Volume VII: Nature — Chapter 3: Marine Ecosystems

"Land is an island. Everything else is the ocean. We call ourselves children of the earth, but our blood is salty, and the first cell from which we descend was born in darkness beneath three thousand measures of water." — Guild of Navigators, "Book of the Depths," +3,640 Y.E.

Related sections: → see [Volume I, Ch.14, "The Oceans of Théxar"], → see [Volume I, Ch.21, "Ecosystems"], → see [Volume I, Ch.20, "The Origin of Life"], → see [Volume VII, Ch.07, "The Biology of the Moryn"], → see [Volume VII, Ch.09, "The Plant Kingdom"], → see [Volume VII, Ch.13, "Biosphere Interactions"], → see [Volume VIII, Ch.05, "The Thalas Ocean"], → see [Volume VIII, Ch.06, "The Gharra Ocean"] Length: 20 pages

Introduction

The oceans cover 67% of the surface of Théxar and support diverse marine communities (→ see [Volume I, Ch.14, "The Oceans of Théxar"]). The two oceans — the northern Thalas and the southern Gharra — are divided by a system of continents and five seas, and each body of water has its own hydrological and biological regime.

The marine biosphere of Théxar differs from the terrestrial one in three fundamental features. First, the metabolic pathways of the most ancient life have survived here: the H₂S chemosynthesis that arose in the hydrothermal fields of Gharra 4.2 billion cycles ago still feeds entire ecosystems (→ see [Volume I, Ch.20, "The Origin of Life"]). Second, reef-building on Théxar is based not on calcite but on a silicon-calcium composite, which allows reefs to exist deeper and in more acidic water. Third, the Gharra Ocean is the homeland of the planet's third sentient species, the Moryn (→ see [Volume VII, Ch.07, "The Biology of the Moryn"]).

This chapter deepens the survey description of marine life given in Volume I and examines marine communities as a system: from the illuminated surface layer to the hadal trenches, where pressure reaches a thousand measures.

3.1 The Ocean as the Planet's Main Biome

The marine biosphere exceeds the terrestrial one roughly twofold in area but yields to it in biomass — a consequence of the fact that oceanic producers are represented chiefly by microscopic phytoplankton with rapid turnover.

The two oceans compared:

The asymmetry of the two oceans has deep biological consequences. Thalas is warmer, better mixed, with broad shelves; the main fishery biomass is concentrated here. Gharra has deeper trenches, colder, isolated by the circumpolar current; endemism is higher here and the planet's most ancient communities have survived here (→ see [Volume VIII, Ch.06, "The Gharra Ocean"]).

3.2 Vertical Zonation

The vertical stratification of the oceans of Théxar is determined above all by the penetration of the light of Khar'Vex. The orange spectrum of the star (a broad visible spectrum) is absorbed by water faster than blue-green, so the euphotic zone of Théxar is shallower than one might expect at the same water transparency.

KEY FACT

The euphotic zone of Théxar extends only to 146 sha (≈120 m) — a third shallower than its terrestrial analogue. Compensation is achieved in two ways: the elevated quantum efficiency of chromophyll (0.92) and the presence of the deep-water pigment Thal phycobilin with an absorption peak at 620 nm, working down to 240 sha (→ see [Volume I, Ch.17, "Flora"]).

3.2.1 The Dysphotic Zone and "Twilight Farms"

The 146–732 sha layer is a zone where ordinary photosynthesis is limited; specialised pigments use the upper part of the layer, but light still carries information. Here is concentrated the diel vertical migration — the largest regular movement of living matter on the planet by mass: up to 4.1 Gt of biomass rises to the surface during the dark hours of the local day (27.3 h) and descends 300–600 sha at dawn.

Because of the greater length of the Théxar day, the migration cycle is stretched: the ascent takes ~4.5 h, the stay near the surface up to 11 h, the descent ~3.8 h. This makes the Théxar "twilight conveyor" a more efficient pump of organic carbon into the depths than on planets with a 24-hour cycle.

3.3 Producers of the Ocean

3.3.1 Phytoplankton

The base of the marine food web is microscopic single-celled producers, providing 28% of the planet's total primary production (→ see [Volume I, Ch.21, "Ecosystems"]).

