NOAA-NMFS-NWFSC TM-5: Seasonal Changes in the Intertidal and Subtidal Macrobenthic Invertebrate Community Structure in Baker Bay, Lower Columbia River Estuary

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NOAA Technical Memorandum NMFS-NWFSC-5

Seasonal Changes
in the Intertidal and Subtidal
Macrobenthic Invertebrate
Community Structure
in Baker Bay,
Lower Columbia River Estuary

Toshio Furota¹ and Robert L. Emmett

National Marine Fisheries Service
Northwest Fisheries Science Center
Coastal Zone and Estuarine Studies Division
2725 Montlake Blvd. E.
Seattle WA 98112-2097
(206) 860-3270

¹Present address:
Toho University
Faculty of Science
Miyania 2-2-1
Funabashi, Chiba, 274 Japan

January 1993

U.S. DEPARTMENT OF COMMERCE
Barbara Hackman Franklin, Secretary

National Oceanic and Atmospheric Administration
John A. Knauss, Administrator

National Marine Fisheries Service
William W. Fox, Jr., Assistant Administrator for Fisheries

NOAA-NWFSC Tech Memo-5: Seasonal Changes in the Intertidal and Subtidal Macrobenthic Invertebrate Community Structure in Baker Bay, Lower Columbia River Estuary

NOAA Technical Memorandum NMFS Series

The Northwest Fisheries Science Center of the National Marine Fisheries Service, NOAA, uses the NOAA Technical Memorandum NMFS series to issue informal scientific and technical publications when complete formal review and editorial processing are not appropriate or feasible due to time constraints. Documents published in this series may be referenced in the scientific and technical literature.

The NMFS-NWFSC Technical Memorandum series of the Northwest Fisheries Science Center continues the NMFS-F/NWC series established in 1970 by the Northwest & Alaska Fisheries Science Center, which has since been split into the Northwest Fisheries Science Center and the Alaska Fisheries Science Center. The NMFS-AFSC Technical Memorandum series is now being used by the Alaska Fisheries Science Center.

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This document should be cited as follows:

Furota, T., and R.L. Emmett. 1993. Seasonal changes in the intertidal and subtidal macrobenthic invertebrate community structure in Baker Bay, lower Columbia River estuary. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-NWFSC-5, 68 p.

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RESULTS AND DISCUSSION

Water Temperature and Salinity

Sediment Characteristics

Benthic Invertebrates

Efficiency of Sampling Method

Vertical Distribution Along the Tidal Gradient

Seasonal Change

Intertidal Community

Subtidal Community

Effect of Salinity on the Benthic Community

Effect of Vegetation on the Benthic Community

ABSTRACT

Macrobenthic invertebrates and sediments at 1 subtidal and 10 intertidal stations along a transect in Baker Bay of the 1ower Columbia River estuary were sampled monthly from November 1980 to October 1981. Water column temperatures and salinities were also recorded at the subtidal station. The intertidal community consisted primarily of estuarine species, whereas the subtidal community had additional marine species. Marine species declined in abundance after the interstitial salinity minimum (June), indicating the important role of salinity in determining benthic community structure.

Filamentous algae, tide pools, and eelgrasses (Zostera spp.) were also important factors determining macrobenthic invertebrate community structure. By altering sediment characteristics, Zostera spp. had a positive effect on deposit feeders and a negative effect on the sand-dwelling amphipod Eohaustorius estuaris.

INTRODUCTION

Changes in the faunal communities of estuaries are caused by physical, chemical, and biological processes. One of the most important physical factors determining the benthic fauna is salinity (Boesh 1977). In intertidal areas where physical conditions are extreme, distinct vertical distributions of benthic fauna have been observed (Brady 1943; Wells and Roberts 1980). Furthermore, benthic infauna diversity and density are affected by vegetative growth, particularly of eelgrass (Zostera spp.), which increases sediment stability and interrupts the movement of burrowing animals (Orth 1977; Ringold 1979).

For much of the year, the Columbia River estuary is characterized by low salinity due to large inputs of fresh water. Salinity is highest during late summer-early fall at the river mouth where surface salinity ranges from 5.8 to 30.5‰. Lowest salinity occurs in early summer as a result of snowmelt from the headwaters (Neal 1972). Large tidal fluctuations (>3.0 m) result in extensive tidal flats in the estuary.

