A Rocky Intertidal Desert at the Head of a Large Macrotidal Estuary in Quebec, Canada

Abstract

This article documents the widespread absence of sessile species in bedrock intertidal habitats at the head of the St. Lawrence Estuary, a large macrotidal estuary located in eastern Canada. Extensive observations revealed that no seaweeds or sessile invertebrates occurred anywhere (including cracks and crevices) on substrate areas that become exposed to the air during low tides. Only one sessile species, a green filamentous alga, was found submerged in tidepools. The lack of truly marine sessile species is likely explained by the very low water salinity of this coast, while the absence of sessile freshwater species on intertidal substrates outside of tidepools likely responds to a combination of oligohaline conditions during high tides and daily exposures to the air during low tides, which freshwater species are typically not adapted to. Influences of winter ice scour and coastal suspended sediments are likely secondary. Experimental research could unravel the interactive effects of these abiotic stressors. Overall, this “intertidal desert” could be a useful model system to further explore the boundaries of life on our planet.

Deserts are land areas with a limited plant cover and extensive bare substrate. Their occurrence is mainly determined by low precipitation, temperature acting as a moderating influence. Extreme cases are the Sahara, Namib, and Atacama deserts, which are devoid of vegetation across large extents of terrain [1].

Where land meets the sea, there is a unique environment known as the intertidal zone. Its ecology is defined by tides as, every day, low tide exposes intertidal habitats to the air while high tide keeps them under water. This daily alternation of periods of emersion and submergence has allowed for the occurrence of many species that are exclusively intertidal. Rocky intertidal habitats, in particular, are common around the world. In rocky intertidal habitats where bedrock is the main substrate, seaweeds and filter-feeding invertebrates such as barnacles and mussels are often the main space occupiers. These organisms are sessile, which means that they remain permanently attached to the substrate. They can cover large intertidal areas and can provide shelter and/or food for other sessile species and for mobile consumers. As a result, intertidal communities frequently exhibit a high biodiversity and complex food webs [2].

Low biodiversity can indeed occur in rocky intertidal habitats, but is restricted to harsh environments. Harsh conditions are common at the high intertidal zone, where organisms can experience intense desiccation, irradiance, and thermal stress during prolonged exposures to the air during the lowest tides. The limited number of species that can tolerate such conditions exhibit specific physiological or morphological adaptations [3,4]. Harsh conditions also occur in intertidal habitats that are exposed to powerful waves, which cause a strong hydrodynamic stress onto organisms. The intertidal species that can survive those conditions possess structural adaptations or grow in dense stands to limit the physical effects of waves [5,6,7]. In subpolar rocky intertidal habitats, a common example of environmental harshness is the scour that winter ice causes across intertidal areas when actively moved by wind and tides. In such habitats, sessile species can survive if they grow in cracks or crevices, where ice movement is low to null, depending on the size of those spaces [8,9].

The above examples are meant to point out that, even in stressful rocky intertidal habitats, a certain amount of biodiversity is still expected to occur. It is worth asking, though, whether exceptions may occur under truly extreme conditions. This article provides visual evidence of extreme rocky intertidal habitats with an almost complete absence of sessile species. The St. Lawrence Estuary is a large macrotidal estuarine system located in the province of Quebec, in eastern Canada. This estuary drains the waters mainly of the St. Lawrence River into the Gulf of St. Lawrence, which is ultimately connected to the Atlantic Ocean. Like all estuaries [10], there is a marked salinity gradient from zero or near zero at the head of this estuary to marine conditions at its mouth [11]. At the head of the St. Lawrence Estuary, on the coast of Quebec city, tidal amplitude is large, with a highest recorded water level of just above 7 m over chart datum [12]. In this part of the estuary, intertidal areas composed of bedrock are common. The intertidal slope of many of these places is relatively shallow, which, combined with the large tidal amplitude, results in extensive rocky intertidal habitats that span tens of meters from low to high elevations (Figure 1). On 28 May 2025, I examined these rocky intertidal habitats along a coastal stretch of hundreds of meters on the south shore of Orleans Island (Île d’Orléans), which is close to Quebec city. Observations were conducted during daytime near the time of the lowest tide of the day (0.1 m over chart datum for Saint-Jean I.O., 46.916° N, 70.897° W; [13]), which allowed for a systematic examination of almost the full vertical extent of the intertidal zone from high to low elevations along this coast.

