SeaGrant

Detailed Descriptions of the Hypotheses Being Tested

Local shoreline productivity (growth rates of both plants and animals) is closely linked in space and time with food supply rather than with other factors such as temperature.

Background: Estuarine waters are very heterogeneous in terms of temperature, nutrients, plankton production, and other factors that may affect growth rates of both producers and consumers in the nearshore. The “textbook” estuary is highly productive because food webs are supported by nutrients entering from watersheds. While nutrients delivered to estuaries in freshwater (e.g., into south Sound) can stimulate phytoplankton population growth, freshwater can introduce osmotic stress and, at high flow, flush phytoplankton out of an estuary (Muylaert et al. 2001). Rivers can also introduce sediment that interferes with light required for phytoplankton growth (Pennock and Sharp 1986). Maurer et al. (1992) suggest that P/B (production per biomass) of secondary consumers is higher inside Delaware Bay than at coastal stations, but this may primarily reflect the dominance of deposit feeders inside the bay. Thus, although resource patterns vary consistently along exposure gradients within estuaries, the predominance of terrestrial inputs in freshwater at the head and of marine inputs at the mouth may not have consistent consequences for the production of benthic suspension feeders that consume pelagic resources. In addition, measuring food supply for benthic species is problematic; phytoplankton biomass generally declines from inner to outer portions of estuaries, but estimates of annual production do not match this pattern (Heip et al. 1995).

Shellfish harvest (wild and aquaculture) is a key industry on Washington state’s shorelines, with millions of dollars in revenues annually. Factors that improve oyster growth may also increase productivity of hardshell clams, geoducks, and other important suspension feeders that are abundant in Puget Sound. Our preliminary data on growth rates of juvenile oysters (Crassostrea gigas, Fig. 4) suggest that south and central Sound is a weak “textbook” estuary. Secondary production is higher at the south (head) end of Puget Sound. Oysters grow significantly faster in Budd Inlet than at other sites, and stable isotope ratios of oyster tissue are enriched in δ15N there (Fig. 5). Stable isotope ratios indicate the structure of food webs and the energetic subsidies entering an area. Many elements on earth include several naturally occurring forms that differ in molecular weight. Carbon-12, for instance, is very common on the earth’s surface, whereas Carbon-13 is rare. For stable isotope analysis, the ratio of 13C to 12C is measured as δ13C, the relative amount of 13C in a sample relative to a standard. Other elements commonly considered in stable isotope analyses are 15N/14N (nitrogen) and 35S/34S (sulfur). Two key points allow stable isotopes to be used to explore feeding relationships. First, stable isotope ratios vary among primary producers, because inorganic carbon and organic nitrogen sources occur in different ratios in different habitats. Atmospheric CO2, for instance, contains relatively little 13C compared to carbon dissolved in seawater (HCO3-). Second, you are what you eat. To a large extent, the stable isotope ratios of consumers reflect the stable isotope ratios of what they have been eating. If a consumer eats two things with different stable isotope ratios, the consumer will have an isotopic signature in between the two resources. Some elements, particularly nitrogen, change directionally from one trophic level to the next. So, 15N becomes slightly more common through the food chain, providing an indication of trophic level, not just energy source (Dawson and Brooks 2001). These isotopic differences seen in our preliminary oyster studies (Fig. 5) suggest that there is spatial variation in food sources among our study sites (Peterson and Fry 1987, Stribling and Cornwell 1997).

Bioenergetic models of Crassostrea gigas emphasize total suspended solids, chlorophyll, and temperature as factors most responsible for variation in growth (Ren and Ross 2001). Growth generally improves with chlorophyll, whereas suspended solids can actually interfere with suspension feeding. A peak in growth generally occurs at temperatures around 19oC. The temperature differences we have observed among our study sites, roughly 1-2oC higher in the south than north, may be sufficient to account for observed differences in growth, but we suspect resource variation is more likely.