The benthic macrofauna in the Columbia River estuary has been well studied (Haertel and Osterberg 1967; Higley and Holton 1975; Sanborn 1975; Higley et al. 1976; Higley and Holton 1978; Higley et al. 1979; Durkin and Enmett 1980; Durkin et al. 1981; Jones 1984). Major components of the benthic macrofauna include: the amphipods Corophium salmonis, C. spinicorne, and Eohaustorius estuaris; the polychaetes Neanthes limnicola and Hobsonia florida; and the bivalve Macoma balthica.

The objectives of this study were to determine the influences of seasonal changes in salinity and the presence of intertidal vegetation on the macrobenthic community and to describe the vertical changes in the benthic fauna along a tidal gradient on an intertidal-subtidal flat in Baker Bay, Columbia River estuary.

METHODS

Study Site

Baker Bay is located on the north side of the lower Columbia River estuary and has broad tidal flats consisting mainly of fine sand (Durkin and Emmett 1980). Eleven stations were sampled along a transect that extended from the mean high water level to the bottom of the adjacent boat channel at the northwest corner of Baker Bay (Fig. 1). Figure 2 depicts a cross section of the study site and shows station location, and the sediment characteristics of each station as observed in February 1981.

At Stations 1 and 2, three-square bullrush (Scirpus americanus) surrounded the sandy flats that were sampled. During late summer to early fall, filamentous algal mats, consisting of blue green algae (Lyngbya spp.) and a chrysophyte of the genus Vaucheria, densely covered the sediment surfaces of some areas at these stations.

Stations 1 through 10 were exposed during extremely low water. Patches of the eelgrasses Zostera japonica (on the upper intertidal area, Stations 3-7) and Z. marina (on the lower intertidal area, Stations 7-10) grew from July through October. Stations 4 through 7 had ripple marks on the sediment surface indicating relatively frequent wave disturbances.

Station 11, which was located at the bottom of the adjacent boat channel (subtidal), was about 4 m deep at mean low water. Sampling Procedure

At each station, three benthic invertebrate samples and one sediment core (10 cm deep by 5 cm in diameter) were taken monthly from November 1980 to October 1981 (see Appendix), usually during the second negative tide series. Intertidal stations were sampled for invertebrates using a 506 cm² (10 cm deep) metal frame. At the subtidal site (and lower intertidal sites) a 506-cm² Ekman-Birge sediment sampler was used. The Ekman-Birge sampler was handled directly by scuba divers, except at Station 11 where it was operated from a research boat using a messenger weight. During the last 2 months, however, this sampler was operated directly by divers at Station 11 (less ship traffic reduced the hazards for divers). Invertebrate samples were placed in a sieve box and washed through a 0.595 mm (No. 30) screen. Samples were then fixed in a 4% formaldehyde solution containing Rose-bengal (a protein stain).

During October 1981, two stations (Stations 3 and 7) were sampled to a depth of 20 cm. These sediments were divided into top (0 to 10 cm) and bottom (10 to 20 cm) portions and analyzed separately to identify the efficiency of the sampling method.

Semples for benthic invertebrate analysis were rinsed of formaldehyde, and invertebrates were then picked from the sample, separated by species or major taxonomic groups, and counted. Wet weights of the invertebrates were determined to the nearest 0.001 g at Toho University, Chiba, Japan, using a Sartorius¹ 2355 balance. The weights of some invertebrates were not obtained because they were damaged during transport from the United States to Japan.

Sediment composition for the February 1981 samples was determined by sieving the sediment through a set of sieves (2.0, 1.0, 0.5, 0.25, 0.125, and 0.063 mm) and then weighing each pre-weighed seive. Interstitial salinities were determined by drawing water from the core sediment using an aspirator with a Millipore HA filter; a silver nitrate method was then used to measure salinity (Strickland and Parsons 1968). At Station 11 (subtidal boat channel), water temperature and salinity were measured monthly at 1-m intervals (surface to bottom) using a Beckman Model RS53 salinometer and probe.

Differences between benthic invertebrate densities in samples from stations with and without eelgrass, were tested for statistical significance using either a Chi-square test (no zeros in the data) or a Mann-Whitney test (some zeros in the data).

