Perspective Chapter: Remarks on the Relationship of Mangrove Recruitment and Thrombolithic Development in Coastal Lagoons

Abstract

Thrombolithic formations are sedimentary structures generated by precipitation, consolidation, and lithification of sediments and caused by the evolution of cyanophyte mats that promote coastal ground formation. These, and unlithified thrombolithic platforms or pro-thrombolites, are closely associated with mangrove trees and forests that use them as an anchoring substrate. So, in contrast with the suggested role of mangroves as coastal ground formers, here, evidence is provided that supports the hypothesis that mangroves are actually opportunists that eventually colonize various phases of the evolution of thrombolites. Photographs of mangrove specimens found affixed on various pro-thrombolithic structures, including the first thrombolite platform described in 2005, constitute palpability for the above. Also, lithified structures have been identified as rhizoliths belonging to mangrove trees as evidence that colonization on thrombolite platforms has occurred also during earlier geological events. It is desirable that specialists on mangrove ecology consider analyzing and assessing this theory from their own perspective.

1. Introduction

Because thrombolites and other microbialites are well represented in the ancient geological record, information regarding their presence and distribution is critical for the understanding of past environments, as well as current landscapes, and may also serve to detect paleoecological issues. Living thrombolites or pro-thrombolites developing today exerts important changes in the geomorphological development of coastal areas, including landscape changes and alteration of coastal water currents and upper tidal limits [1]. Moreover, fossil and living microbialites are important references for explaining midterm environmental changes, such as the processes leading to the formation of coastal lagoons, coastal ground formation, and establishment of mangrove forests [2].

2. Thrombolithic and pro-thrombolithic formations discovered in lacunar environments

Thrombolites are defined as biosedimentary structures generated by the entrapment, binding, and consolidation of sediments as a result of the metabolic activity and growth of microorganisms, particularly cyanobacteria, but unlike the structure of stromatolites, in thrombolites no lamination is present, instead, there is a conglomerated matrix of cemented sediments; hence, the term “trombo” meaning clotted [3, 4, 5, 6]. As with stromatolites, thrombolites are not difficult to recognize, especially by the curious look of science students, who immediately observe certain irregularities in a landscape. However, the sedimentary structures described and discussed here-on required a bit more heuristic zest in order to be recognized, because of their peculiar forms of development.

In 2005, a serendipitous observation triggered an exploration that revealed that most of the beaches of Ensenada de La Paz (coastal lagoon), Baja California Sur (BCS) were fringed by extensive unlithified sedimentary platforms, or irregular solidified mud spills (Figure 1) that were deemed as “living (active) thrombolites” [7]. Thus, it was inferred that these sedimentary structures were generated also by the entrapment, binding, and consolidation of sediments as a result of the metabolic activity and growth of filamentous cyanophytes, and were later called pro-thrombolites, inasmuch they were considered precursors of thrombolithic formations.

Figure 1.

First thrombolithic platform recorded at El Conchalito, La Paz lagoon in 2005 that shows seaward accretion.

The clotted sediments of the pro-thrombolithic platforms at La Paz lagoon exhibited inclusions of shell fragments, irregularly packed, and surrounded by a sheet of active cyanobacteria [1] that some thrombolithic platforms lack because of prolonged exposure or being covered by more recent sediments [8]. However, most pro-thrombolithic and many thrombolithic formations may still present a conspicuous sheet of filamentous cyanobacteria, mainly Microcoleus chthonoplastes Thuret ex Gomont and Lyngbya aestuarii Liebman ex Gomont. Other common cyanophyte taxa are Spirulina sp., Oscillatoria spp., Calothrix, and unicellular forms, such as Chroococcus sp. and Aphanotece sp. [1]. Also, a diverse arrangement of benthic diatoms living on the surface of the thrombolithic platforms was described [9]. However, further inspection of the clotted matrix showed no traces of embedded diatoms [2].

From a wider view, soft extensive benthic mats observed to be formed roughly by the same filamentous cyanobacteria species cover most of the intertidal and subtidal shores (Figure 2a and b) of the La Paz lagoon, BCS., Mexico [9]. These are associated with soft, conglomerated (vertically accreted) mats and more or less consolidated platforms.

Figure 2.

a) Extensive conglomerated cyanophyte mats located at El Centenario in the La Paz lagoon, BCS. b) Cross-section of a cyanophyte mat from El Conchalito, La Paz lagoon. c) Panoramic view of an extensive uncovered pro-thrombolithic platform, showing an anchored individual of Laguncularia racemosa. d) View of the same pro-thrombolithic platform covered by macroalgae.

