Eelgrass Distribution Research Articles

Does the recruitment of a non‐native mussel in native eelgrass habitat explain their disjunct adult distributions?

ABSTRACTWe studied variability in the abundance of small individuals of an invasive mussel (Musculista senhousia) across the depth distribution of a native marine angiosperm, eelgrass (Zostera marina). Adult mussels and eelgrass have a disjunct local distribution, each limiting the other in complex ways. To assess whether eelgrass also influenced the distribution of juvenile mussels, we sampled inside and outside eelgrass beds in one site in Mission Bay and two in San Diego Bay, California, USA. We sampled mussels in size classes 0.26–0.50 mm, 0.51–1.00 mm, 1.10–2.00 mm and > 2.00 mm from September 1997 to April 1999. We also monitored gonad development in larger mussels and in situ growth of mussels ≤ 2 mm tagged with the chemical marker calcein. Spatial and temporal variations in mussel abundances were high but seasonal patterns were roughly similar at San Diego Bay sites; very few mussels were found in Mission Bay. Mussels with full gonads were found year‐round in San Diego Bay, as were mussels in the smallest size class (with a large peak in fall and a smaller secondary one in spring), suggesting that many of the smallest mussels represent recruitment. The observation that most, although not all, tagged mussels increased in size provides further support for recruitment. Some of the highest numbers of mussels in the smallest size class were found inside eelgrass beds, indicating that eelgrass does not restrict and may actually enhance the distribution of very small mussels. The disjunct distribution of adult mussels and eelgrass thus is apparently established primarily postrecruitment. M. senhousia is capable of year‐round reproduction, recruitment and growth, and thus is poised to preempt space from eelgrass following any disturbance that results in eelgrass declines, such as habitat fragmentation, eutrophication, or disease.

Desiccation is a limiting factor for eelgrass (Zostera marina L.) distribution in the intertidal zone of a northeastern Pacific (USA) estuary

Intertidal irradiance, temperature, and aerial exposure were measured for two years in intertidal Zostera marina beds located in Yaquina Bay (Newport, OR, USA). These physical data were correlated with plant growth and other metrics measured at intervals during the study. Photosynthesis vs. irradiance (P vs. I) curves were determined by PAM fluorometry on plants along an intertidal gradient, and were used to estimate saturating irradiance (I k ) and daily hours of saturating irradiance (H sat ). Mean I k values were ∼64 μmol photons m -2 s -1 and H sat increased with increasing tidal position. Measurements taken during the second winter showed that H sat values in the low intertidal zone were within a critical range requiring eelgrass to use rhizome reserves to maintain carbon balance. However, plants continued to grow. Temperature differences across tidal gradients were not found to be physiologically important. These results suggested that temperature and irradiance were not limiting factors for intertidal Z. marina growth. High intertidal eelgrass, exposed to aerial conditions more frequently and for longer durations, exhibited signs of desiccation damage and appeared to have faster blade turn-over rates in spring and summer than their low intertidal neighbors. Thus, we concluded that the major factor limiting Z. marina in the upper intertidal zone was desiccation stress, which was acute and episodic rather than chronic.

葉上浮泥による光量低下と砂面変動がアマモ場の分布に及ぼす影響―広島湾でのケーススタディ―

The objective of this study is to clarify the effect of light reduction by sediment deposition on leaves and sand movement on eelgrass distribution for obtaining fundamental knowledge on the formation of the eelgrass (Zostera marina L.) habitat. We investigated light regime and sediment deposition on leaves near the lower edge of an eelgrass bed, and monitored sand movement and transplanted eelgrass in an unvegetated site near the higher edge. The average light intensity through sediment deposition on eelgrass leaves was estimated to be approximately 3 E·m-2·d-1 at a low depth limit. As sediment deposition on leaves was 0.7 mg·cm-2 <, the light intensity required by eelgrass might be 4 E·m-2·d-1 <. Sand movement at the unvegetated site ranged from -10 cm to 12 cm and was more unstable than that at a natural site. Scouring for two months reached up to 10 cm, and seedlings disappeared at the unvegetated site near the higher edge. However, the number of vegetative shoots in the unvegetated site steadily increased. These results suggest that vegetative shoots can survive within 10 cm washouts and the growth of eelgrass near the higher edge of an eelgrass bed is restricted to vegetative reproduction.