RESULTS AND DISCUSSION

Water Temperature and Salinity

Water column temperatures at Station 11 ranged from 6.0°C in January to 17.7°C in August (Table 1). Vertical changes in temperatures varied little throughout the year.

Water column salinities tended to be lower during late winter to early summer with the lowest salinity, 3.5‰, occurring near the surface in February (Table 2). The highest salinity, 22.4‰, was recorded at 5 m in December. Though salinity tended to increase with water depth, vertical differences in salinity were not great, suggesting that water in the study area was well mixed.

Throughout the year, interstitial salinities were lower at the intertidal sites than at the subtidal site (Table 3). Intertidal interstitial salinities were lowest during the spring and early summer, with the lowest levels occurring in late June (<2.0‰). Sanders et al. (1965) showed that in an Atlantic coast estuary, interstitial salinities remained relatively constant even though overlying water salinities varied diurnally. In Baker Bay, however, low interstitial salinities occurred in June as a result of the large input of fresh water from the Columbia River that consistently overlaid the tidal flat during this period.

Seasonal fluctuations of subtidal interstitial salinities were relatively small but followed a pattern similar to that of the intertidal stations.

Sediment Characteristics

The sediment of the tidal flat (Stations 1 through 9) consisted primarily of medium (0.500 mm £ 0.250 mm in diameter) and fine sand (0.250 mm < 0.125 mm in diameter), (Fig. 2). Subtidal sediment was composed primarily of fine and with some silt and clay (< 0.063 = in diameter) at Station 10 and very fine sand (0.125 mm £ 063 mm in diameter) and silt-clay at Station 11.

Benthic Invertebrates

Thirty-seven different benthic invertebrates were identified (Table 4). The polychaetes Neanthes limnicola and Hobsonia florida, the bivalve Macoma balthica, and the amphipods Corophium salmonis and Eohaustorius estuaris were abundant in the study area throughout the year (Figs. 3 and 4). Gravimetrically, M. balthica was the dominant species in the intertidal community followed by N. limnicola. These two species comprised more than 90% of the total weight of the middle-low intertidal community (Fig. 4). At the subtidal site, M. balthica was the most important species numerically and gravimetrically, followed by the spionid polychaete Pseudopolydra kempi (Fig. 5).

In a previous Baker Bay study, Higley and Holton (1978) found oligochaetes and M. balthica abundant in the bay as a whole, with the haustorid amphipod E. estuaris and the capitellid polychaete Mediomastus californiensis abundant only in shallow and deep habitats, respectively. In a nearby intertidal mudflat in Baker Bay, Jones (1984) observed that Macoma balthica and the polychaetes H. florida and P. kempi were the dominant members of the benthic community, followed by oligochaetes and the polychaete N. limnicola. Oligochaetes were not abundant in our study. This may be related to sieve sizes: Higley and Holton (1978) used 0.425 screens, Jones (1984) used 0.125 mm screens, and we used 0.595 mm screens. Efficiency of Sampling Method

Holton et al. (1984) reported that benthic invertebrates in Baker Bay were most abundant in the top 10 cm of the sediment. Similarly, our analysis showed that a 10-cm depth sample was sufficient to describe the macrobenthic community at our study site, although several species would be slightly underestimated (Table 5). For example, while sampling, we observed many adult soft-shell clams, Mya arenaria, in the lower intertidal areas. However, because they lived more that 20 cm below the sediment surface, we did not sample this species quantitatively. Thus, estimations of these undersampled species were excluded from our analysis. Vertical Distribution Along the Tidal Gradient

The vertical distributions of abundant benthic invertebrates along the tidal gradient in winter (February 1981) and summer (August 1981) were similar (Fig. 3), suggesting that the distribution patterns did not change throughout the year. In the upper two stations (Stations 1 and 2) insect larvae (Dolichopodida and Chironomidae) were dominant; their distributions were restricted to this area. Most other invertebrates were distributed below this zone showing a clear zonal separation from the insect larvae, especially in summer. Neanthes limnicola and H. florida were also abundant in the upper tidal zone if tide pools existed. This agrees with Lewis (1961) who showed that physical conditions, such as desiccation, are important factors determining the upper limit of intertidal animals on rocky shores.

