top of page

NATURAL HISTORY

NATURAL HISTORY
Science & Nature

Science & Nature

Watch Now

DUNE GEOLOGY

The key ingredients needed to build a dune system include a source of sand, a shoreline perpendicular to the prevailing winds and a low landscape over which dunes can migrate. In addition, plant species that are adapted to survive the drying winds and shifting sands are needed to help shape and build the dunes. Here in the Humboldt Bay area, the Mad and the Eel rivers supply most of the sand. Winter storms flood these rivers and transport sand to the ocean. Sand is carried by currents along the coast and pushed up on to the beach by gentle summer waves. Once dry, the sand is moved by the prevailing summer winds from the northwest. This dynamic process has created of a variety of dune habitats within a narrow stretch of coastline.

The waveslope is the area of the beach that shows evidence of having been washed by waves during the last tidal cycle. Beachcombing here you can find everything from eelgrass, Pacific razor clam shells, sand dollars, to the carcass of a gray whale. This is also the feeding area for a number of shorebirds. They feed on small invertebrates in the sand and in the seaweeds washed on shore. The upper end of the waveslope is the strand. Here you may find plants such as sea rocket and native dune grass starting to colonize the bare sands.

 

The series of dunes and ridges paralleling the beach are collectively called the foredunes. In areas that are relatively undisturbed by invasive species, you can find a unique community of plants referred to as the dune mat. Here, a wide array of wildflowers adapted to the drying conditions of the dunes help stabilize the shifting sand. The dune mat is home to two federally listed endangered plant species, the Humboldt Bay wallflower and the beach layia. Take care to stay on the main trails through these areas.

 

Hollows/Swales form when the summer wind has removed the sand down to the water table, allowing water-loving plants to move in. During winter storms the water table rises and forms seasonal ponds in these areas. Tadpoles of the tree frog and the red-legged frog can be found here. Eventually, forests may develop in these areas.pacific

 

Few plants can survive the open sands of moving dunes. Look for mammal tracks of nocturnal animals like gray fox, skunk and raccoon crossing from the different areas. In some places, you can see that these moving dunes are slowly covering the dune forest. Large beach pine and Sitka spruce create an area that is surprisingly different and diverse.

 

Developed soils allow for thick plant growth, with huckleberry, silk tassel, red-flowering currant, salal. The forest is also home to many species of lichens, including puffy mats of reindeer lichens, more characteristic of northern forests.

 

Salt marshes and estuaries are recognized by biologists as among the most productive habitats in the world. The nutrient-rich waters form the basis of the salt marsh food chain. Plants such as pickleweed and salt grass are specially adapted to tolerating the salty conditions of a tidal area.

PHYSICAL GEOLOGY

HUMBOLDT BAY

Humboldt Bay, known to Wiyot People as Wigi, is the second largest enclosed bay in California. It is 14 miles long and 4.5 miles across at its widest point. The channel, approximately 30 feet deep, is located near the north end of the South Bay and connects the bay to the ocean providing daily interchanges of seawater. The bay consists of two wide but shallow northern and southern arms connected by a relatively narrow channel.

 

Arcata Bay, also called North Bay, is the larger of the two bay segments and covers a surface area of about 8,000 acres. Most of Arcata Bay is extremely shallow and about 4,500 acres of mudflats are exposed at low tide. The shallow mudflats are traversed with deeper channels formed by tributaries and erosion from tidal drainage. The South Bay is about 4,600 acres in size and is similar to Arcata Bay in character. It is also mostly shallow, and 2,400 acres of mudflats are exposed at low tide. In addition to abundant mudflats, lush eelgrass beds are located throughout the South Bay.

 

Humboldt Bay is an estuary, which is an area where saltwater and freshwater come together and mix. The bay receives most of its freshwater as runoff from a drainage area of approximately 288 square miles surrounding the Bay. The main tributary streams are: Jacoby Creek, Freshwater Creek, Elk River, and Salmon Creek. Many smaller streams also enter the Bay directly or by way of one of its tributary sloughs.

ECOLOGY

The two main habitats of the Humboldt Bay are mudflats and eelgrass beds. Eelgrass (Zostera marina) is not an alga but an angiosperm (flowering plant). It grows mostly in shallow water bays and is the main food source for black brant and other waterfowl species. Eelgrass beds also serve as important habitat and nursery areas for many species, a lot of which are important for sport and commercial fisheries. Some species that use eelgrass beds during part of their life include lingcod, Dungeness crab, halibut, Cabazon, English sole, Pacific herring, and several species of nearshore rock fish. There are also many animals that spend their entire life in the eelgrass such as bay pipefish, nudibranchs, and sea hares. The bay is also home to some marine predators, including brown smooth hounds, leopard sharks, and seven-gill sharks.

 

The intertidal mudflats of the bay are fully exposed at low tide and entirely covered by water at high tide. Plant life on the mudflats consists of eelgrass, and several species of algae including sea lettuce (Ulva spp.), rockweed (Fucus sp.), and polysiphonia (a red alga). Invertebrates are abundant in the mudflats and many birds depend on them food. Most invertebrates occur in the top 6 inches of the mud with approximately 97% being found in the upper 2 inches. The species and numbers of invertebrates present are determined by the sediment composition and location in relation to tidal submergence time. Some invertebrates that burrow in the mudflats include polycheate worms, clams, ghost shrimp, and fat innkeeper worms.

