set juveniles (Kreeger and Langdon 1993) or for adults at crucial points in the reproductive cycle, such as during gametogenesis (Kreeger 1993, Kreeger et al 1995) Regardless of which biochemical constituent limits production at different life stages andseasons, modeling work has provided support for the belief that both food quantity and food quality are very important for larval success (e.g., Bochenek et al., 2001; Powell et al., 2002, 2004; Hofmann et al., 2004) and these modeling studies have drawn upon a range of

experimental literature (e.g., Thompson and Harrison, 1992; Strathmann et al., 1993; Thompson

et al., 1996; Baldwin and Newell, 1995; Hendriks et al., 2003) in aggregate supporting this contention, but direct field evidence is limited (e.g., Bos et al., 2006)

Oysters feed on microparticulate material suspended in the water column (seston) Oyster productivity and reproductive condition can be affected by both food quantity and food quality

To assess food conditions for oysters, water samples were collected from representative areas containing oyster reefs, filtered, and then the filtered material was analyzed for total suspended

solids (TSS) and particulate organic material (POM, represents food quantity) Additional

filtered samples were fractionated for their proximate biochemical composition to determine particulate concentrations of protein, lipid and carbohydrate (represents food quality)

Particulate protein, lipid and carbohydrate concentrations were contrasted among each other and as percentages of TSS and POM as further expressions of the bioavailable fractions available

to support oyster growth and production Data collected from this study were then used by Rutgers staff to update and refine hydrodynamical models of oyster production that includes food supply along with other factors that drive oyster population productivity (Powell et al 2012)

The design of the 2009-2011 sampling program was modeled after a similar earlier assessment that occurred between May 2000 and March 2001, led by Versar In the 2000 Versar study, seston quantity and quality, which were referred to as “nutrients,” were analyzed using methodsdeveloped by Dr D Kreeger, Academy of Natural Sciences The 2009-2012 sampling program repeated the earlier assessment using the same seston analysis methods as in 2000, but with more stations (up to 18) and months (up to 9) being sampled to provide better resolution of temporal and spatial variation

Methods

Field Sampling Program To examine seston quantity and quality, water samples were collected

by Rutgers staff at eighteen sites in the Delaware Bay and River once every month in 2009, 2010and 2011, with the exception of February and December Sites were accessed via the F/V Dredge Monster At each site three replicate 1-gallon jugs of water were retrieved from 30.5 cmbelow the surface with an Eheim Universal Model 1048 submersible pump and flexible rubber tubing Jugs of water were kept at ambient temperature in coolers while being transported back to the laboratory Detailed station locations and notes on the 2009-2011 field sampling can

be obtained from Rutgers

Seston Collection Seston is defined as microparticulate material too small to be seen by the

human eye These are suspended particles that are large enough to be retained on a glass fiber filter having an approximate retention of 0.7 µm (particle diameter) and small enough to pass through a 100-µm sieve Depending on TSS concentrations, typically between 100 and 1000 ml

of water are needed to obtain enough seston per filter for accurate analysis

Methods for collecting seston from water samples are described in detail in DK-SOP-23 (Rev 2, 8/06) This is a standard operating procedure prepared by D Kreeger and it is available upon request In summary, seston was collected on prepared glass fiber filters using vacuum filtration

of water collected in 4-L jugs from field sampling stations Rutgers staff performed the

filtrations within 24 hours of collection, and filters containing seston were added to Petrislides™ for storage in a freezer at -20oC until laboratory analysis at Drexel University

For each sample (collection station sampled at a given time,) three replicate jugs were filled andthen filtered From each jug, four replicate seston samples were collected on 0.7-µm retention glass fiber filters (47 mm diameter; Whatman type GF/F or equivalent.) The replicate filtration

Figure 1 Diagram showing how replicate water

subsamples from a single water sample are filtered onto four separate glass fiber filters

of each water sample (jug) onto four filters allowed for the separate analysis of seston weight, protein content, carbohydrate content and lipid content (Fig 1)