Due to their rocky appearance with a dark greenish covering [1], these seaward extensions had been hitherto unnoticed, and may still be overlooked when covered by filamentous macroalgae (Figure 2c and d). The said sedimentary structures may be in the form of muddy extensions, lithified platforms (Figure 3), and/or fragments of assorted sizes resembling mudstone, but in all cases, they occur as seaward soft or hardground accretions. In the case of lithified structures, microscopic analysis of their clotted matrix indicated that they were thrombolithic in nature [1]. Although these were first described around the shores of La Paz lagoon; thrombolites (rocky) had been recorded earlier further north in BCS [10].

Figure 3.

Smooth lithified thrombolithic platform at El Mogote, La Paz lagoon.

Following the identification and description of these microbialithic structures [1, 9], several issues were addressed, such as How do they originate? Are they a product of lacunar processes? How do they evolve? Are they solely evidence of past microbial activity? Or do they play a current ecological/geological role? And mainly, the one concerning the importance of pro-thrombolithic processes in the formation of coastal lagoons [1]. All these and other issues were further supported with evidence from various localities. But the most recent concern addressed their role as the primary coastal ground forming agents, a function commonly attributed to mangroves, but which were actually used opportunistically by mangrove recruits as anchoring substrate [2, 8, 11].

2.1 Association of thrombolithic formations in mangroves ecosystems

The above observations comprise the inspection of several coastal lagoons in the NW region of Mexico (Figure 4). The shores of the explored coastal lagoons are characteristically populated by marsh vegetation, mainly Salicornia spp. and three species of mangrove: Avicennia germinans (Linnaeus) Stearn, Rhizophora mangle Linnaeus, and Laguncularia racemosa (Linnaeus) Gaertner, as mentioned above, closely associated to the pro-thrombolithic grounds. Particularly targeted were the ecosystems at La Paz lagoon and Bahía Magdalena, because in the former the initial observations were made and have been more closely studied. While in the latter it is where the (big-time) hypothesis that “thrombolites and pro-thrombolites should be found in other coastal lagoons” was first favorably contrasted. Eventually, the said structures were found later elsewhere, also associated with mangroves, including San Ignacio lagoon [1], Bahía Concepción (BCS), and more conspicuously in Sonora. Likewise, thombolithic platforms were found in extra-lacunar environments, where pre-ichnofossil evidence of past mangrove presence was also observed [2].

Figure 4.

Localities where observations on thrombolithic structures associated with mangrove forests have been made: 1) La Paz lagoon and surrounding sites (inset); 2) Magdalena bay; 3) Concepcion bay; 4) San Ignacio lagoon; and 5) Sonora.

In the La Paz lagoon, located on southeastern (gulf) coast of the Baja California Peninsula, Avicennia germinans is the most common (black) mangrove though red mangrove (R. mangle) is also conspicuous, while white mangrove (L. racemosa) is scarce. In most cases, where thrombolithic and pro-thrombolithic platforms are present, mangroves are found associated with them mainly as anchoring substrate (Figure 5a). Also, consolidated pro-thrombolithic blocks of 50 cm high (approx.) were discovered (at Marina Sur) associated with well-established A. germinans trees, nearby the first platform observed in the lagoon (Figure 5b). These blocks were broken-off from a platform structure, and those more exposed to wave energy appeared whimsically eroded, while others seemed worn particularly around the lower part, reminding the bun shaped stromatolites of Shark Bay, Australia [2].

Figure 5.

a) First observed association between mangrove (Avicennia germinans) and a thrombolithic platform at El Conchalito, La Paz lagoon. b) Pro-thrombolithic platform, broken down into blocks, associated with mangrove trees at La Marina, La Paz, lagoon. c) Broken-off thrombolithic blocks from extensive platforms at Calerita, outside La Paz, lagoon in Bahia de La Paz, Mexico, which appears to have rested on an earlier pro-thrombolithic platform. d) Ichnofossils, allegedly of anchored mangrove roots on thrombolithic platforms at Calerita outside La Paz lagoon.