The densities of Pygospio elegans, H. florida, and C. salmonis were low in the middle tidal zone (Stations 5 through 8) where ripple marks on the sediment surface indicated that the substrate was relatively unstable. In contrast to polychaetes, the amphipod Eohaustorius estuaris was relatively abundant on this unstable sediment. Pseudopolydora kempi was one of the most important members of the intertidal flat community in Baker Bay (Jones 1984) and was often abundant from the low tidal zone to the subtidal bottom where the sediment had a high silt-clay content. This suggests that the stability of the sediment surface plays an important role in determining the species composition and abundance of intertidal invertebrates.

Seasonal Changes

Intertidal Community—Numerically dominant species throughout the year in the intertidal community were N. limmicola, H. florida, E. estuaris, Macoma balthica, and C. salmonis (Fig. 4). Peak densities were related to recruitment and were observed in May for M. balthica, June for E. estuaris, June and August through October for C. salmonis, July for N. limnicola, and July and October for H. florida. These population peaks coincided closely with peaks found in another Baker Bay intertidal flat, with the exception of M. balthica and E. estuaris (Jones 1984). Eohaustorius estuaris was not present in the other flat here M. balthica spat settled in late summer or early fall (Jones 1984). The reason for the different seasons of spat settlement on the two flats is unknown. Also of note, the decreases in densities of two spionid polychaetes, Pygospio elegans and Pseudopolydora kempi, coincided with the decline of interstitial salinities (Fig. 4).

More than 90% of the weight of the intertidal benthic community through the year consisted of M. balthica and N. limnicola. The community weight of M. balthica increased during warmer seasons (spring to fall), even after its population numbers had decreased in summer. This suggests the increase was related to individual growth.

The intertidal community biomass increased gradually during this study with a maximum in the final month, October 1981. Because observations were not made during the following winter, it is not known whether the community weight declined to the level observed in the previous winter.

Subtidal Community—Macoma balthica was the most numerous species for 8 months from May to October and accounted for the largest portion of the community biomass throughout the year at the subtidal site (Fig. 5). Total biomass of M. balthica increased in late summer and early fall. Because there were increases in both number and individual weights of M. balthica at the subtidal site, it is unclear whether the increases in total weight resulted more from growth or recruitment.

At the subtidal station, Pseudopolydora kempi was abundant until the interstitial salinity minimum of 7.2‰ occurred in June. The P. kempi population recovered in September but dropped again in numbers in October, perhaps due to predation or disturbance of the bottom by crabs and fishes (Virnstein 1977). The subtidal station is located in a channel known to have high densities of Dungeness crabs, Cancer magister (McCabe at al, 1986). Jones (1984) observed a sharp peak in the density of P. kempi in a mudflat in Baker Bay in August 1981. In contrast to P. kempi, densities of H. florida increased in June (Fig. 5). The cumacean of the genus Hemileucon was relatively more abundant in winter through spring.

Effect of Salinity on the Benthic Community

By identifying their preferred habitat, invertebrates collected in this study can be classified into three categories: 1) species of unknown salinity preferences, 2) estuarine species, or 3) marine species (Banse and Hobson 1974; Sanborn 1975; Smith and Carlton 1975; Higley and Holton 1978; Bousfield 1979) (Table 6). In the intertidal benthic community, estuarine species were dominant throughout the year except for October (Fig. 6). Marine species were an important component from November until June, when interstitial salinities reached a minimum and apparently reduced their numbers. Recovery of the marine species took about 4 months. During the recovery period the intertidal community was composed almost entirely of estuarine species and those with unknown salinity preferences. In October, high densities of chironomid larvae (unknown salinity preferences) at the upper intertidal stations (1 and 2) contributed almost half of the total community densities (Fig. 6).

Marine species were relatively more abundant in the subtidal community, especially during winter and spring before the June salinity decline. After the salinity minimum occurred, the population density of marine species remained low throughout the summer, recovering, at least temporarily, in September. The recovery of the marine species in the subtidal area occurred earlier than on the intertidal flat. This suggests that salinity (average and minimum) was an important factor affecting the population density of the marine species at the study area. However, densities of marine species also declined in October due primarily to lower densities of P. kempi. With just one year's data, it is not possible to determine unequivocally that the seasonal salinity decline regulated marine species abundances. Nevertheless, the dramatic decline in marine species densities after the salinity minimum strongly suggests a cause-and-effect relationship.