Long-term changes in area distribution of eelgrass ( Zostera marina) in Danish coastal waters

For five shallow-water eelgrass stands in Denmark, changes in area distribution over the period 1940/50s–1990s were assessed from aerial photographs. In areas where the wasting disease had decimated eelgrass meadows in the 1930s, this included determination of the time scale of recolonization. In addition, we aimed at evaluating whether fluctuations in eelgrass area distribution were related to various natural and human-induced disturbances, namely storm events, ice-cover, water temperature and nutrient loading. We found that populations affected by the wasting disease exhibited a time lag of more than 10 years before substantial recolonization began, probably reflecting long distances to seed-producing populations and extreme climatic events in the period. After the initial time lag, eelgrass area distribution increased rapidly, and a substantial recovery had taken place in the 1960s. All eelgrass populations showed marked inter-annual fluctuations. Declines were often rapid with reductions of about 60% occurring in less than 6 years. Recoveries occurred over similar time scales and documented that recolonization may take place relatively fast when suitable environmental conditions are present. Fluctuations in eelgrass area distribution tended to be larger in enclosed, protected bays as compared to open coasts, probably because enclosed sites are often more eutrophic. Changes in shallow-water eelgrass area distribution did not correlate with the available long-term records of natural and human-induced disturbance parameters. Thus, while deep-water eelgrass populations have declined markedly over the last century in response to eutrophication, long-term changes in shallow-water populations are less equivocal and seem more stochastic.

Spatial and temporal variation in eelgrass ( Zostera marina) landscapes: influence of physical setting

The distribution of eelgrass ( Zostera marina) was mapped from aerial photographs at three Danish sites with different levels of physical exposure by digital image analysis at 1 m × 1 m resolution. A set of indices from landscape ecology were used to quantify various components of the eelgrass landscapes such as patch shapes and landscape aggregation in photographs from 1995. Furthermore, the long-term fluctuations in the spatial distribution of eelgrass were investigated during the period 1954–1995/1999. Eelgrass landscapes exposed to wave dynamics were less aggregated with more elongated patch shapes than the sheltered eelgrass areas. The outline of the larger patches also tended to be more complex at exposed sites. Patch size distribution at all sites was highly skewed with 75–95% of the patches being <10 m 2. Although the majority of the patches were small, they only constituted a small proportion of the total eelgrass area because most of the eelgrass-covered area was contained in a few, large patches. Large fluctuations in the spatial distribution of eelgrass occurred at both exposed and protected sites in the course of the investigation period. In the relatively short intervals of ∼7 years between two consecutive photographs, on average 39–62% of the total eelgrass area covered in the photographs had changed (i.e., disappeared or recolonized) and the largest changes occurred at exposed sites. Overall, this study showed that shallow eelgrass populations form characteristic landscapes with a configuration that is highly related to the level of physical exposure and that the size and position of eelgrass beds changes substantially among years.

Long-term change in eelgrass distribution at Bahía San Quintín, Baja California, Mexico, using satellite imagery

Seagrasses are critically important components of many marine coastal and estuarine ecosystems, but are declining worldwide. Spatial change in distribution of eelgrass,Zostera marina L., was assessed at Bahia San Quintin, Baja California, Mexico, using a map to map comparison of data interpreted from a 1987 Satellite Pour l'Observation de la Terre multispectral satellite image and a 2000 Landsat Enhanced Thematic Mapping image. Eelgrass comprised 49% and 43% of the areal extent of the bay in 1987 and 2000, respectively. Spatial extent of eelgrass was 13% less (−321 ha) in 2000 than in 1987 with most losses occurring in subtidal areas. Over the 13-yr study period, there was a 34% loss of submerged eelgrass (−457 ha) and a 13% (+136 ha) gain of intertidal eelgrass. Within the two types of intertidal eelgrass, the patchy cover class (<85% cover) expanded (+250 ha) and continuous cover class (≥85% cover) declined (−114 ha). Most eelgrass losses were likely the result of sediment loading and turbidity caused by a single flooding event in winter of 1992–1993. Recent large-scale agricultural development of adjacent uplands may have exacerbated the effects of the flood. Oyster farming was not associated with any detectable losses in eelgrass spatial extent, despite the increase in number of oyster racks from 57 to 484 over the study period.

