Shell growth in estuaries

Shell growth in estuaries is an aspect of marine biology that has attracted a number of scientific research studies. Many groups of marine organisms produce calcified exoskeletons, commonly known as shells, hard calcium carbonate structures which the organisms rely on for various specialized structural and defensive purposes. The rate at which these shells form is greatly influenced by physical and chemical characteristics of the water in which these organisms live. Estuaries are dynamic habitats which expose their inhabitants to a wide array of rapidly changing physical conditions, exaggerating the differences in physical and chemical properties of the water. Estuaries have large variation in salinity, ranging from entirely fresh water upstream to fully marine water at the ocean boundary. Estuarine systems also experience daily, tidal and seasonal swings in temperature, which affect many of the chemical characteristics of the water and in turn affect the metabolic and calcifying processes of shell-producing organisms. Temperature and salinity affect the carbonate balance of the water, influencing carbonate equilibrium, calcium carbonate solubility and the saturation states of calcite and aragonite. The tidal influences and shallow water of estuaries mean that estuarine organisms experience wide variations in temperature, salinity and other aspects of water chemistry; these fluctuations make the estuarine habitat ideal for studies on the influence of changing physical and chemical conditions on processes such as shell deposition. Changing conditions in estuaries and coastal regions are especially relevant to human interests, because about 50% of global calcification and 90% of fish catch occurs in these locations. A substantial proportion of larger marine calcifying organisms are molluscs: bivalves, gastropods and chitons. Cnidarians such as corals, echinoderms such as sea urchins, and arthropods such as barnacles also produce shells in coastal ecosystems. Most of these groups are benthic, living on hard or soft substrates at the bottom of the estuary. Some are attached, like barnacles or corals; some move around on the surface like urchins or gastropods; and some live inside the sediment, like most of the bivalve species. Minute pelagic species in the phyla Foraminifera and Radiolaria also produce ornate calcareous skeletons. Many benthic mollusks have planktonic larvae called veligers that have calcareous shells, and these larvae are particularly vulnerable to changes in water chemistry; their shells are so thin that small changes in pH can have a large impact on their ability to survive. Some holoplankton (organisms that are planktonic for their full lives) have calcareous skeletons as well, and are even more susceptible to unfavorable shell deposition conditions, since they spend their entire lives in the water column.

Details of carbonate usage

There are several variations in calcium carbonate (CaCO3) skeletons, including the two different crystalline forms, calcite and aragonite, as well as other elements which can become incorporated into the mineral matrix, altering its properties. Calcite is a hexagonal form of CaCO3 that is softer and less dense than aragonite, which has a rhombic form. Calcite is the more stable form of CaCO3 and is less soluble in water under standard temperature and pressure than aragonite, with a solubility product constant (Ksp) of 10−8.48 compared to 10−8.28 for aragonite. This means that a greater proportion of aragonite will dissolve in water, producing calcium (Ca2+) and carbonate (CO2−3) ions. The amount of magnesium (Mg) incorporated into the mineral matrix during calcium carbonate deposition can also alter the properties of the shell, because magnesium inhibits calcium deposition by inhibiting nucleation of calcite and aragonite. Skeletons with significant amounts of magnesium incorporated into the matrix (greater than 12%) are more soluble, so the presence of this mineral can negatively impact shell durability, which is why some organisms remove magnesium from the water during the calcification process.

Influencing factors Food availability can alter shell growth patterns, as can chemical cues from predators, which cause clams, snails and oysters to produce thicker shells. There are costs to producing thicker shells as protection, including the energetic expense of calcification, limits on somatic growth, and reduced growth rates in terms of shell length. In order to minimize the significant energetic expense of shell formation, several calcifying species reduce shell production by producing porous shells or spines and ridges as more economical forms of predator defense. Temperature and salinity also affect shell growth by altering organismal processes, including metabolism and shell magnesium (Mg) incorporation, as well as water chemistry in terms of calcium carbonate solubility, CaCO3 saturation states, ion-pairing, alkalinity and carbonate equilibrium. This is especially relevant in estuaries, where salinities range from 0 to 35, and other water properties such as temperature and nutrient composition also vary widely during the transition from fresh river water to saline ocean water. Acidity (pH) and carbonate saturation states also reach extremes in estuarine systems, making these habitats a natural testing ground for the impacts of chemical changes on the calcification of shelled organisms.

Carbonate and shell deposition

Calcification rates are largely related to the amount of available carbonate (CO32−) ions in the water, and this is linked to the relative amounts of (and reactions between) different types of carbonate. Carbon dioxide from the atmosphere and from respiration of animals in estuarine and marine environments quickly reacts in water to form carbonic acid, H2CO3. Carbonic acid then dissociates into bicarbonate (HCO−3) and releases hydrogen ions, and the equilibrium constant for this equation is referred to as K1. Bicarbonate dissociates into carbonate (CO2−3), releasing another hydrogen ion (H+), with an equilibrium constant known as K2. The equilibrium constants refer to the ratio of products to reactants produced in these reactions, so the constants K1 and K2 govern the relative amounts of different carbonate compounds in the water.

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