Filters were prepared in advance

Filters were pre-combusted at 450oC

for at least 24 hr prior to seston

filtration A sufficient number of

pre-weighed (to 0.01 mg) filters were also

prepared in advance of sampling For

each bottle/jug of water, one of the

four filter replicates was used for

weight-on-ignition and the remaining

three filter replicates were used for

biochemistry The replicate to be used

for weight-on-ignition was

pre-weighed The same balance was used

before and after filtering seston

Weights were measured only on

desiccated samples

Seston Analysis One of the four

replicate seston-coated filters per

water jug was used for

weight-on-ignition analysis, one for protein

content, one for lipid content and one

for carbohydrate content The weight-on-ignition assay assessed the total seston weight per unit volume (i.e., TSS concentration) and the seston organic content Calculations for seston particulate material (PM, aka TSS), particulate organic matter (POM), and the percentage organic content (%OC) are given below Seston filters for weight analysis were dried at 60oC for

>2 days and weighed (±0.01mg; Sartorius M-9001) Filters were then combusted in a muffle furnace for 48 hr at 450oC and weighed again on the same balance

Concentrations of PM (a.k.a total suspended solids, TSS) and POM were calculated with the following formulae:

PM (mg/L) = [(dry filter+seston weight) – (dry filter weight)] / (filtered volume)

POM = PM – [(ash filter+seston weight) – (dry filter weight)] / (filtered volume)

Organic Content = [POM] / [PM] * 100%

The concentration of particulate protein, carbohydrate and lipid was measured on separate replicate filters from each water sample This proximate biochemical composition was

determined using published methods that have been adapted by Kreeger et al (1997) Protein was measured spectrophotometrically using the bicinchonic acid modification (Pierce test kit, 23225) of the procedure of Lowry et al (1951), standardized with bovine serum albumen

(Pierce 23210) A microplate reader was used for spectrophotometry at a wavelength of 640

nm

Carbohydrates were quantified spectrophotometrically (wavelength 480 nm) using the method

of Dubois et al (1956), standardized with potato starch (Sigma S 4561)

Lipids were measured gravimetrically according to a modification of the technique of Folch et al.(1957), whereby dried seston filters were suspended in 10 ml of 2:1 v/v chloroform/methanol, ground for 1 min in a Potter Elvehjem tissue grinder tube with PTFE pestle (Wheaton #358039), and then centrifuged at 1000 x g for 5 min The supernatant (containing lipid) was collected andreceived a 20% v/v (final concentration) aliquot of 0.88% KCl to promote phase separation The bottom layer was transferred by pipette to a pre weighed vial, dried at 37oC until constant weight was achieved, and weighed Hexadecanone was used to standardize the lipid procedure

The concentration of particulate protein, lipid and carbohydrate in each water sample was expressed relative to the filtered volume to calculate concentrations Each concentration was then divided by the particulate material concentration to calculate the percentage protein, lipid and carbohydrate contents, respectively

Statistical analyses were performed with Statgraphics Centurion XVI.I Results from analysis of variance tests are reported as statistical means generated from the pooled variance in each model, and hence these means may differ slightly from arithmetic means

Results and Discussion

Data collected from a total of 1,465 seston samples are summarized in the Appendix Each row

in the Appendix corresponds to one water sample (i.e jug of water) taken from 18 stations (Fig 2) from which were successfully assessed all four main seston attributes (weight, protein, lipid, carbohydrate) As noted above, up to three replicate jugs of water were collected at each station and month, hence, in most cases there are three rows of data per station and month sampling In a few cases, only two jugs of water were collected and filtered successfully, or the full complement of four filters per jug was not obtained (one filter each for weight, protein, lipid, carbohydrate) In a few cases, there was a problem with the laboratory analysis of one of the four metrics Data was only accepted if all four seston metrics were collected successfully per water sample In 2011, some stations were substituted Despite these occasional

limitations with collecting, filtering or analyzing samples, >97% of the intended samplings were completed