Decline in eelgrass ( Zostera marina L.) in Long Island Sound near Millstone Point, Connecticut (USA) unrelated to thermal input

Eelgrass ( Zostera marina L.) shoot density, seed-bearing shoot abundance, shoot length, and standing stock biomass were monitored during summer months from 1985 to 2000 at three locations in eastern Long Island Sound (LIS) near Millstone Power Station (MPS), Waterford, Connecticut, USA. Short-term declines in eelgrass abundance were directly associated with fouling and overgrowth of eelgrass on two occasions; once by blue mussels ( Mytilus edulis) and once by a bloom of green algae ( Cladophora spp.). Analysis of long-term trends indicated some degree of decline in most of the parameters examined at all three areas monitored. The spatial relationship of the long-term eelgrass declines suggests primary causal factors other than the power plant discharge or regional climate change. Two populations to the east of MPS and near the fringes of the thermal plume (<1.5 km from the MPS discharge to LIS) exhibited only slight declines over the 16-y study period and thermal input from MPS to these sites was minimal (<1 °C above ambient conditions). By comparison, heavy eelgrass losses were documented in the Niantic River, located >2 km from the power plant thermal plume. Die-offs of entire individual eelgrass study beds in the Niantic River were observed on five separate occasions during the study with no sign of recovery. While the causes were not determined, anthropogenic influences such as nutrient loading from surface run-off and groundwater sources may have contributed to observed declines. The Niantic River has a more restricted tidal inlet and is closer to sources of nutrient enrichment than Jordan Cove. Historically, eelgrass has ranged to far western reaches of LIS, but over the last century has become restricted to the easternmost third of the Connecticut coastline due to nutrient loading and eutrophication of the western portions. This study suggests that the west-to-east declining trend in eelgrass distribution in LIS may be further progressing.

Long-term Changes of a Brackish-water Eelgrass (Zostera marina L.) Community Indicate Effects of Coastal Eutrophication

The distribution and importance of eelgrass (Zostera marina L.) meadows for associated faunal communities in the coastal waters of the Baltic Sea are still poorly known. In these low-saline (5–7), shallow coastal areasZ. marina grows at its limit of distribution, forming mostly patchy meadows. In June 1993, a seagrass locality (Tvärminne, SW Finland) thoroughly studied in 1968–71 was revisited in order to detect possible long-term changes in both vegetation structure (distribution, density, biomass) and benthic fauna (species composition, abundance, biomass, distribution and diversity patterns). The same sampling design as in the 1970s was used in both sparse (<20 shootsm−2) and dense (>150 shootsm−2) Z. marina. In addition, the feeding-efficiency of adult flounder (Platichtys flesus L.) on infauna was measured by counting feeding pits in vegetated and bare sand. The analysis shows that the shoot density had increased in sparse Z. marina, while dense Z. marina patches showed similar biomass values (20gAFDWm−2) as in the 1970s. In contrast to the vegetation, where little apparent change could be recorded, the total abundance and biomass of zoobenthos has increased significantly between 1968–71 and 1993 in the dense Z. marina patches. These changes are mainly attributed to significant increases of the bivalve Macoma balthica L., mudsnails Hydrobia spp. and oligochaetes. In sparseZ. marina diversity in terms of number of taxa exhibited minor changes over time, whereas in dense Z. marina patches the mean number of taxa has increased from 16 to 20. The vegetation cover was sufficient to reduce significantly the predation effects of flounder on seagrass infauna. This study represents a rare example of long-term persistence of seagrass communities in an area where the negative effects of nutrient enrichment are evident. The faunal changes in the Z. marina community indicate increased food availability, which is associated with positive effects of coastal eutrophication.

