Changes in Marine Salinity Levels

Introduction

Salinity refers to the dissolved salt content of a body of water. Marine salinity levels are influenced by a number of factors including rainfall, evaporation, inflow of river water, wind, and melting of glaciers. Salinity can have a great impact on the type of organisms that live in a body of water, as examined in the Case Studies section below. Additionally, salinity plays a critical role in the water cycle and ocean circulation. The exact influence of salinity on these two phenomena is explained in this video by NASA. Scientists use robotic floating devices that collect information about ocean salinity and temperature in order to study the water cycle. A recent study by Durack et al. on the intensification of the water cycle as a result of ocean salinity changes in the past fifty years shows that the water cycle has since intensified by 4%, twice the rate predicted by models. Furthermore, the study predicts that a 2°C increase in global temperature would result in a 16% intensification of the water cycle. Similarly, a 3°C increase would see a 24% intensification. The following figure, which shows surface salinity changes from 1950 to 2000, was published along with the study. Regions becoming saltier are depicted in red while blue represents regions becoming fresher.

Case Studies

• A Change in the Freshwater Balance of the Atlantic Ocean over the Past Four Decades
• The Impact of Climate Change on the Adaptation of Marine Fish in the Baltic Sea
• Effects of Salinity on Oxygen Consumption of Cyprinodon Variegatus
• Effect of Salinity on Cadmium Uptake by the Tissues of the Shore Crab Carcinus Maenas
• Effect of Salinity on Freshwater Ecosystems in Australia
  

A Change in the Freshwater Balance of the Atlantic Ocean over the Past Four Decades

The salinity of oceans, particularly the Atlantic, has changed over the past four decades. When researchers measured the change in salinity levels of a “long transect (50° S to 60° N) through the western basins of the Atlantic Ocean” (Curry, 2003). They found that the oceans freshened in areas north of 40° N and south of 25° S. The region between 35° N and 25° S, on the other hand, experienced an increase in salinity. The freshening of the ocean can be attributed to “enhanced wind-driven exports of ice or fresh water from the Arctic, increased net precipitation rates, and/or elevated volumes of continental runoff from melting ice” (Curry, 2003). Conversely, the increase of salinity levels could be the result of altered circulation between surrounding waters, intensified trade winds and/or higher evaporation rates due to warmer ocean surfaces. Salt cannot be gained nor lost through the atmosphere, so an increase in salinity can only occur via (1) a transfer of additional salt from surrounding waters or (2) the removal of fresh water. Because surrounding waters do not exhibit the corresponding decrease in salinity, the increase in salinity levels must be due to higher evaporation rates.

The Impact of Climate Change on the Adaptation of Fish in the Baltic Sea

Climate change has influenced the development and evolution of marine species in the Baltic Sea since the last glaciation. The distribution and abundance of marine fish in this area is heavily dependent on the salinity, temperature, and oxygen content of the different water layers. The salinity and oxygen content of the Baltic Sea are determined by the intensity of water exchange between it and the North Sea. Water exchange, in turn, is controlled by meteorological processes which could change along with the climate. Because salinity and oxygen content can vary significantly from year to year, successful establishment of the Baltic Sea by marine species is difficult. For instance, “of about 120 marine fish species in the North Sea, fewer than 30 occur permanently and can reproduce in [the Baltic Sea]” (Ojaveer, 2005). These numbers are quiet alarming since they show that less than one quarter of marine species in the vicinity of the Baltic Sea have been able to adapt to the salinity levels which are constantly changing as a result of climate change. The adaptations employed by the few marine species that have been able to establish themselves in the Baltic Sea are very interesting. Species with floating eggs have altered their specific gravity/egg diameter ratios effectively changing the density of their eggs. The spawning period of some species in the Baltic Sea starts earlier and lasts longer than that of their counterparts in the west. Additionally, these species mature faster and exhibit higher reproductive ability. The few marine species that have been able to adapt may soon see those adaptations become obsolete as the salinity levels change beyond the normal year-to-year range. This is a great case study of how climate change may semi-directly affect marine species in the Baltic Sea.

Effects of Salinity on Oxygen Consumption of Cyprinodon Variegatus

The sheepshead minnow (Cyprinodon varuegatus) is euryhaline, meaning it is able to adapt to a wide range of salinities. The minnows are commonly found in the shoreline (1-2 cm deep) of saline lakes in San Salvador Island. Freshwater influx and evaporation can cause changes in the salinity of the lakes. In particular, heavy rains can quickly dilute the shallow waters. Researchers studied the extent to which changes in osmotic regulation (as a result of salinity differences) are accommodated by shifts in metabolic rates (measured by changes in oxygen consumption). The experiment was carried out by placing the minnows in water of varying salinity (10% and 35%) and measuring changes in dissolved oxygen content to determine the amount of oxygen consumed. Results showed that oxygen consumption was greatest among the minnows exposed to the dilute (10%) environment. Researchers concluded that reduced salinities are associated with increased metabolic rates, as these elevated rates are required for osmotic adjustment. This study is of importance as it quantifiably demonstrates how changes in salinity can affect the metabolism of marine organisms such as minnows.