Figure 2 Fifteen of the 18 sampling stations shown with stars Three

additional stations were located further upbay in the tidal Delaware River

zone

Seston quantity and quality varied widely in space and time across the many stations sampled inthis study, seasonally, and to a lesser extent among the three survey years 2009-2011 In several cases, the spatial variation followed the salinity gradient along the axis of the Delaware Estuary, and some differences were apparent between inshore and offshore sampling stations

In other cases, specific sites appeared to be consistent outliers, having special characteristics Temporal variation was reasonably consistent, reflecting the expected seasonal variability associated with spring blooms of phystoplankton and fall-winter outwelling of detritus from marshes and rivers These patterns were punctuated by anomalous weather events associated with storms (e.g Hurricane Irene) and record rainfall and runoff during 2011

Seston Quantity - Spatial Variation

Averaged among all months and the three years, the concentration of total suspended solids (TSS =particulate matter) varied widely among stations (Fig 3) This spatial variation was examined in several ways First, stations were grouped according to established oyster

monitoring zones which tend to span the salinity gradient from lower bay to upper river: 1=lower bay, 2= high mortality beds, 3=medium-high mortality beds, 4= low mortality beds, 5= very low mortality beds, 6=Delaware sites, and 7=upper river sites More seston was present in the upper estuary than in the lower Delaware Bay sites (Fig 4), presumably due to inputs of sediment and particulates from rivers as well as the expected higher concentrations that get trapped in the Estuary Turbidity Maximum zone

Figure 3 Concentration of particulate matter averaged (± SE) for all years and

months at 22 stations in the Delaware Estuary, sampled 2009-2011

Figure 4 Concentration of particulate matter averaged (natural logarithm

transformed; ± SE) for all years and months at 7 oyster monitoring zones that extend from the lower bay (1) to upper tidal river (7) in the Delaware Estuary, sampled 2009-2011

Figure 5 Sum of monthly mean concentrations of particulate

matter during the 2009 sampling year at 18 sampling stations Forcomparison, average fall oyster condition index is shown at those stations where assessed during 2009

In addition to the inverse relationship between particulate matter concentration and salinity, exemplified along the axis of the estuary (Fig 4), we found significantly greater concentrations

of particulates at stations nearer to shorelines than in the middle of the channel or bay (Fig 5), presumably due to landward sources of particulates in river runoff or marshes Of particular interest was the Nantuxent Station, which had the greatest overall concentration of total particulates (mean = 53.6 mg/L, n=86 samples) among all stations (overall mean = 30.0 mg/L, n=1465), averaged among all years and months

the water column,

which is a proxy for the

marshes and typified by

fewer algal blooms than

other major American

estuaries because of

light limitation by

turbidity Therefore,

much of the particulate

material can be in the

form of suspended

inorganic sediments and

the organic fraction can

have lower food value

due to higher amounts

of refractory detritus

(see below) These data

suggest that overall

food quantity (PM concentrations) for oysters is inversely related to oyster condition in the Delaware Estuary due to these unusual natural conditions

Figure 6 Concentration of particulate matter averaged (± SE) per month for all years

and stations in the Delaware Estuary, sampled 2009-2011

Seston Quantity - Temporal Variation

The concentration of particulate material varied significantly among the three years and also among months (2-way ANOVA, p<0.05), when averaged among all stations sampled Highest annual concentrations of particulates occurred during 2011 (mean = 35.1 mg/L, n=469), which was significantly greater than in 2009 (mean = 28.3 mg/L, n=490), which was in turn significantlygreater than in 2010 (mean = 24.8 mg/L, n=506) Highest concentrations tended to occur in March (mean = 49.6 mg/L, n=151) and November (mean = 38.4 mg/L, n=94); whereas, lowest concentrations occurred during June (mean = 22.2 mg/L, n=167) and July (mean = 25.6 mg/L, n=157) (Fig 6) Low concentrations also were found in December (mean = 14.6 mg/L, n=46), but that month was only sampled in one of the years