Distribution of Eelgrass, Zostera marina L. on Coasts of the Korean Peninsula: Preliminary Study for Eelgrass Restoration

Eelgrass, Zostera marina L. widely spreads throughout all the coastal areas of the Korean Peninsula. However, some previously reported eelgrass populations disappeared. The disappearance was probably caused by anthropogenic disturbance such as reclamation and pollutant or exceeded nutrient release. Eelgrass beds occurred from the intertidal to the subtidal zone, mainly in lagoon, estuaries, ports, barrier reef and bays. Eelgrass beds were also found at the intertidal mud and sand flats, subtidal mud and sandbank in more exposed areas. Habitat characteristics of eelgrass beds distributed on the coasts of the Korean Peninsula varied among coast areas. Eelgrass distributed constantly throughout the southern coast of Korea, while the distribution was limited at lagoon, bay, port, or barrier reef on the eastern coast, because of steep water depth and high wave energy in that coast. On the western coast, eelgrass mainly appeared at the intertidal and subtidal zones in islands. Sediment characteristics of the Z. marina beds varied with locality, tidal current and water motion. Sediments of Z. marina beds were composed of sand, muddy sand, sandy mud and mud. Mean grain size ranged from 1.5 to 4.1 phi.

Distribution and stability of eelgrass beds at Izembek Lagoon, Alaska

Spatial change in eelgrass meadows, Zostera marina L., was assessed between 1978 and 1987 and between 1987 and 1995 at Izembek Lagoon, Alaska. Change in total extent was evaluated through a map to map comparison of data interpreted from a 1978 Landsat multi-spectral scanner image and 1987 black and white aerial photographs. A ground survey in 1995 was used to assess spatial change from 1987. Eelgrass beds were the predominant vegetation type in the lagoon, comprising 44–47% (15 000–16 000 ha) of the total area in 1978 and 1987. Izembek Lagoon contains the largest bed of seagrass along the Pacific Coast of North America and largest known single stand of eelgrass in the world. There was a high degree of overlap in the spatial distribution of eelgrass among years of change detection. The overall net change was a 6% gain between 1978 and 1987 and a < 1% gain between 1987 and 1995. The lack of significant change in eelgrass cover suggests that eelgrass meadows in Izembek Lagoon have been stable during the 17-year period of our study.

Community structure and spatial variation of benthic invertebrates associated with Zostera marina (L.) beds in the northern Baltic Sea

The distribution and bed structure of eelgrass ( Zostera marina L.), and its importance for associated faunal communities in the coastal areas of the northern Baltic Sea are poorly known. The spatial distribution of the fauna associated with Zostera was studied at five localities in SW Finland in 1993–1994. Zostera was common on all localities, but the beds varied in terms of area (1–5 m diameter), density (50–500 shoots/m 2) and blade length (20–110 cm). A total of about 40 species or taxa were recorded. The zoobenthic infauna showed significant spatial differences, and total abundance and species diversity were significantly higher in the Zostera beds than in adjacent bare sand. The total abundance in Zostera ranged from 25 000 to 50 000 ind/m 2 and in sand from 2500 to 15 000 ind/m 2 The mean number of species in Zostera ranged from 5.9 to 8.8 spp ( H′ = 1.76–2.54) and in sand from 2.2 to 5.5 spp ( H′ = 1.67–2.31). The epifauna in Zostera was numerically dominated by grazing gastropods (Hydrobiidae) and copepods. The epifauna is an important community component, which contributes to the total diversity of the Zostera assemblage. These systems are among the most species-rich components of the shallow soft-bottom ecosystems in the northern Baltic Sea. The mechanisms structuring both the Zostera and the ambient sand-bottom habitats are presented.