On substrate areas that were exposed to the air during the low tide, seaweeds and sessile invertebrates were completely absent (Figure 1). These areas represent the vast majority of the substrate at these rocky intertidal sites, as tidepools (which retain water at low tide) are small and infrequent on this coast. Submerged in these tidepools, only one sessile species was found, a filamentous green alga (Figure 2). Permanent wooden structures that remain submerged during high tides but that are fully exposed to the air during low tides were also devoid of any sessile species (Figure 3). All in all, the nearly total absence of sessile organisms makes these rocky intertidal environments ecologically equivalent to the most extreme deserts known on land [1].

What could limit intertidal sessile life in such a dramatic way on this coast? Every winter, the St. Lawrence Estuary and the Gulf of St. Lawrence experience widespread freezing of surface waters [11,14], with a thick layer of ice covering the intertidal zone for weeks [15]. When the ice breaks into multiple pieces that are moved around by wind, tides, and currents, a great deal of intertidal scour results [16]. This severe form of disturbance removes the sessile organisms that developed on exposed intertidal areas during the preceding growth season. Thus, this phenomenon may contribute to the absence of sessile organisms on exposed substrate areas described above. In general, however, ice scour is weak or null in cracks and crevices so, on typical ice-scoured shores, sessile organisms often survive the winter by growing in those protected places [8,9]. However, on the coast of Orleans Island, sessile species were also entirely missing in cracks and crevices (Figure 1 and Figure 4), without even small specimens recolonizing the substrate as seen at the same time of the year on ice-scoured shores in the outer Gulf of St. Lawrence (Figure 5). Thus, another factor seems to be primarily responsible for the widespread lack of sessile organisms in the studied habitats. Suspended sediments are common in local coastal waters [11] and thus might contribute in some way, but likely not as the main driver either, as other rocky intertidal habitats washed by waters with a high sediment load do host abundant sessile species, as seen in the upper Bay of Fundy, also in eastern Canada (Figure 6).

Water salinity should be a more important factor. Due to the continuous freshwater input from the large St. Lawrence River, the head of the St. Lawrence Estuary exhibits waters with a very low salinity. At Orleans Island, for example, salinity is lower than 2–4 PSU under a regime of complete vertical mixing [11], which ensures that local intertidal habitats consistently experience very low water salinities during high tides. Thus, the absence of sessile species of marine origin in the studied habitats is likely a result of salinity levels below their tolerance limits [10,17]. These salinity levels, in turn, may also limit the occurrence of sessile freshwater species because most have no tolerance for even low salinities [18], which may be especially stressful for the pelagic larvae of freshwater bivalves [19]. Some freshwater organisms, however, do tolerate low water salinities to a certain extent [10]. Thus, another factor could play a more critical role in the absence of sessile freshwater species on intertidal substrates that are exposed to the air during low tides on this coast. In fact, the daily exposure to aerial conditions may alone represent a lethal physiological barrier for these organisms. This may be the case because freshwater species may have mechanisms to limit excessive water intake, but a strong ability to retain water if exposed to the air does not characterize them in general [20,21,22,23,24,25,26,27], especially if aerial exposures occur every day. Some freshwater bivalves (zebra mussel) living in tidal rivers can tolerate brief daily emersions but under a freshwater regime during high tides [28], which the studied habitats do not experience. Also, some freshwater invertebrates seek humidity during dry periods through burrowing [29], but that behavior cannot be displayed on rocky substrates like those examined here.

Overall, very low water salinity during high tides and (for freshwater sessile species) daily exposures to the air during low tides seem to underlie the occurrence of this “intertidal desert”. Influences of winter ice scour and coastal suspended sediments are likely secondary. Given the unique nature of these environments, experimental research to understand interactive effects of these abiotic stressors would be interesting. In that way, these extreme intertidal environments could constitute a useful model system to further explore the boundaries of life on our planet.