Methods: In years 1 and 2, we will put out TidbiT temperature dataloggers and transplant juvenile oysters across the north-south estuarine gradient at the 9 sites discussed above. Temperature data from the TidbiTs will serve as local proxy allowing us to tie nearshore seasonal and interannual trends into those at the 40 offshore stations where DOE (2003) gathers monthly chlorophyll data. At each site, we will place one Tidbit in the shallow subtidal zone to record nearshore temperatures, one on the surface of the beach to record beach-surface (including groundwater runoff) temperatures, and one buried in the sediment. Monthly growth rates of oysters will then be correlated with beach and water temperatures and with offshore productivity. Oysters (Crassostrea gigas) will be settled on individually-labeled ceramic tiles (11 x 11 cm) in a commercial hatchery. After two weeks, when individuals are about 2 mm in shell length, they will be thinned to 5-20 per tile and outplanted to study sites. Tiles will be attached through pre-drilled holes to PVC stakes, and pushed into the substrate until the tile is just above the surface at mean lower low water. Five tiles will be outplanted to each beach. Photographs will be taken of each tile prior to outplanting and at 1-2 month intervals in the field to determine growth and survival. Transplants will be carried out in June-September (summer growth) and September-June (winter growth) over two years.

Stable isotope analyses of outplanted juvenile oyster tissue can provide an integrated picture of estuarine food webs, translated through the feeding habits of an economically and ecologically important species. We will focus on δ15N and δ13C because these inform us about sewage inputs and terrestrial-marine inputs, respectively (McKinney et al. 2001). δ35S will be measured in selected samples, because this element best distinguishes food derived from marine macrophytes such as eelgrass. Isotopes will be analyzed from homogenized individuals from each of the 5 tiles per site, as well as from water samples at the southern and northern ends of the estuarine gradient. We will also test potential benthic sources of organic C, especially ulvoid algae and benthic diatoms.
Sites with higher shoreline productivity have higher species diversity and higher biomass of organisms.

Background: Theory suggests a nonlinear relationship between diversity and productivity (Huston 1994), such that an increased resource base first promotes coexistence, but, at very high resource levels, monodominant stands develop and diversity declines. Proulx and Mazumder (1998), summarizing studies of consumer impacts across a range of resource levels, found that results differed consistently between natural gradients in productivity (where consumers tended to reduce diversity) and anthropogenic gradients (where consumers tended to increase diversity). Interestingly, our preliminary data with juvenile oysters suggest a negative correlation between productivity (somewhat higher in the south, Fig. 4) and overall benthic diversity (clearly higher in the north, Fig. 1).

Methods: At each of the 9 sites we will measure both primary productivity, using growth of algae, and secondary productivity, using growth of suspension feeders. We will transplant rocks with small Fucus gardneri (brown algae) and tiles with juvenile oysters and barnacles to each site and quantify growth through the spring and summer of years 1 and 2. We will gather rocks with young Fucus (less than 5 cm length) from other sites, thin the plants to leave approx. 10 per rock, and place at least 3 marked rocks per site. Fucus growth can easily be measured non-destructively in the field by quantifying the maximum length and total number of tips, which correlate closely with biomass (Dethier et al. in review). Barnacles (Balanus glandula) will be allowed to recruit onto small ceramic tiles hung from the breakwater at the Friday Harbor Labs; juveniles will be thinned and the tiles attached to stakes (5 per site) hammered into the substrate. Barnacle tiles will be photographed monthly in the field, and survival and growth measured digitally. Simultaneous oyster growth rates will be obtained from the tiles described in H1.