Twentyfive years of changes in the distribution and biomass of eelgrass,Zostera marina, in Grevelingen lagoon, The Netherlands

The wax and wane of the eelgrass (Zostera marina L.) population in Grevelingen lagoon (East Atlantic; The Netherlands) has been documented for over 25 years, together with quantitative and semi-quantitative data on environmental variables. The population expanded after the closure of the Grevelingen estuary in 1971, but declined from 4600 ha surface area in 1978 to less than 100 ha in 1993. There is little causal evidence which factors are responsible for the observed dynamics of the population. The incomplete picture emerging from the data is that of an extremely impoverished eelgrass population, living under constant oligo-mesotrophic marine conditions. Both the sexual and the vegetative modes of reproduction are severely stressed by environmental variables, most likely a combination of low temperatures, high salinity, low dissolved silicate and low ammonium concentrations. Survival of the population asks for the restoration of moderate estuarine conditions.

Quantifying Eelgrass Habitat Loss in Relation to Housing Development and Nitrogen Loading in Waquoit Bay, Massachusetts

Change analysis of eelgrass distribution in Waquoit Bay demonstrated a rapid decline of eelgrass habitat between 1987 and 1992. Aerial photography and ground-truth assessments of eelgrass distribution in the Waquoit Bay National Estuarine Research Reserve documented progressive loss in eelgrass acreage and fragmentation of eelgrass beds that we relate to the degree of housing development and associated nitrogen loading, largely via groundwater, within various sub-basins of the estuary. The sub-basins with greater housing density and higher nitrogen loading rates showed more rapid rates of eelgrass decline. In eelgrass mesocosm studies at the Jackson Estuarine Laboratory, excessive nitro- gen loading stimulated proliferation of algal competitors (epiphytes, macroalgae, and phytoplankton) that shade and thereby stress eelgrass. We saw domination by each of these three algal competitors in our field observations of eelgrass decline in Waquoit Bay. Our study is the first to relate housing development and nitrogen loading rates to eelgrass habitat loss. These results for the Waquoit Bay watershed provide supporting evidence for management to limit development that results in groundwater nitrogen loading and to initiate remedial action in order to reverse trends in eelgrass habitat loss.

Tides, light and the distribution of Zostera marina in Long Island Sound, USA

The disappearance of Zostera marina L. (eelgrass) in western Long Island Sound has been attributed to the eutrophication-induced increase in light attenuation in the waters of that area. In this work we explore whether the much higher tidal range in the western (3 m) than in the eastern (1 m) Sound could further reduce light availability and, therefore, restrict the vertical distribution of eelgrass. Assuming that the spring low water level determines the upper limit of distribution and the depth of minimum light required for growth determines the lower limit, then the vertical zone for growth in the western Sound is limited to a 1 m fringe. Eelgrass within this narrow range would be vulnerable to exposure during storm events. In the eastern Sound, the viable range for eelgrass growth is 4 m, and similar disturbances would be less likely to affect the seagrass population (since deeper growing shoots may provide energy for shallow-growing ones). A further evaluation of tidal effects on the light availability for Z. marina in Long Island Sound was pursued by allowing surface irradiance and depth of the water column above seagrass canopies to fluctuate over 24 h periods in a Lambert-Beer Law based model. It revealed that the diel benthic light curves were skewed or had a bimodal (rather than sinusoidal) shape and that the number of hours of growth-saturating (about 300 μmol quanta m −2 s −1) light was smaller as light attenuation and tidal ranges increased in the western Sound. Therefore, the large tidal ranges may have contributed to the disappearance of eelgrass in the western Sound. We suggest that, due to the significant influence of tides on light availability resulting in light restrictions for benthic vegetation, tides should be taken into account when managing coastal waters with the aim of allowing for the successful growth of seagrasses.