This seasonal pattern in particulate material is considered typical for coastal systems in

temperate climates, which receive periodic runoff during wetter times of year, combined with spring blooms of phytoplankton

Seston POM Quality – Spatial Variation

Figure 7 Concentration of particulate organic matter averaged (± SE) for all years

and months at 22 stations in the Delaware Estuary, sampled 2009-2011

Seston quality is governed by the concentration and relative proportions of essential nutritional constituents for consumers which are also bioavailable (able to be captured and digested) Much of the overall TSS is expected to be inorganic material (e.g suspended sediments), but even the particulate organic fraction might contain compounds that are not bioavailable for consumers such as oysters Nevertheless, the concentration of particulate organic material (POM), and the seston organic content (POM/PM x 100%) are often used to assess seston quality, and so they are reported here

When averaged across all stations, months and years, the concentration of particulate organic material averaged 5.95 mg/L (n=1465), and the organic content averaged 21.8 % (n=1465) Therefore, about 78% of suspended matter is inorganic material, not usable to meet the nutrition of suspension-feeding animals such as oysters

Averaged among all months and the three years, the concentration of POM varied widely among stations (Fig 7), much like PM In general, the concentration of POM tracked the

concentration of PM, being consistently much greater at inshore locations (e.g Nantuxent) but

it was not significantly correlated (linear regression, p>0.05) with salinity

Seston POM Quality – Temporal Variation

Figure 8 Concentration of particulate organic matter averaged (± SE) per month for

all years and stations in the Delaware Estuary, sampled 2009-2011

Similar to PM, the concentration of POM showed a repeatable seasonal pattern among the three study years, with greatest concentrations occurring in spring (Fig 8) The concentration of POM was significantly greater (ANOVA, p<0.0001) in 2011 (mean = 6.97 mg/L, n=469) than in

2009 (mean = 5.65 mg/L, n=490) or 2010 (mean = 5.23 mg/L, n=506), which were statistically similar Inter-annual variation in the percentage organic content was less variable, and it was actually greater (ANOVA, p=0.002) in 2010 (22.8%) than in 2009 (21.6%) or 2011 (21.1%)

Seston Biochemical Quality – Spatial Variation

An interesting finding from this study was that a majority of POM is not actually comprised of bioavailable protein, lipid or carbohydrate Averaged across all stations, years and months, the percentage of POM that was either protein, lipid or carbohydrate was only 41.6% Typically, the dry weight of live food items of oysters such as phytoplankton are comprised of about 94% protein, lipid and carbohydrate with other compounds such as nucleic acis making up the difference This finding that suspended particulates of the Delaware are nearly 80% inorganics, and more than half of the remaining organics are also not bioavailable indicates that the

nutritional value of Delaware Estuary seston is very low, on average (see pink pie slices in Figure 9)

Figure 9 Relative differences in the summer mean concentration

of particulate material in the form of bioavailable organics (POM

as P+C+L), non-bioavailable organics (POM Other) or inorganic matter (Ash) during 2009 at most sampling stations

As shown in Figure 9,

both seston quantity

(size of pies, =PM) and

quality (colored slices

of pies) vary greatly

stations have high POM

quantity and quality

(e.g Nantuxent), some

have low POM quantity

and quality (e.g

Crossledge), some have

higher POM quantity

but low quality (e.g.,

Hope Creek) and some

have lower POM

quantity but higher

quality (Ship John

Channel) (Fig 10)

Seston Biochemical Quality – Temporal Variation

Concentrations of proteins, lipids and carbohydrates, as well as overall POM concentrations, tended to be greater in spring (March to May) (Fig 11) However, the bioavailable fraction (percentage of POM in the form of protein, lipid and carbohydrate) was also significantly greater(ANOVA, p<0.0001) in the spring to early summer than at other times of the year (Fig 11): April (50.7%), June (50.4%), March (48.5%) and May (48%) were significantly greater than all other months (means ranged 28.7-42.8%) The lowest proportion of bioavailable POM was during September (28.7%)