Also, because lagoon-like environments are evident on the island coves in the southern Gulf of California, explorations were done at Isla Espiritu Santo and Isla San Jose, where mangrove forests are well established [2]. Additionally, observations on thrombolithic-like platforms and blocks outside lacunar environments in the Gulf of California is graphically documented northwest (Las Brisas) of the La Paz lagoon [2], and to the north (Calerita), where two distinct geological levels of formation have occurred (Figure 5c) inside Bahia de La Paz in the southern Gulf. In this case, an issued null hypothesis stated that thrombolithic structures would not be found, inasmuch as the explored sites were not lacunar environments. However, as mentioned above, although no mangroves currently occur in these two extra-lacunar localities, evidence of their past presence on the thrombolithic platforms as ichnofossils is present (Figure 5d).

In the case of Bahia Magdalena lagoon, located on the southwestern (Pacific) coast of the Baja California peninsula, its shores are densely populated by mangrove forests of the same three species, though much bigger, also closely associated with thrombolithic structures [2]. But where, unlike in the La Paz lagoon, L. racemosa is very common. There, several sites were explored, specifically around Puerto San Carlos where thrombolithic platforms were exposed (Figure 6), while structures assumed to represent transitional stages between conglomerated mats and thrombolites were recorded landward (Figure 7a) among and behind the mangrove trees line [2].

Figure 6.

First thrombolithic platforms recorded at San Carlos, Bahía Magdalena, BCS. These findings are backed up by the posed “big time” hypothesis on their expected occurrence within other coastal lagoons.

Figure 7.

a) Partially uncovered pro-thrombolithic platforms found behind the mangrove line at San Carlos, Bahia Magdalena. b) Thrombolithic blocks at Santa Rosa estuary, Sonora, associated with extensive mangrove forests. Image by Diana Luque (Ecoturismo Seri, CIAD and CtamCoyai, a.C.). c) Author and godson, with conspicuous fresh-water stromatolites peeking out from behind at Bacalar lagoon. d) the author on trombolithic blocks at Bacalar, Quintana Roo long ago excavated to make way for the local Malecon road.

Observations on the coast of Sonora were based both on direct inspection and indirect evidence. In the first case, a quick exploration was carried out at Estero El Soldado in Guaymas, where the same conglomerated formations were noted along with thrombolithic-like platforms, also associated with mangroves [2]. However, the above initiative was triggered out of viewing a slide, during a symposium, from Estero Santa Rosa, Sonora shows Seri natives seating on thrombolithic blocks within a copious mangrove environment (Figure 7b). The said blocks are quite similar to those found along the Malecon road at Bacalar, Quintana Roo, Mexico, where mangrove forests are extensive, even though more than 50% is reported to be have been destroyed. There, stromatolites have become celebrities since being described [12], but not thrombolites (Figure 7c and d) for which a single indirect study (on epilithic diatoms) is known [13]. Unfortunately, the Santa Rosa site has not yet been accessed and samples from either site in Sonora are not available, and neither from Bacalar where they lack the required minimum attention (Figure 8). Notwithstanding, in spite that sedimentological and other direct examinations are pending, the association of pro-thrombolithic formations and mangroves is clearly confirmed.

Figure 8.

Faith for thrombolithic remains? And a consequence of lack of minimum care?

This constant correspondence between thrombolithic structures and mangroves suggested the hypothesized dependence of the latter. However, it was reinforced by the occurrence of actual recruiting of three mangrove trees (two Avicennia germinans and one L. racemosa) during the term of the present investigation on the first thrombolithic formation recorded (Figure 9a), where rhizolith-like structures were observed below the level of the current platform [11]. Likewise, another recruit of black mangrove was documented on the second recorded platform, where a much older specimen of A. germinans was present (Figure 9b), and by the incipient ichnofossils-like structures from El Mogote, the sand-bar defining the La Paz lagoon (Figure 9c and d). These complex structures seemed derived from the interaction between dead mangrove root systems and pro-thrombolithic activity and are most likely incipient rhizoliths. As mentioned above, in the northern part of BCS, in Bahia Concepción, fossil deposits include stromatolites and thrombolites [10] as well as rhodolith beds, muddy sandstone, rhizoliths, and mangroves [14].

Figure 9.

a) Updated images (2022) of successful mangrove recruiting of two Avicennia germinans individuals and one Laguncularia racemosa on the first thrombolithic platform recorded at El Conchalito, La Paz lagoon, around 2015. b) Recruiting of an individual of A. germinans on the second thrombolithic platform recorded at El Conchalito, La Paz lagoon occurring around 2015. c) Conglomerate of ichnofossils-like structures of mangrove roots (rhizoliths) topped by a thrombolithic plate at El Mogote, La Paz lagoon. d) Incipient ichnofossils assumed to be rhizolith-precursors of mangrove roots from El Mogote, La Paz lagoon.