Effect of Salinity on Cadmium Uptake by the Tissues of the Shore Crab Carcinus Maenas

Cadmium is a naturally occurring element which is present in ocean water in trace quantities. Although cadmium does not have any physiological functions, many marine organisms are known to accumulate the element in their bodies. The rate of accumulation is affected by both temperature and salinity. D. A. Wright used Carcinus maenas, a euryhaline shore crab, to study the effects of salinity on the accumulation of cadmium by marine organisms. The experiment consisted of exposing the specimens to two solutions of differing salinity and measuring their cadmium uptake throughout a 68-day period. Wright examined cadmium concentration in tissue samples taken from the carapace, hepatopancreas, gills, and chelae muscle. According to the data, salinity had no effect on the cadmium concentration of the hepatopancreas or the chelae muscle. However, cadmium accumulation in the carapace and gills was significantly higher in dilute sea water. These results are consistent with those of other studies. Similar studies conducted on Mytilus edulis (blue mussel) and Artemia franciscana (brine shrimp) also show an increase in cadmium accumulation at lower salinities. Additionally, a study published by David Engel in an issue of Environmental Health Perspectives affirms that decreased cadmium toxicity is observed at higher salinities. While these studies may not address climate change directly, their findings can be combined with the ideas discussed in the previous case studies to formulate some predictions. The study by Ruth Curry showed that certain bodies of water are becoming less saline as a result of increased runoff from melting ice. The present case study showed that decreased salinity leads to increased cadmium accumulation across a wide array of marine organisms. Humans likely consume some of the marine organisms whose cadmium uptake is influenced by ocean salinity. Therefore, one can predict that humans may become exposed to more cadmium as bodies of water become more dilute. This prediction is terrifying considering cadmium is toxic and may cause kidney and developmental disorders.

Effect of Salinity on Freshwater Ecosystems in Australia

Under normal circumstances, salt enters bodies of water either through groundwater or the atmosphere, the latter occurring simply when salt is transported by wind and rain. The groundwater route involves the erosion and weathering of soils and rocks as underground water flows. As underground water levels rise, more salt is dissolved and deposited in nearby bodies of water. Large scale clearing of native vegetation is one reason why the underground water levels may rise. Native vegetation would normally trap water before it made its way to the underground watertable. The lack of native vegetation, often replaced by pastures, increases the amount of water that reaches the watertables causing underground water levels to rise and salinity to increase. Changes in salinity levels affect not only marine life but also the physical component of aquatic ecosystems. From a physical environment standpoint, increased salinity levels may help remove particles from the water column, which in turn increases water clarity. Clearer water means more light penetration and therefore higher rates of photosynthesis. Increases in photosynthesis may cause blooms of cyanobacteria which could alter dissolved oxygen levels. Changes in salinity levels can also affect the abundance and solubility of some minerals, particularly the major ionic species. The ionic composition of water “modifies the way biota respond to high salinities,” and may also have a slight effect on water acidity (Nielsen et al., 2003). As for marine life, increased salinity levels may slow organismal growth and reduce species richness. Models predict the Australian freshwater rivers and wetlands will experience salinity levels ranging from 500 mg L-1 to above 10,000 mg L-1 over the next 50 years. These increased salinity levels affect each form of life differently. Algae, for example, cannot tolerate salinity levels above 10,000 mg L-1 which means diversity would be severely decreased if salinity levels reach this level. Plants, on the other hand, can only tolerate salinity levels up to 1,000 mg L-1 before their growth and reproduction is impaired. At levels above 4,000 mg L-1 , halophytic species replace all non-halophytic plant species, reducing diversity. Invertebrates are also affected by increased salinity levels, displaying a rapid decrease in diversity when salinity levels exceed 1,000 mg L-1 but a less rapid decline at levels above 10,000 mg L-1. Lastly, most Australian fish appear to be tolerant of salinity levels well above 3,000 mg L-1. Unfortunately, salinity levels above 3,000 mg L-1 do affect juveniles and eggs, often reducing survival rate by 50%. Although the study by Nielsen has some upsetting results, the findings can help set salinity level limits that could give conservation agencies a more quantitative value to work with in their efforts to save Australian marine life.

Further Reading:

References:

Barton, M. "Effects of Salinity on Oxygen Consumption of Cyprinodon variegatus." Copeia , Vol. 1987, No. 1 (1987), 230-232. Web

Curry, R., B. Dickson, and I. Yashayaev. "A Change in the Freshwater Balance of the Atlantic Ocean over the past Four Decades." Nature 426.6968 (2003): 826-29. Web.

Ojaveer, E., and M. Kalejs. "The Impact of Climate Change on the Adaptation of Marine Fish in the Baltic Sea." ICES Journal of Marine Science 62.7 (2005): 492-500. Web.

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