A dynamic model describing the seasonal variations in growth and the distribution of eelgrass ( Zostera marina L.) I. Model theory

In shallow coastal areas eelgrass ( Zostera marina L.) can have an important influence on the biological structure. As a reflection of this a general eutrophication model for coastal marine areas has been extended to include the seasonal variations in growth of eelgrass. The basic model describes the growth of phytoplankton and zooplankton and the nutrient dynamics. The important factors controlling eelgrass growth and distribution are included in the model: water transparency, water temperature, and water depth/topography of the water bed. Factors such as growth of epiphytes on eelgrass leaves and grazing birds and invertebrates are omitted. The eelgrass submodel describes seasonal and regional variations in production and biomass of above and below ground parts. The phytoplankton and eelgrass submodels interrelate, especially at the point of water transparency, since water transparency depends on the phytoplankton (chlorophyll- a) concentration. The model has been developed and calibrated based on data from a shallow area around the island of Masnedø in the southern part of Denmark. Data on phytoplankton, zooplankton and eelgrass production and biomass as well as nutrient status and important forcing functions were collected over a period of 1 year. The developed model was found to be able to describe the variations in the data material in a satisfying way.

Effects of anaerobiosis on root metabolism of Zostera marina (eelgrass): implications for survival in reducing sediments

The temperate seagrass Zostera marina L. typically grows in highly reducing sediments. Photosynthesis-mediated O2 supplied to below-ground tissues sustains aerobic respiration during photosynthetic periods. Roots, however, experience daily periods of anoxia and/or hypoxia at night and under conditions that reduce photosynthesis. Rhizosphere cores of Z. marina were collected in August 1984 from Great Harbor, Massachusetts, USA. We examined short-term anaerobic metabolism of [U-14C]sucrose in excised roots and roots of intact plants. Under anaerobic conditions roots showed appreciable labeling of CO2, ethanol and lactate, and slight labeling of alanine and other metabolites. Over 95% of the 14C-ethanol was recovered in the root exudate. Release of other metabolites from the roots was minimal. Ethanol was also released from hypoxic/anoxic roots of intact plants and none of this ethanol was transported to the shoot under any experimental conditions. Loss of ethanol from roots prevented tissue levels of this phytotoxin from increasing during anaerobiosis despite increased synthesis of ethanol. Anaerobic metabolism of [U-14C]glutamate in excised roots led to appreciable labelling of γ-aminobutyrate, which was known to accumulate in eelgrass roots. Roots recovered to fully aerobic metabolism within 4 h after re-establishment of aerobic conditions. The contributions of these root metabolic responses to the ability of Z. marina to grow in reducing marine sediments are related to light-regulated interactions of shoots and roots that likely dictate depth penetration, distribution and ecological success of eelgrass.

A Simulation Model of Production, Seasonal Changes in Biomass and Distribution of Eelgrass (Zostera marina) in Lake Grevelingen

A Simulation Model of Production, Seasonal Changes in Biomass and Distribution of Eelgrass (Zostera marina) in Lake Grevelingen

A simulation model of production, seasonal changes in biomass and distribution of eelgrass (Zostera marina) in Lake Grevelingen

A simulation model has been described, based on data from Lake Grevelingen, The Netherlands, as a predictive tool for lake management. The model has been developed as part of a large-scale aquatic modelling effort in Lake Grevelingen, carried out in close cooperation with the Delft Hydraulics Laboratory and the Delta Department, Environmental Research Division of the Ministry of Transport and Public Works. Available data on growth rates per unit eelgrass biomass, obtained with the leaf-marking technique, and on above- en below ground biomass and shoot density changes per unit area have been used. A space limitation depending on density of the above ground biomass and a growth limitation due to shortage in below ground biomass have been introduced. The seasonal changes in eelgrass production, both above and below ground, have been simulated as functions of the external forcing variables light, water temperature, wind generated water movements and of the internal control variables due to aging of the plant material. The vertical distribution of eelgrass can be partly explained from the modelling results on space, light and below ground biomass limitations. From the shore down to about 1 m waterdepth the above ground eelgrass biomass suffers from space limitation. Between 1 and 2 m production and biomass reach maximum values. Between 2 and 3 m waterdepth the above ground eelgrass growth is limited by the availability of below ground biomass. Between 3 and 5 m waterdepth both below ground biomass and light are the growth-limiting factors. Below 5 m waterdepth light is not sufficient to sustain net growth of eelgrass from rhizomes. Together with additional data — not used in the model — on seed production and growth of eelgrass shoots from seeds the vertical and horizontal distribution of the dominant macrophyte in the lake can be explained.