Figure 10 Concentration of particulate organic matter fractionated as either protein,

lipid, carbohydrate or uncharacterized other material at 18 regular stations in the Delaware Estuary, sampled 2009-2011

Figure 11 Monthly mean concentration of particulate organic matter fractionated as

either protein, lipid, carbohydrate or uncharacterized other material, averaged among

18 regular stations in the Delaware Estuary, sampled 2009-2011

Seston Protein per Month (All Stations)

Figure 12 Monthly mean concentration of particulate protein averaged among all

sampling stations in the Delaware Estuary, sampled 2009-2011

The bioavailable proportion of POM (and also PM) is likely governed by seasonal primary

production dyanamics whereby spring blooms of phytoplankton lead to a higher index of nutritional quality at that time, depletion occurs during summer, and by fall refractory organics begin to outwell from marshes, derived from senescing plant matter

Seston Biochemical Quality – Protein and Carbohydrate Variation

This seasonal depletion of nutritionally important constituents was most apparent for seston protein, which declined in concentration after a peak in March (3.23 mg/L, n=151) to the lowestprotein month, November (0.76 mg/L, n=94) (Fig 12) Protein (and particulate nitrogen) is essential for bivalve filter-feeders, necessary to sustain all biosynthesis, including growth and reproduction

Seston protein concentration also followed an interesting spatial pattern, being greatest both in the upper estuary and the lower Delaware Bay, but being depressed in the middle, when

averaged across all months and years (Fig 13) This pattern was visible when stations were simply grouped by their latitude since the axis of the Delaware Estuary is generally north-south (Fig 13) The sampling station that had the greatest protein concentrations was in the DelawareBayshore area of New Jersey, Nantuxent (2.90 mg/L, n=86) Multiple stations had average

Seston Protein % Content versus Latitude

Figure 13 Monthly mean concentration of particulate protein versus latitude of

sampling station, averaged among all months and years in the Delaware Estuary,

is helpful to look at both the absolute concentrations (mg/L) and relative contents (% w/w) of various dietary constituents that are essential

Carbohydrate concentrations in the seston declined during the year, similar to proteins (Fig 14).Seston carbohydrates also were least abundant in a middle estuary area, being greatest upbay and at the lower most stations (Fig 15), similar also to seston protein concentrations Over all sites and times, carbohydrate concentrations averaged 0.37 mg/L, and lipid concentrations averaged 0.73 mg/L) Lipid concentrations were much more variable and did not exhibit as marked seasonal or spatial patterns as proteins or carbohydrates

Seston CHO by Month (2009-2011)

Figure 14 Monthly mean concentration of particulate carbohydrate averaged

among all sampling stations in the Delaware Estuary, sampled 2009-2011

Figure 15 Monthly mean concentration of particulate carbohydrate at stations

along the upbay-downbay gradient, averaged among all months and years in the Delaware Estuary, sampled 2009-2011

Figure 16 Summer mean seston protein content compared to the fall condition index

of oysters at selected stations where data for both was collected in the Delaware

Estuary, sampled 2009-2011

Seston Biochemistry versus Oyster Condition

Oyster condition index was assessed in the fall by Rutgers at many of the same seston sampling locations during the 3 seston study years Fall oyster condition index is a useful metric to gauge the health of local populations after the peak of the growing season and prior to winter when energy stores are needed for overwintering Typically, higher fall condition reflects better growing conditions and is an index of oyster population fitness for that time of the year

A series of exploratory statistical tests were completed to contrast various seston quantity and quality metrics with fall oyster condition across the 2009-2011 seston study period and for those stations where both metrics were collected Importantly, fall oyster condition was found

to be inversely correlated (linear regression, LSD) with seston particulate matter (i.e., particle quantity) In contrast, several food quality metrics were found to be statistically correlated with fall oyster condition in a positive manner One of the most significant positive relationships was between the percentage content of protein in the seston during summer (protein:PM w/w x100%) and fall oyster condition index (Fig 16) In contrast, the inorganic ash percentage ([PM-POM]/PM x100%) was negatively correlated with oyster condition These results suggest that particle quality (relative percentage of bioavailable compounds per unit seston) is more related

to oyster fitness than particle quantity (concentrations of seston) Indeed, there might even be

