.............................................................. PAGE 157 ▼ T8.2 FRESHWATER ENVIRONMENTS Surface fresh water can be described according to with the abiotic environment forms the basis of an two distinct categories: lentic (standing water) and ecosystem. lotic (running water). Lentic environments include Lake ecosystems are dominated by autotrophic surface waters, such as lakes, ponds and wetlands. organisms (i.e., self-nourishing organisms with the Lotic environments comprise rivers or streams. ability to produce organic material from simple Freshwater environments also occur as continuously chemical compounds and sunlight) and are domi- moving groundwater which has percolated through nated by phytoplankton, periphyton and aquatic the upper layer of soil to underground storage areas plants. Lakes can also receive nutrients from outside (aquifers). Groundwater can naturally reach above sources and may include meteorological, geological ground as a spring. or biological inputs to the system.1 This Topic deals with surface and underground River ecosystems are dominated by heterotrophic aquatic freshwater environments. Wetlands are dis- organisms (i.e., those requiring organic material or cussed in T8.3. food from other sources) and are characterized by input of detritus from terrestrial sources. Larger sys- SURFACE WATER tems which are exposed to sunlight can have au- totrophic components. Primary production from al- Lakes and ponds are formed where an interruption gae and rooted plants then becomes an important of the drainage pattern, either by the formation of a energy source. basin or by a barrier, restricts the flow of water. Water in surface channels forms the rivers and Water Coverage streams which ultimately discharge into the ocean. Approximately 5 per cent of the land area of Nova Scotia is covered by fresh water in the form of lakes, T8.2 Ecology of Surface Water rivers and wetlands, representing a total of 2408 km2. Freshwater Both lentic and lotic environments support diverse There are 6674 lakes in the province which are greater Environments biotic communities or groups of organisms living in than one hectare in surface area, with a mean surface a given area. The interaction of a biotic community area estimated at 34 hectares.2 Water coverage is not Chignecto Bay Unit 581 Kejimkujik Park Unit 311 Unit 710 Advocate Bay Minas Channel Figure T8.2.1a: Abundant surface water in the granite and quarzite areas Figure T8.2.1b: Surface water coverage of the permeable rocks of the of southwestern Nova Scotia (Region 400). Chignecto Peninsula where lakes are scarce. © Nova Scotia Museum of Natural History Topics: Natural History of Nova Scotia, Volume I PAGE .............................................................. 158 ▼ T8.2 Freshwater Environments Plate T8.2.1: Mavilette, Digby County (District 820). A small stream flowing through lowlands, showing meander and tidal marsh development. Photo: LRIS evenly distributed throughout the province, as the LAKES southwestern section has many more lakes than the northern portion. In general, surface water is more Origin of Lakes abundant on the granite and quartzites of the Atlan- The majority of lakes in the province are of glacial tic Interior (Region 400) than on the more permeable origin and are generally aligned with the direction of sedimentary formations in the Carboniferous and the ice movement. The lakes of Nova Scotia may be Triassic lowlands (Regions 500 and 600) and basalt categorized by their origin as follows: formations of the Bay of Fundy (Region 700) (see • Glacial Lakes—formed either as a basin scoured Figure T8.2.1a and b). out by ice movement or by the interruption of Table T8.1.1 in Freshwater Hydrology indicates the original drainage by the deposition of the amount of water coverage per primary water- glacial till. shed across the province. In some watersheds the • Oxbow Lakes—formed when river meanders are freshwater coverage has been substantially increased cut off from the main channel, examples can be by the construction of dams. found in the Stewiacke Valley (District 510). • Levee Lakes—formed when river sediments Natural History of Nova Scotia, Volume I: Topics © Nova Scotia Museum of Natural History .............................................................. PAGE 159 ▼ deposited during periods of high water cause a Classification of Lakes separate waterbody to be formed. Lakes may be classified according to their trophic • Solution Lakes—formed from dissolving status (i.e., their available phosphate content and materials such as salt, limestone and gypsum. biological productivity): These lakes can be alkaline and have a higher salt content than most other lakes and occur • Oligotrophic Lakes—those with low phosphate