 

The mud of the bay has a rotten egg odor, and that the yellow-brown color of the mud surface differs from the blackish color of the mud underneath. Beneath the thin surface layer, little oxygen can penetrate and bacterial respiration is anaerobic. The by-product, hydrogen sulfide, smells like rotten eggs. The hydrogen sulfide reacts with iron sulfide to produce hematite, which is yellow-brown.

 

 

Invasive dense-flowered cordgrass (Spartina densiflora) has infested an estimated 90% of salt marshes in Humboldt Bay and the adjacent Eel and Mad River estuaries. The invasion of Spartina threatens the natural diversity and structure of salt marshes in Humboldt Bay as well as other estuaries along the west coast. You can read more about Spartina and salt marsh ecology, restoration methods, the status of Humboldt Bay National Wildlife Refuge's two-year refuge eradication project, and links to other west coast and international Spartina eradication efforts at: Spartina Invasion and Management.

SOLITARY BEES

Take a leisurely stroll through the local dunes on a warm day between April and September and your eyes will be met with a feast of wildflowers scattered across the sand in low-growing mats. Take a closer look, and you may notice a plethora of insects attending the blooms. Many insects, including beetles, flies, wasps, and bees, visit flowers to gather food in the form of pollen and nectar, and, while foraging, transfer pollen from one flower to another. This process, called pollination, results in fruit/seed production for the plant.

 

At least 67 percent of flowering plants today depend on insects for pollination. Bees in particular are important pollinators since their life history is closely linked to the flowers they service. Unlike wasps, which are carnivorous, bees rely on flower resources for all of their food, continuously foraging to fulfill both their own nutritional needs and those of their offspring.

 

If you are like most people, bee conjures up one of two images: the hard-working European honeybee (Apis mellifera), or its furry cousin, the bumblebee (Bombus spp.). However, most of the 20,000+ bee species in the world are solitary bees. In contrast to social bees, solitary bees have no colony or hive, no worker caste, and no stored resources. Instead, nests are the result of short-lived females working alone. During their 3-4 week lifespan, individual females provide 7-12 cells, each of which contains a single egg. These will then develop underground, and emerge the following spring to begin the cycle again.

 

There are about a dozen solitary bee species commonly found in our local dunes. Most are "generalists", visiting a wide variety of flowers to meet their nutritional needs. However, some species are "oligolectic". Bees of this type are picky about the plants they visit; they specialize in collecting pollen from only a few closely related plants. One of the few oligolectic species in the local dunes is the leaf-cutter bee (Megachile wheeleri).  This bee forages exclusively on flowers in the aster family. This bee is abundant in late summer when dune goldenrod blooms abundantly.

 

In addition to floral resources, solitary bees in this system are also dependent on the dunes for nest sites and materials. In the early spring, you may notice "silver bees" (Emphoropsis miserabilis) swarming over the sand and among the beach pea flowers. These are males searching for newly emerged females to mate with. The females, once mated, will dig nests up to a meter in depth in the open sand in which to lay their eggs.

 

Other solitary bees nest in the cryptogamic mats (mossy crust) of the foredune and utilize specific plant materials to line their shallow nests. For example, a "wool-carder" bee (Anthidium palliventre), gathers plant hairs from beach buckwheat to line its nests. Check out the leaves of this plant beginning in April when Anthidium emerges, and you may notice bare spots where the "wool" has been removed, a sign that this bee is nesting nearby. Another bee, Megachile wheeleri, also nests in the cryptogamic mat, but this bee uses leaf pieces cut from dune goldenrod to line its nests. Look for clumps of goldenrod with "missing" leaf pieces in late summer and you’re likely to find this bee foraging on neighboring plants.

 

Ceratina acantha is a bee that’s easy to miss since it is quite un-bee-like in appearance: tiny (5-9mm), black, and hairless. This "carpenter" bee hollows out old dune goldenrod stems to use as nest sites. If you are lucky enough to spot an active nest, you will most likely see a tiny black face peering up at you from inside the hollow stem.

Aside from the important service they provide as pollinators, bees are fascinating creatures. The next time you’re out enjoying the floral display in the dunes, remember to keep an eye out for our local bees and give them a nod of "thanks" for the part they play in maintaining floral diversity.

SOLITARY BEES

WESTERN SNOWY PLOVER

The western snowy plover (snowy plover) is a small shorebird, approximately the size of a sparrow. The Pacific Coast population of the snowy plover is a distinct sub-population that is Federally listed as threatened under the Endangered Species Act (ESA). The snowy plover breeding season extends March through September. Locally, snowy plovers can be seen nesting and rearing their young along the sandy shores, estuaries, and the lower stretches of the Eel River. Nests usually contain three tiny eggs, which are camouflaged to look like sand and barely visible to even the well-trained eye. While a variety of pressures make survival difficult for these birds, four factors are the most significant within Recovery Unit 2 (Del Norte, Humboldt and Mendocino Counties).

 

Predators are the most significant factor affecting reproductive success of snowy plovers. Small mammals, such as fox and raccoon, and birds, particularly the common raven, prey on the eggs and chicks of snowy plovers. Occasionally, birds of prey also take adults decreasing the entire population.

 

Loss and degradation of breeding habitat is another factor affecting snowy plovers. They prefer open, sandy areas with sparse and low lying vegetation for nesting. Due in large part to the spread of introduced plant species, principally European beach grass, the number of suitable nesting areas has diminished.