Recruitment vs Summer Seston Protein

Percent Protein (transformed)

Figure 17 Summer mean seston protein content

compared to summer oyster recruitment at selected stations where data for both was collected in the Delaware Estuary, sampled 2009-2011

negative consequences associated with high particle concentrations if those particles are of poor nutritional value

Seston Biochemistry versus Oyster Recruitment

Similar to oyster condition, data from Rutgers-led oyster recruitment monitoring were

contrasted with various seston quality and quantity metrics for sites and times where both wereassessed Oyster recruitment is assessed every summer at a subset of the seston study stations

to gauge reproduction success and estimate future production at different reef locations Oyster recruitment was more

positively correlated with seston

protein content (Fig 17) than any

other seston metric, and was

negatively correlated with

absolute seston concentrations,

similar to fall oyster condition

Interesting, the relative balance of

biochemically important

compounds was also found to

relate to oyster recruitment For

example, the ratio of proteins to

carbohydrates was also strongly

correlated with summer oyster

recruitment (Fig 18), perhaps

because newly set oysters need to

grow rapidly to escape predation

pressure and rapid growth is

constrained by protein-intensive

anabolic processes associated

with biosynthesis The

importance of low C:N ratios and

high percent protein contents has

been reported for juvenile clams

(Gallagher & Mann 1982) and

mussels (Kreeger and Langdon

1993) These results suggest that oyster success in the Delaware Estuary might be associated with nutritional balance as well as the bulk availability of important bioavailable and essential nutritional compounds

Figure 18 Summer mean ratio of protein to carbohydrates in the seston compared to

summer oyster recruitment at selected stations where data for both was collected in the Delaware Estuary, sampled 2009-2011

Conclusions

Taken together, results of the 2000 Versar study and the 2009-2011 Delaware Estuary Study indicate that seston quantity and quality varies widely in space and time across prime growing areas of Delaware Bay The quantity of seston is represented by the total suspended solids, TSS (= particulate material, PM) Typically, TSS is higher upbay and in nearshore areas, whereas it is lower in main channel areas Comparisons of TSS with oyster condition and recruitment

suggest that oyster production is negatively associated with TSS concentration at most times in the Delaware Estuary, a finding that has also been seen in other areas of the United States (e.g., Texas; ANSP 2012) This is because TSS typically includes a high percentage content of inorganic matter (averaged 78% in this study) that is of no direct nutritional value to oysters, and excess turbidity can impair or reduce efficiency of oyster feeding and particle sorting

Food quality, on the other hand, was found to strongly correlate with oyster condition, as evidenced by positive relationships between several food quality metrics and oyster condition

index in the fall Recruitment success was also positively correlated with food quality and tests comparing food quantity (TSS concentration) with summer recruitment were negatively

associated There are many ways to examine food quality, such as by assessing the total

particulate organic matter concentration, the organic matter percent content, the sum

concentration of bioavailable constituents (examined here as protein, lipid and carbohydrate), concentrations of individual essential biochemicals (e.g., protein, lipid), the relative balance of different essential biochemicals (e.g percentage content of protein and lipid contrasted with either PM or TSS), or ratios of different biochemicals (e.g., protein:carbohydrate ratio, an inverseproxy for C:N stoichiometric ratio) Hence, further data analysis is possible from the results of this study However, preliminary analyses summarized here indicate that the summer protein and carbohydrate content of seston is of greatest predictive value for fall oyster condition (fall conditioning is critical for oyster overwintering and subsequent reproductive conditioning in spring) The sum of seston proteins, lipid and carbohydrate is also strongly correlated to oyster condition in spatial analyses, helping to explain why certain areas of Delaware Bay typically sustain the highest and lowest oyster production