Silodiatoma — silica-flagellate algae with shells of amorphous SiO₂ — play on Théxar the role analogous to terrestrial diatoms, but their shells are twice as thick: a consequence of the excess of dissolved silicon in the ocean (0.14 mmol/L against terrestrial 0.07). The settling of their shells forms siliceous oozes covering 31% of the abyssal floor and is a key link in the silicon cycle (→ see [Volume VII, Ch.13, "Biosphere Interactions"]).

3.3.2 Macroalgae — Division Thalophyta

The division Thalophyta numbers ~32,000 described species (→ see [Volume VII, Ch.09, "The Plant Kingdom"]). It reaches its greatest development in the cold shelf waters of Gharra and in the Ven-Khal Strait.

Thal-Ribbon is the largest organism of Théxar by linear size: individual thalli reach 104 sha (≈85 m), and clone fields in the Serrin Sea occupy up to 340 mo². Growth — up to 0.6 sha per local day.

3.4 Pelagic Communities

3.4.1 Trophic Structure of the Pelagial

The energy transfer efficiency in the marine chains of Théxar (~12%) is slightly higher than on Earth — a consequence of higher photosynthetic efficiency (4.2%) and lower thermoregulation losses among the predominantly poikilothermic marine Thermochromadae.

3.4.2 Key Pelagic Species

Thal-mor is the largest animal of Théxar. It migrates up to 1,626 mo per cycle, following the phytoplankton bloom. Its role in the ecosystem is not limited to consumption: the vertical movements of the Thal-mor carry ~0.9 Gt of biogenic elements per year from the depths to the surface (a "biological pump in reverse").

Ven-Krill is the mass species of the polar Koss Sea; its total biomass is estimated at 1.4 Gt, making it the base of all high-latitude marine communities and the main food of migrating filter feeders.

3.5 Benthos and Reef Systems

3.5.1 Silicon-Calcium Reefs

Reef-building on Théxar differs fundamentally from the terrestrial: reef-building organisms build their skeleton from a silicon-calcium composite (SiO₂·CaCO₃ in roughly 2:3 ratio), significantly less soluble in weakly acidic water (→ see [Volume I, Ch.14, "The Oceans of Théxar"]).

Double symbiosis is the key innovation of the Théxar reefs (→ see [Volume VII, Ch.10, "Symbiosis and Coevolution"]). In addition to photosynthetic symbionts, reef polyps contain in their tissues chemoautotrophic bacteria that oxidize H₂S. This allows the reefs to maintain a positive energy balance at depths where light is already insufficient and to survive periods of water turbidity.

3.5.2 The Largest Reef Systems

The Ven-Khal Deep Reef is a unique system, existing almost entirely on chemoautotrophic symbionts at a depth of 90–190 sha, where illumination is less than 1.4% of the surface. It serves as the main argument for the hypothesis that the first reefs of Théxar were chemosynthetic, and photosymbiosis is a secondary acquisition (→ see [Volume VII, Ch.11, "Evolution Patterns"]).

3.5.3 Soft Grounds and Infauna

The abyssal plains of both oceans are covered with siliceous and clayey oozes. Life density here is low, but diversity is high: up to 1,200 species per mo² at a biomass of only 0.02 ru/mo².

3.6 Chemosynthetic Ecosystems

3.6.1 Hydrothermal Fields

12 fields of hydrothermal vents have been discovered on the floor of the oceans of Théxar (→ see [Volume I, Ch.14, "The Oceans of Théxar"]). They are direct heirs of the environment in which the planet's life arose.

Community structure of a hydrothermal field:

KEY FACT

The productivity of a hydrothermal field is up to 12 ru/mo² — 600 times higher than the surrounding abyss. Yet the fields occupy only 1.1 mo² of total area — less than one millionth of the floor. They are not literal "oases" but refugia of ancient metabolic pathways (→ see [Volume VII, Ch.12, "Extinction Events"]).

3.6.2 Cold Seeps and Sulfide Banks

In addition to hot vents, the shelves and slopes of Théxar are widely dotted with cold seeps — seepages of methane and hydrogen sulfide from sedimentary strata. More than 340 of them have been described; total area — about 940 mo², almost a thousand times the area of the hydrothermal fields.