Effect of Vegetation on the Benthic Community

Filamentous Algae

Filamentous algae were first observed covering the sediment at the upper intertidal sites in January. They formed dense green mats in May and remained until at least October. Insect larvae (Chironomidae and Dolichopodidae) and the sabellid polychaete, Manayunkia aestuarina were abundant with the occurrence of the agal mat. However, the agal filaments reduced the effective size of the sieving screen, and without the agal mat a considerable number of these invertebratescould have passed through the 0.595 mm screen. It is unclear whether the presence of the algal mat provided habitat for the insect larvae and the abellid polychaete.

A sacoglossan gastropod occurred with the filamentous algae starting in August and showed a maximum density of 1,680/m² in October. This gastropod feeds on algae and stores their chloroplasts in its cerata. Some Sacoglossa feed on the chrysophyte Vaucheria (Smith and Carlton 1975), one of the major components of the filamentous algal mat, suggesting that the algal mat provides habitat and food for this gastropod.

Lewis (1961) suggested that the upper distribution limit of intertidal animals is determined by their physiological tolerance to desiccation. Despite the capillary effect of the algae filaments, which prevents desiccation of the sediment surface, the abundance of most benthic animals, except insects, was lower in the algal mat zone in late summer (Table 7). The filamentous algal mat may, under certain conditions, eliminate some of the invertebrates that live below the sediment surface, perhaps by preventing feeding activity or reducing dissolved oxygen. The presence of a shallow tide pool at Station 2 apparently elevated the upper limits of some intertidal animals (Table 8).

Eelgrasses

The eelgrasses Zostera japonica and Z. marina formed patchy beds on the tidal flat during the summer. Eelgrass beds increase sediment stability, resulting in high infaunal diversity and density (Orth 1977), but the rhizomes of Zostera spp. physically hinder the movement of burrowing organisms (Ringold 1979). The amphipods Eogammarus confervicolus and Corophium spinicorne tended to be more abundant at stations with eelgrasses (Table 9), suggesting that eelgrass provides substratum for these epiphytic amphipods. The polychaetes N. limnicola and H. florida were more abundant in eelgrass beds than outside the beds between Stations 4 and 6. The positive effect of Zostera spp. beds on these surface-feeding polychaetes is probably related to increased sediment stability. However, the gammarid amphipod Eohaustorius estuaris, which prefers unstable sediment (Bosworth 1973), was less abundant on the Zostera spp. beds.

CONCLUSIONS

The macrobenthic community structure in a fine-sand tidal flat and adjacent muddy subtidal site in Baker Bay showed few seasonal changes in species composition. The intertidal community consisted mainly of estuarine species, such as M. balthica, N. limnicola, C, salmonis, H. florida, and E. estuaris, which were abundant throughout the year. Marine species, such as P. kempi, tended to increase in population densities from the intertidal to subtidal sites and declined in abundance after the June salinity minimum. This indicates that a seasonal reduction in salinity is probably an important factor maintaining the community structure of an estuarine-species dominated intertidal-subtidal benthos. This reduction was caused by the large spring freshet of the Columbia River.

Two zones of benthic invertebrate communities were observed in the tidal flat. The first was an high upper tidal one (Stations 1 and 2) dominated by insect larvae (Chironomidae and Dolichopodidae). A filamentous algal mat covered the sediment surface here during summer. The second zone was the middle-lower tidal zone. Here, the surface deposit feeders, such as M. balthica, N. limnicola, and H. florida were abundant on the relatively stable sediments of the lower tidal areas and on eelgrass beds in the middle tidal level. The sand-dwelling amphipod E. estuaris was abundant in the unstable sediments of this middle tidal level.

At the subtidal site, M. balthica was the dominant species while P. kempi was more abundant here than at the other sites. The reduced densities of estuarine species in the subtidal site may be related to salinity, sediment texture, and perhaps intense fish or crab predation.

Our results suggest that one of the most important factors determining the macrobenthic animal community structure in the Columbia River estuary is salinity. Also, increased sediment stability and Zostera spp. have a positive effect on densities of deposit feeders and a negative effect on the densities of E. estuaris.

ACKNOWLEDGMENTS

We would like to give special thanks to Bill Muir for his help in sampling and George McCabe, Jr. for his review of this paper.

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