In all years, seston quality declined from early summer on, characterized by low organic content(% of TSS) and low protein content (% of TSS) from August to November (~2%) compared to May to June (>4%) Carbohydrate content was also lowest in late summer Late summer is an important time for oysters because adults begin to sequester nutrients and energy for

overwintering, and young newly recruited spat (July) must grow as quickly as possible This late summer decline in food quality was consistent among sampling years in Delaware Bay

Interestingly, a large proportion of Delaware Bay seston was of no or poor nutritional value for suspension-feeding animals such as oysters Protein, carbohydrate and lipid concentrations were 1.4, 0.37, and 0.73 mg/L, respectively, when averaged across all sites, months and years, totaling 2.53 mg/L On the other hand, particulate material (TSS) and particulate organic

material (POM) concentrations averaged 31.0 and 6.2 mg/L across all samples Therefore, the combined biochemical nutrients represented only 9.1% w/w of TSS and 41.6% of POM Most ofthe particulate organic matter was therefore likely to be non-nutritious, uncharacterizable flocs and organic aggregates of low to no nutritional value In addition, 78.4% of TSS was inorganic material

While not unique to coastal ecosystems, these findings of unusually low organic and nutritional content in the Delaware Estuary affirm the widespread characterization of this system

(especially nearest the mixing zone) as especially sediment-laden, high turbidity and limited light for phytoplankton (Kreeger et al 2006, PDE 2012) Here, even subtle spatial and temporal variation in seston quality is likely to contribute substantially to variation in fitness of

suspension-feeders such as oysters due to the marginal nature of the typical diet

Powell et al (2012) reported that these seston quantity and quality data were very useful in predicting oyster productivity in Delaware Estuary using hydrodynamic models Cluster analysis

on residuals identified two large groups of sites, one comprising most sites on the eastern side

of the bay including all of the sites on the New Jersey oyster beds south of the uppermost beds and one including most of the sites along the central channel and waters on the western side (Delaware) Food values over the New Jersey oyster beds were often depressed by as much as 50% relative to the bay-wide mean As shown in this study, food quality tended to also be

Figure 19 Conceptual diagram depicting some of

the important physical, chemical and biological factors that govern oyster productivity in the Delaware Estuary Oyster survivorship associated with lower disease pressure is greater upbay Food quality, recruitment, and oyster condition is greater downbay The balance of these factors is thought to lead to greatest oyster population production in the middle

depressed in the middle estuary over the main New Jersey oyster production areas The oyster reefs of Delaware Bay are dominantly sited on the New Jersey side, where food supply was mostdepressed and where passive particle residence times were longest While not conclusive, Powell et al (2012) concluded that these results suggest that oysters can influence seston composition on the New Jersey side of the estuary where they are currently most abundant, and this would explain the cross-bay gradient in food values as an outcome of oyster feeding and top-down grazing pressure on the nutritional components (i.e., associated with

phytoplankton)

The ramifications of this study and the

analysis by Powell et al (2012) are that

oyster food supply is likely to be an

important limitation on overall

production and population dynamics in

the naturally turbid Delaware Estuary

Hence, future studies of oyster

population dynamics in this system

should monitor seston composition

and quantity along with other key

drivers of production (mortality,

disease, predation, fishing pressure,

condition, recruitment) In a simplistic

analysis (Fig 19), oyster productivity is

greatest in the middle estuary zone

along the New Jersey side of the

Delaware Estuary because of the

balance of different production

constraints, including better

survivorship of disease upbay (due to

lower salinity curtailing disease

agents), and better food conditions

downbay (due to separation from

estuary turbidity maximum)

In summary:

• Seston quantity and quality vary widely in time and space within Delaware Bay

• Seston quality is more strongly correlated with oyster condition and recruitment than seston quantity

• Summer concentrations of key biochemical constituents (especially protein) was most correlated with oyster condition and recruitment

• Oysters could be food-limited in turbid Delaware Bay, with grazing pressure possibly explaining local depletion of food over some productive reefs during summer