2.2 Putative role of mangroves in coastal ground formation

The relationship between the role of mangroves and the morphodynamic response of the shoreline as it has been outlined [15], indicates that sedimentation modifies the geomorphological setup and influences the soil characteristics, groundwater reach, and substrate salinity, which determines mangrove zonation and species distribution. The said process includes deposition of fine-grained, clay-dominant particles within the forest floor, which is considered to be one of the driving factors of land-building and shoreline progradation. Likewise, coastal sedimentation is favored by mangrove vegetation resisting the tidal water flow and trapping the sediments through the network of their roots. In this way, mangroves, acting as traps for both mineral and organic sediments, control sedimentation and thus form their own survival ground, and eventually, the result permits a clear differentiation between coasts with and without mangroves [16].

In accordance with the above, it has been commonly accepted that mangroves functioned as sediment traps and eventually causing ground formation [17, 18]. Although other observations have suggested that ground formation is actually a preexisting process that is accelerated by mangroves after colonizing suitable areas [15]. In general, however, the origin of ground available for mangrove recruiting is scarcely addressed. Here, mangroves are proposed as being opportunists that colonize an already available substratum, in this case, thrombolithic structures.

Our observations thus provide evidence that thrombolite and pro-thrombolite platforms are, along with thick cyanophyte mats, the substrates most likely to promote mangrove colonization [2], evolving from an organic mat, and passing through the stages of conglomerate mats, all constructed mainly by long multiseriate filaments of Microcoleus chthonoplastes having thick mucilaginous sheaths that remain long after death (Figures 10 and 11a), and pro-thrombolite platforms, into sedimentary rock by micrite deposition [1, 2].

Figure 10.

a, b, and c) different magnifications for a multiseriate filament of Microcoleus chthonoplastes showing thick mucilaginous sheath; and d) Oscillatoria limosa. Both were from a cyanophyte mat collected at El Conchalito, La Paz bay.

Figure 11.

a) Close-up of empty Microcoleus chthonoplastes sheaths from a desiccated mat behind the mangrove line at San Carlos, Bahia Magdalena. Microphotograph is taken at 100×. b) Recruit of Rhizophopra mangle affixed in a cyanophyte mat in front of the mangrove line in the intertidal of San Carlos, Bahia Magdalena. c) Recruit of Laguncularia racemosa anchored on a pro-thrombolithic (unlithified) block at El Mogote, La Paz lagoon. d) Recruits of Rhizophora mangle anchored on a seaward extended thrombolithic platform at San Carlos, Bahia Magdalena.

During their evolution, the various thrombolithic phases are opportunistically colonized by mangrove recruits (Figure 11b, c, and d). It is thus a preexisting process that is accelerated by mangroves after colonizing suitable areas as suggested previously [15], such as those provided by the pro-thrombolite and thrombolite platforms.

3. Conclusion

The present thesis had a serendipitous origin. During an exploration for appropriate sites to collect epipelic diatoms in the La Paz lagoon, I came upon an irregularity on the shore that looked like a disgusting land dump, darkened by extreme irradiance, which got me thinking about what was I to do in terms of the expected distribution of the diatom assemblages. Fortunately, a quick turn of thought focusing on the sedimentary structure gave a start to a series of questions and hypotheses based on the above observations that ended in the proposal of a complementary theory for the origin of coastal lagoons. Likewise, this was followed by yet another complementary theory, the present one on the origin of coastal ground that explained the availability of substratum for the opportunistic recruiting and establishment of mangrove forests. Albeit somewhat iconoclastic, it is still an incipient theory in need of much work to fill in the many gaps that individual creativity can observe. Let us hope that specialists on mangrove ecology and geomorphology of coastal lagoons consider this theory from their own perspective for proper continuance, conceiving ideas for giving way to various research problems and enriching it.

Acknowledgments

This study is the basis for a periodic presentation in class and seminars at Cicimar-IPN, addressing the role of scientific hypothesis in the scientification of serendipitous observations. During a great part of this investigation, several colleagues eagerly aided in gathering the evidence referred to above: Oscar U. Hernández Almeida, Uri Argumedo Hernández, Rubén García Gómez, and Janette Murillo Jiménez. I thank my colleague José Borges who recently rescued the draft of this paper from cybernetic mayhem. Francisco López Fuerte formatted the photographic material.