Brine pools — "lakes under water" with salinity up to 18% — serve as reservoirs of the most extremophilic forms and, according to the Guild of Natural History, contain lineages that diverged from the rest of the biosphere more than 900 million cycles ago.

3.6.3 Carrion Communities

The carcass of a Thal-mor descending to the abyssal floor creates a local ecosystem that persists for 40–90 cycles and passes through four stages: scavengers (0.5–2 cycles), gnawers (2–8), sulfophilic (8–70), and reef (70–90). The sulfophilic stage is the most important: the decomposition of bone lipids releases H₂S, supporting a chemosynthetic community related to the hydrothermal one. These "islands" serve as stepping stones of dispersal between the divided hydrothermal fields.

3.7 Hadal Ecosystems

The hadal zone (below 8,537 sha) occupies 1.3% of the ocean volume, but represents a set of isolated "pockets" with the highest endemism.

The Thash Trench is the deepest point of the planet. The temperature of the bottom water here is 1.6°X higher than in comparable trenches, because of the elevated geothermal flux in the subduction zone (→ see [Volume I, Ch.12, "Geological Evolution"]). This creates conditions for an unusually rich community for a hadal setting: 610 species have been described, of which 555 occur nowhere else.

Adaptations of the hadal fauna:

• Silicon-organic membranes (the same solution as in the Moryn and the Kel'vash — → see [Volume VII, Ch.11, "Evolution Patterns"]);
• Piezolytes — low-molecular stabilizers of proteins, concentration rising linearly with depth;
• Complete abandonment of gas cavities, including the auditory ones;
• Slowed metabolism: 6–9 times lower than in shelf relatives;
• Lifespans up to 240 cycles at small body size.

3.8 The Moryn as an Ecological Factor

The Gharra Ocean is not only the habitat of the third sentient species, but also an ecosystem modified by it (→ see [Volume VII, Ch.07, "The Biology of the Moryn"]).

The Moryn are the only sentient species of Théxar whose agriculture is built entirely on chemosynthesis. Their "fields" are artificially widened seeps, into which H₂S flows from the sedimentary strata are directed. This practice, no less than 40,000 cycles old, makes the Moryn the planet's oldest farmers — even though they have never seen either the sun or plants.

3.9 Productivity and Biogeochemistry of the Ocean

3.9.1 Geographical Distribution of Production

3.9.2 The Ocean's Role in the Sulfur Cycle

The ocean is the main reservoir and the main processor of sulfur on Théxar (→ see [Volume VII, Ch.13, "Biosphere Interactions"]).

The biogenic emission of hydrogen sulfide from the ocean is the source of the 0.0008% H₂S contained in the atmosphere of Théxar and serving as the planet's key biomarker (→ see [Volume I, Ch.15, "Atmosphere and Climate"]).

3.10 Threats and Conservation

The marine ecosystems of Théxar survived a period of heavy anthropogenic pressure during the Crisis Era (−2,100 – −750) and the Rebirth (→ see [Volume VII, Ch.12, "Extinction Events"]). The modern policy of the three species rests on the Law of Silence and the Law of Consent.

Protected waters: 8% of the ocean area has reserve status. The largest zone — Gharra Deep — covers the deep trenches of the southern ocean and the entire main settlement area of the Moryn. Since +2,100 Y.E. not a single species extinction has been recorded in the marine biosphere of Théxar.

Chapter Summary

1. The oceans of Théxar (67% of the surface) contain ~980,000 described species and yield ~36 Gt C/cycle — 30% of the planet's primary production; Thalas is warmer and more productive, Gharra with deeper trenches and more endemic.
2. The euphotic zone is shallow (0–146 sha) because of the orange spectrum of Khar'Vex; compensated by chromophyll quantum efficiency (0.92) and the deep-water Thal phycobilin (620 nm).
3. Reefs are built of silicon-calcium composite and use double symbiosis — photosynthetic and chemoautotrophic — which allows them to grow down to 190 sha.
4. Chemosynthetic communities (12 hydrothermal fields, 340 cold seeps, carrion communities) are refugia of the planet's most ancient metabolic pathways; productivity up to 12 ru/mo².
5. The Gharra Ocean is the homeland of the Moryn, the only civilization of Théxar built on chemosynthesis; 8% of the waters are protected, no marine species extinctions have been recorded since +2,100 Y.E.