Overall community diversity will be measured in surface quadrats and sediment cores using our standard long-term monitoring techniques (see Approach). Species-specific biomasses will be measured by randomly collecting samples of epibiota and large infauna (clams) and returning them to the lab to get wet and dry masses; small infauna will have biomasses measured following laboratory identification. To link biomass data to our broader database on abundance, biomasses of sessile species (whose abundances are quantified by percent cover) will be assessed on a per-area, not per-individual basis. If growth of our target species (Fucus, oysters, barnacles) is positively correlated with their biomass across sites, then productivity is likely to influence community structure; otherwise, recruitment or post-recruitment survival rates more strongly influence community structure. Post-recruitment survival (an indicator of top-down processes, such as herbivory and predation) can be assessed in part by measuring survival of our transplanted target species over each monthly interval.
Sites with higher rates of recruitment, i.e. colonization of the shoreline by larvae and spores of marine animals and plants, have higher diversity.

Background: In areas where recruitment of organisms is high because supply and survival of propagules (larvae and spores or zygotes) is high and regular, we might expect overall diversity on the beach to be high. This prediction would only hold, however, if recruitment (supply-side) rather than biological interactions (top-down) limits a variety of species in this system; while this dichotomy has been debated extensively for rocky shore communities, no such literature exists for estuaries. A meta-analysis of marine studies suggests that consumers reduce diversity when overall recruitment rates are low and increase diversity when overall recruitment rates are high (Moore et al. in revision). Patterns may depend in part on whether or not species have planktonic propagules, and in estuaries, on whether species are primarily of marine or brackish origin. We can test these relationships in Puget Sound. In addition, by clarifying the timing of recruitment with frequent sampling, we can characterize the physical (or biological, e.g. food supply) conditions under which recruitment occurs.

Methods: Beginning in late winter of years 2 and 3, we will embed small flower pots of sterile sediment at each sampling site. Pots will be approx. 1.5L and will be filled with a natural size mix of sand, gravel, and cobbles; these will be embedded flush with the natural sediment. Pots will be collected and replaced to quantify colonization by infauna on a monthly basis, corresponding to physical measurements in H1. Samples will be sieved with 2 and 0.5 mm sieves to capture young recruits; our normal monitoring procedures only use 2 mm sieves.

At the same sites and sampling periods we will also measure monthly rates of colonization by epibiota (algae, barnacles, etc) on settling tiles; ceramic tiles (11 x 11 cm, 5 per site) will be attached to PVC poles embedded in the beach. Tiles will be collected and replaced at each sampling interval, and species will be identified and counted in the laboratory.

Rates and timing of recruitment of both infauna and epibiota will then be related to monthly local physical conditions (as quantified above), and to local diversity as quantified in H2.
Sites with more fine sediments (i.e. more sand and mud, fewer pebbles and cobbles) increase recruitment of organisms that live in sediment but reduce recruitment of surface species. Surface species are negatively affected by the scouring or smothering action of loose sediment.

Background: The high diversity of these mixed-sediment beaches is due in large part to there being essentially 3 habitats: relatively stable surface cobbles, on which algae and sessile invertebrates settle; spaces under cobbles and pebbles where mobile invertebrates
and small fishes hide; and sediments below, containing enough fines to support a rich infauna of polychaetes, clams, and other invertebrates. The amount of fine sediment subsurface probably contributes positively to infaunal recruitment and diversity, but our monitoring data suggest that too much sand on the surface decreases epibiota, due to scour or smothering. Sediment supply and transport are key and controversial topics in the nearshore of Puget Sound, because watershed changes and shoreline hardening (with seawalls) have clearly altered these processes. Our research will investigate how changes in nearshore sediment processes can affect communities on this common beach type.

Methods: To quantify the effect of higher proportions of sand on infaunal recruitment, pots of sterile sediment (as in H3) will be prepared with 3 differing proportions of fines (sand) characteristic of each of the 3 sampling regions, and set out at the three northern sites in years 2 and 3. Pots will be collected and replaced at each sampling interval. In addition to quantifying recruitment, we will do final grain size analyses to determine how much the pots changed between sample periods. To quantify surface scour, painted rocks will be established at all sites each spring; at each sampling interval we will measure the amount of paint removed from each (N=5 rocks per beach). We will correlate the amount and seasonality of scour with abundance and seasonality of epibiota, both on these beaches and on our settling tiles (H3).