particularly in the Windsor (Sub-Unit 511a) and concentrations and resulting low productivity Margaree (Unit 591) areas. Gypsum sinkholes • Mesotrophic Lakes—those with moderate found in karst topography may contain solution phosphate concentrations and resulting lakes (see T3.4). moderate productivity • Barrier or Barachois Lakes—these are created • Eutrophic Lakes—those with high phosphate mainly behind sand or gravel bars in coastal concentrations and resulting high productivity areas and have higher salinity than fresh water. This brackish water condition either is due to Highly coloured, low pH lakes resulting from high tidal flow or is the result of salt spray. Prime humic concentrations are termed dystrophic. These examples are found between Gabarus and lakes can have nutrient concentrations and produc- Fourchu Harbour on Cape Breton Island tivity within the trophic states defined above. (District 870). Both clearwater and highly coloured • Beaver Ponds—these are constructed and (brownwater) lakes exist in Nova Scotia; however, maintained by beavers but are regularly the latter predominate. Most lakes in the province abandoned if and when the local food supply are oligotrophic, although some have become eu- (i.e., poplar, willow, birch and alder) within trophic due to the impacts from agricultural runoff, reach of the water is exhausted. Some persist domestic sewage and other human activities. for years; others is only used during the winter Eutrophication is the natural process of lake “aging,” (see Plate T11.11.1). whereby a lake shows a gradual increase in nutrient Human-made Lakes concentrations and productivity, slowly infilling with • Dammed reservoirs—created principally to accumulated organic material. In extreme cases, regulate water flow for lumbering, flood the decay of the plant biomass can remove oxygen to T8.2 control, municipal water supplies or power the detriment of the other organisms. Hydrogen sul- Freshwater generation and usually contain substantial phide and other gas production can cause unpleas- Environments quantities of coarse to fine woody debris. Lake ant odours. The process can be greatly accelerated Rossignol, in Sub-Unit 412a, is a good example by human activities and is known as cultural of this occurrence. Waterfowl impoundments eutrophication. can increase the area of open water in marshes such as those found in the Tantramar Marsh Succession and Zonation area near Amherst (Unit 523). In geological terms, lakes in Nova Scotia are only • Excavated lakes—formed through quarrying temporary features on the landscape, generally and other related activities for example in the infilling to become wetlands, such as peat bogs. This gravel quarries at North River near Truro. is the primary or initial successional stage which may lead to the development of an organic mat within a wetland system.3 Peatlands in Nova Scotia were formed in this manner. In some systems (e.g., marshes), the deposition is primarily comprised of mineral sediments. Refer to T8.3 for continued dis- cussion on wetlands and succession. Typically, lakes have three distinct zones: the lit- toral or lake edge (hydrosere), the limnetic or surface water, and the profundal or deep water (see Figure T8.2.2). In deep-water lakes, the water column can © Nova Scotia Museum of Natural History Topics: Natural History of Nova Scotia, Volume I PAGE .............................................................. 160 ▼ Depth (m) 0 Limnetic (Open Water) Epilimnion 2 Metalimnion (Summer thermocline) 4 Hypolimnion 6 Profundal 8 10 Littoral Bottom 12 (Edge/Hydrosere) 14 16 Benthic Figure T8.2.2: Ecological features of a large lake, seen in diagramatic cross section. also be divided into three zones: the surface water or the sediment it carries is deposited as silt, sand or epilimnion, the bottom water or hypolimnion, and mud.4 At this stage, a river is referred to as mature, the area of transition in between called the with meanders which form in large and small loops metalimnion. Thermal stratification can occur where as the channel erodes its floodplain (e.g., Meander the surficial zone becomes much warmer than the River in Sub-Unit 511a). Meanders sometimes be- profundal zone during the summer, due to atmos- come so exaggerated that they cut off from the main T8.2 pheric warming of surface waters and lack of mixing. river channel to form oxbow lakes (see Figure T8.2.3). Freshwater This creates an area with a steep temperature gradi- The rejuvenation of a river sometimes occurs Environments ent between the two zones, called the thermocline. when the it encounters a geological obstacle pro- When these conditions exist, the surface waters can- not readily mix with the bottom layer and thus the lake bottom may become oxygen deficient. Larger