Distribution and biomass of eelgrass (Zostera marina L.) and other seagrasses in Odawa Bay, Central Japan

In Odawa Bay, central Japan, three species of seagrass occur, Zostera marina L., Z. japonica Aschers. et Graebner and Halophila ovalis Hook. f., of which the eelgrass, Z. marina, forms dense and wide beds in shallow littoral areas. The ecological distribution of these seagrasses was studied by diving observations from which it was determined that depth distribution was closely related to transparency of water and/or degree of insolation. The area of Odawa Bay covered with the eelgrass and the total eelgrass biomass in May 1977 were estimated to be ca. 68.2 ha, and ca. 101.2 tons dry weight.

The Growth of Eelgrass in Relation to Tidal Depth

The growth of eelgrass (Zostera marina) in relation to tidal depth was studied on three areas of South Humboldt Bay, California, in 1960 and 1962. Six contours of bay bottom between +1.0 and -1.5 feet in relation to mean lower-low tide were sampled. The upper limit of eelgrass growth was at or slightly above +1.0 foot. This contour was exposed to air about 15 percent of the time. The percent of eelgrass coverage, mean turion length, and eelgrass biomass per square foot all increased with increases in tidal depth. The density of turions per square foot and the number of leaves per turion did not vary with depth. The optimum depth for eelgrass production at Humboldt Bay appears to be at about -1.0 foot. Over 90 percent of the total eelgrass biomass and about 60-70 percent of the eelgrass acreage in South Humboldt Bay occur at or below mean lower-low tide. The importance of eelgrass to waterfowl is well documented (Addy and Aylward 1944, Cottam et al. 1944, Cottam and Munro 1954). Humboldt Bay on the northern California coast supports extensive beds of eelgrass (Keller 1963, Waddell 1964) which supply the winter food for large numbers of black brant (Branta bernicla orientalis) and American widgeon (Anas americana), as well as other waterfowl (Yocom and Keller 1961, Denson 1961, Murrell 1962). Modern trends in use of tidal basins for such things as channels, oyster culture, reclamation for industrial development and agriculture, and sewage outlets tend to reduce the acreage of eelgrass beds and to disturb the conditions for growth of remaining beds. To evaluate the impact of such activities on the supply and growth of eelgrass, the effects of environmental factors on eelgrass in relatively undisturbed situations must be known. This paper describes the growth of essentially undisturbed eelgrass in South Humboldt Bay in relation to tidal (water) depth. Acknowledgment is made to Dr. Charles Cooper for his help in planning parts of this study, and to Dr. Charles F. Yocom, Dr. Richard E. Genelly, and Dr. Ernest O. Salo for reviewing early copies of this paper. Irel D. Lowe, Donald Kemp, Kenneth Sohler, and Dale Gardner assisted in the surveying of South Bay, and Kenneth Sohler assisted in the laboratory work. DEFINITIONS AND METHODS Eelgrass growth was sampled on a study ar a in South Humboldt Bay between August 7 and 22, 1960, and between July 26 and August 19, 1962. Two additional widely separated study areas were sampled during the 1962 period. On all three areas, transects were established on contour lines at each 6-inch decrease in elevation by surveying from local benchmarks. At each area, a total of six contours were chosen for study as follows (all measurements expressed in feet in relation to mean lower-low tide): +1.0, +0.5, 0.0, -0.5, -1.0, and -1.5. These contours were selected because +1.0 foot represented the elevation of the upper limit of eelgrass distribution at Humboldt Bay, and it was not physically practical to sample contours below -1.5 feet because such contours were remotely located and water depths were excessive, even at low tide. The following types of data were recorded: